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THE ELEMENTS OF GEOLOGY

BY WILLIAM HARMON NORTON

PROFESSOR OF GEOLOGY IN CORNELL COLLEGE

PREFACE

Geology is a science of such rapid growth that no apology is

expected when from time to time a new text-book is added to those

already in the field. The present work, however, is the outcome of

the need of a text-book of very simple outline, in which causes

and their consequences should be knit together as closely as

possible,--a need long felt by the author in his teaching, and

perhaps by other teachers also. The author has ventured,

therefore, to depart from the common usage which subdivides

geology into a number of departments,--dynamical, structural,

physiographic, and historical,--and to treat in immediate

connection with each geological process the land forms and the

rock structures which it has produced.

It is hoped that the facts of geology and the inferences drawn

from them have been so presented as to afford an efficient

discipline in inductive reasoning. Typical examples have been used

to introduce many topics, and it has been the author's aim to give

due proportion to both the wide generalizations of our science and

to the concrete facts on which they rest.

There have been added a number of practical exercises such as the

author has used for several years in the class room. These are not

made so numerous as to displace the problems which no doubt many

teachers prefer to have their pupils solve impromptu during the

recitation, but may, it is hoped, suggest their use.

In historical geology a broad view is given of the development of

the North American continent and the evolution of life upon the

planet. Only the leading types of plants and animals are

mentioned, and special attention is given to those which mark the

lines of descent of forms now living.

By omitting much technical detail of a mineralogical and

paleontological nature, and by confining the field of view almost

wholly to our own continent, space has been obtained to give to

what are deemed for beginners the essentials of the science a

fuller treatment than perhaps is common.

It is assumed that field work will be introduced with the

commencement of the study. The common rocks are therefore briefly

described in the opening chapters. The drift also receives early

mention, and teachers in the northern states who begin geology in

the fall may prefer to take up the chapter on the Pleistocene

immediately after the chapter on glaciers.

Simple diagrams have been used freely, not only because they are

often clearer than any verbal statement, but also because they

readily lend themselves to reproduction on the blackboard by the

pupil. The text will suggest others which the pupil may invent. It

is hoped that the photographic views may also be used for

exercises in the class room.

The generous aid of many friends is recognized with special

pleasure. To Professor W. M. Davis of Harvard University there is

owing a large obligation for the broad conceptions and luminous

statements of geologic facts and principles with which he has

enriched the literature of our science, and for his stimulating

influence in education. It is hoped that both in subject-matter

and in method the book itself makes evident this debt. But besides

a general obligation shared by geologists everywhere, and in

varying degrees by perhaps all authors of recent American text-

books in earth science, there is owing a debt direct and personal.

The plan of the book, with its use of problems and treatment of

land forms and rock structures in immediate connection with the

processes which produce them, was submitted to Professor Davis,

and, receiving his approval, was carried into effect, although

without the sanction of precedent at the time. Professor Davis

also kindly consented to read the manuscript throughout, and his

many helpful criticisms and suggestions are acknowledged with

sincere gratitude.

Parts of the manuscript have been reviewed by Dr. Samuel Calvin

and Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W.

Beyer of the Iowa College of Agriculture and Mechanic Arts; Dr. U.

S. Grant of Northwestern University; Professor J. A. Udden of

Augustana College, Illinois; Dr. C. H. Gordon of the New Mexico

State School of Mines; Principal Maurice Ricker of the High

School, Burlington, Iowa; and the following former students of the

author who are engaged in the earth sciences: Dr. W. C. Alden of

the United States Geological Survey and the University of Chicago;

Mr. Joseph Sniffen, instructor in the Academy of the University of

Chicago, Morgan Park; Professor Martin Iorns, Fort Worth

University, Texas; Professor A. M. Jayne, Dakota University;

Professor G. H. Bretnall, Monmouth College, Illinois; Professor

Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable,

instructor in the Iowa State Normal College; Principal C. C. Gray

of the High School, Fargo, North Dakota; and Mr. Charles Persons

of the High School, Hannibal, Missouri. A large number of the

diagrams of the book were drawn by Mr. W. W. White of the Art

School of Cornell College. To all these friends, and to the many

who have kindly supplied the illustrations of the text, whose

names are mentioned in an appended list, the writer returns his

heartfelt thanks.

WILLIAM HARMON NORTON

CORNELL COLLEGE, MOUNT VERNON, IOWA

JULY, 1905

INTRODUCTORY NOTE

During the preparation of this book Professor Norton has

frequently discussed its plan with me by correspondence, and we

have considered together the matters of scope, arrangement, and

presentation.

As to scope, the needs of the young student and not of the expert

have been our guide; the book is therefore a text-book, not a

reference volume.

In arrangement, the twofold division of the subject was chosen

because of its simplicity and effectiveness. The principles of

physical geology come first; the several chapters are arranged in

what is believed to be a natural order, appropriate to the

greatest part of our country, so that from a simple beginning a

logical sequence of topics leads through the whole subject. The

historical view of the science comes second, with many specific

illustrations of the physical processes previously studied, but

now set forth as part of the story of the earth, with its many

changes of aspect and its succession of inhabitants. Special

attention is here given to North America, and care is taken to

avoid overloading with details.

With respect to method of presentation, it must not be forgotten

that the text-book is only one factor in good teaching, and that

in geology, as in other sciences, the teacher, the laboratory, and

the local field are other factors, each of which should play an

appropriate part. The text suggests observational methods, but it

cannot replace observation in field or laboratory; it offers

certain exercises, but space cannot be taken to make it a

laboratory manual as well as a book for study; it explains many

problems, but its statements are necessarily more terse than the

illustrative descriptions that a good and experienced teacher

should supply. Frequent use is made of induction and inference in

order that the student may come to see how reasonable a science is

geology, and that he may avoid the too common error of thinking

that the opinions of "authorities" are reached by a private road

that is closed to him. The further extension of this method of

presentation is urged upon the teacher, so that the young

geologist may always learn the evidence that leads to a

conclusion, and not only the conclusion itself.

W. M. DAVIS

HARVARD UNIVERSITY, CAMBRIDGE, MASS.

JULY, 1905

CONTENTS

INTRODUCTION.--THE SCOPE AND AIM OF GEOLOGY

PART I

EXTERNAL GEOLOGICAL AGENCIES

I. THE WORK OF THE WEATHER

II. THE WORK OF GROUND WATER

III. RIVERS AND VALLEYS

IV. RIVER DEPOSITS

V. THE WORK OF GLACIERS

VI. THE WORK OF THE WIND

VII. THE SEA AND ITS SHORES

VIII. OFFSHORE AND DEEP-SEA DEPOSITS

PART II

INTERNAL GEOLOGICAL AGENCIES

IX. MOVEMENTS OF THE EARTH'S CRUST

X. EARTHQUAKES

XI. VOLCANOES

XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN

XIII. METAMORPHISM AND MINERAL VEINS

PART III

HISTORICAL GEOLOGY

XIV. THE GEOLOGICAL RECORD

XV. THE PRE-CAMBRIAN SYSTEMS

XVI. THE CAMBRIAN

XVII. THE ORDOVICIAN AND SILURIAN

XVIII. THE DEVONIAN

XIX. THE CARBONIFEROUS

XX. THE MESOZOIC

XXI. THE TERTIARY

XXII. THE QUATERNARY

INDEX

THE ELEMENTS OF GEOLOGY

INTRODUCTION

THE SCOPE AND AIM OF GEOLOGY

Geology deals with the rocks of the earth's crust. It learns from

their composition and structure how the rocks were made and how

they have been modified. It ascertains how they have been brought

to their present places and wrought to their various topographic

forms, such as hills and valleys, plains and mountains. It studies

the vestiges which the rocks preserve of ancient organisms which

once inhabited our planet. Geology is the history of the earth and

its inhabitants, as read in the rocks of the earth's crust.

To obtain a general idea of the nature and method of our science

before beginning its study in detail, we may visit some valley,

such as that illustrated in the frontispiece, on whose sides are

rocky ledges. Here the rocks lie in horizontal layers. Although

only their edges are exposed, we may infer that these layers run

into the upland on either side and underlie the entire district;

they are part of the foundation of solid rock which everywhere is

found beneath the loose materials of the surface.

The ledges of the valley of our illustration are of sandstone.

Looking closely at the rock we see that it is composed of myriads

of grains of sand cemented together. These grains have been worn

and rounded. They are sorted also, those of each layer being about

of a size. By some means they have been brought hither from some

more ancient source. Surely these grains have had a history before

they here found a resting place,--a history which we are to learn

to read.

The successive layers of the rock suggest that they were built one

after another from the bottom upward. We may be as sure that each

layer was formed before those above it as that the bottom courses

of stone in a wall were laid before the courses which rest upon

them.

We have no reason to believe that the lowest layers which we see

here were the earliest ever formed. Indeed, some deep boring in

the vicinity may prove that the ledges rest upon other layers of

rock which extend downward for many hundreds of feet below the

valley floor. Nor may we conclude that the highest layers here

were the latest ever laid; for elsewhere we may find still later

layers lying upon them.

A short search may find in the rock relics of animals, such as the

imprints of shells, which lived when it was deposited; and as

these are of kinds whose nearest living relatives now have their

home in the sea, we infer that it was on the flat sea floor that

the sandstone was laid. Its present position hundreds of feet

above sea level proves that it has since emerged to form part of

the land; while the flatness of the beds shows that the movement

was so uniform and gentle as not to break or strongly bend them

from their original attitude.

The surface of some of these layers is ripple-marked. Hence the

sand must once have been as loose as that of shallow sea bottoms

and sea beaches to-day, which is thrown into similar ripples by

movements of the water. In some way the grains have since become

cemented into firm rock.

Note that the layers on one side of the valley agree with those on

the other, each matching the one opposite at the same level. Once

they were continuous across the valley. Where the valley now is

was once a continuous upland built of horizontal layers; the

layers now show their edges, or OUTCROP, on the valley sides

because they have been cut by the valley trench.

The rock of the ledges is crumbling away. At the foot of each step

of rock lie fragments which have fallen. Thus the valley is slowly

widening. It has been narrower in the past; it will be wider in

the future.

Through the valley runs a stream. The waters of rains which have

fallen on the upper parts of the stream's basin are now on their

way to the river and the sea. Rock fragments and grains of sand

creeping down the valley slopes come within reach of the stream

and are washed along by the running water. Here and there they

lodge for a time in banks of sand and gravel, but sooner or later

they are taken up again and carried on. The grains of sand which

were brought from some ancient source to form these rocks are on

their way to some new goal. As they are washed along the rocky bed

of the stream they slowly rasp and wear it deeper. The valley will

be deeper in the future; it has been less deep in the past.

In this little valley we see slow changes now in progress. We find

also in the composition, the structure, and the attitude of the

rocks, and the land forms to which they have been sculptured, the

record of a long succession of past changes involving the origin

of sand grains and their gathering and deposit upon the bottom of

some ancient sea, the cementation of their layers into solid rock,

the uplift of the rocks to form a land surface, and, last of all,

the carving of a valley in the upland. Everywhere, in the fields,

along the river, among the mountains, by the seashore, and in the

desert, we may discover slow changes now in progress and the

record of similar changes in the past. Everywhere we may catch

glimpses of a process of gradual change, which stretches backward

into the past and forward into the future, by which the forms and

structures of the face of the earth are continually built and

continually destroyed. The science which deals with this long

process is geology. Geology treats of the natural changes now

taking place upon the earth and within it, the agencies which

produce them, and the land forms and rock structures which result.

It studies the changes of the present in order to be able to read

the history of the earth's changes in the past.

The various agencies which have fashioned the face of the earth

may. be divided into two general classes. In Part I we shall

consider those which work upon the earth from without, such as the

weather, running water, glaciers, the wind, and the sea. In Part

II we shall treat of those agencies whose sources are within the

earth, and among whose manifestations are volcanoes and

earthquakes and the various movements of the earth's crust. As we

study each agency we shall notice not only how it does its work,

but also the records which it leaves in the rock structures and

the land forms which it produces. With this preparation we shall

be able in Part III to read in the records of the rocks the

history of our planet and the successive forms of life which have

dwelt upon it.

PART I

EXTERNAL GEOLOGICAL AGENCIES

CHAPTER I

THE WORK OF THE WEATHER

In our excursion to the valley with sandstone ledges we witnessed

a process which is going forward in all lands. Everywhere the

rocks are crumbling away; their fragments are creeping down

hillsides to the stream ways and are carried by the streams to the

sea, where they are rebuilt into rocky layers. When again the

rocks are lifted to form land the process will begin anew; again

they will crumble and creep down slopes and be washed by streams

to the sea. Let us begin our study of this long cycle of change at

the point where rocks disintegrate and decay under the action of

the weather. In studying now a few outcrops and quarries we shall

learn a little of some common rocks and how they weather away.

STRATIFICATION AND JOINTING. At the sandstone ledges we saw that

the rock was divided into parallel layers. The thicker layers are

known as STRATA, and the thin leaves into which each stratum may

sometimes be split are termed LAMINAE. To a greater or less degree

these layers differ from each other in fineness of grain, showing

that the material has been sorted. The planes which divide them

are called BEDDING PLANES.

Besides the bedding planes there are other division planes, which

cut across the strata from top to bottom. These are found in all

rocks and are known as joints. Two sets of joints,

running at about right angles to each other, together with the

bedding planes, divide the sandstone into quadrangular blocks.

SANDSTONE. Examining a piece of sandstone we find it composed of

grains quite like those of river sand or of sea beaches. Most of

the grains are of a clear glassy mineral called quartz. These

quartz grains are very hard and will scratch the steel of a knife

blade. They are not affected by acid, and their broken surfaces

are irregular like those of broken glass.

The grains of sandstone are held together by some cement. This may

be calcareous, consisting of soluble carbonate of lime. In brown

sandstones the cement is commonly ferruginous,--hydrated iron

oxide, or iron rust, forming the bond, somewhat as in the case of

iron nails which have rusted together. The strongest and most

lasting cement is siliceous, and sand rocks whose grains are

closely cemented by silica, the chemical substance of which quartz

is made, are known as quartzites.

We are now prepared to understand how sandstone is affected by the

action of the weather. On ledges where the rock is exposed to view

its surface is more or less discolored and the grains are loose

and may be rubbed off with the finger. On gentle slopes the rock

is covered with a soil composed of sand, which evidently is

crumbled sandstone, and dark carbonaceous matter derived from the

decay of vegetation. Clearly it is by the dissolving of the cement

that the rock thus breaks down to loose sand. A piece of sandstone

with calcareous cement, or a bit of old mortar, which is really an

artificial stone also made of sand cemented by lime, may be

treated in a test tube with hydrochloric acid to illustrate the

process.

A LIMESTONE QUARRY. Here also we find the rock stratified and

jointed (Fig. 2). On the quarry face the rock is distinctly seen

to be altered for some distance from its upper surface. Below the

altered zone the rock is sound and is quarried for building; but

the altered upper layers are too soft and broken to be used for

this purpose. If the limestone is laminated, the laminae here have

split apart, although below they hold fast together. Near the

surface the stone has become rotten and crumbles at the touch,

while on the top it has completely broken down to a thin layer of

limestone meal, on which rests a fine reddish clay.

Limestone is made of minute grains of carbonate of lime all firmly

held together by a calcareous cement. A piece of the stone placed

in a test tube with hydrochloric acid dissolves with brisk

effervescence, leaving the insoluble impurities, which were

disseminated through it, at the bottom of the tube as a little

clay.

We can now understand the changes in the upper layers of the

quarry. At the surface of the rock the limestone has completely

dissolved, leaving the insoluble residue as a layer of reddish

clay. Immediately below the clay the rock has disintegrated into

meal where the cement between the limestone grains has been

removed, while beneath this the laminae are split apart where the

cement has been dissolved only along the planes of lamination

where the stone is more porous. As these changes in the rock are

greatest at the surface and diminish downward, we infer that they

have been caused by agents working downward from the surface.

At certain points these agencies have been more effective than

elsewhere. The upper rock surface is pitted. Joints are widened as

they approach the surface, and along these seams we may find that

the rock is altered even down to the quarry floor.

A SHALE PIT. Let us now visit some pit where shale--a laminated

and somewhat hardened clay--is quarried for the manufacture of

brick. The laminae of this fine-grained rock may be as thin as

cardboard in places, and close joints may break the rock into

small rhombic blocks. On the upper surface we note that the shale

has weathered to a clayey soil in which all traces of structure

have been destroyed. The clay and the upper layers of the shale

beneath it are reddish or yellow, while in many cases the color of

the unaltered rock beneath is blue.

THE SEDIMENTARY ROCKS. The three kinds of layered rocks whose

acquaintance we have made--sandstone, limestone, and shale--are

the leading types of the great group of stratified, or

sedimentary, rocks. This group includes all rocks made of

sediments, their materials having settled either in water upon the

bottoms of rivers, lakes, or seas, or on dry land, as in the case

of deposits made by the wind and by glaciers. Sedimentary rocks

are divided into the fragmental rocks--which are made of

fragments, either coarse or fine--and the far less common rocks

which are constituted of chemical precipitates.

The sedimentary rocks are divided according to their composition

into the following classes:

1. The arenaceous, or quartz rocks, including beds of loose sand

and gravel, sandstone, quartzite, and conglomerate (a rock made of

cemented rounded gravel or pebbles).

2. The calcareous, or lime rocks, including limestone and a soft

white rock formed of calcareous powder known as chalk.

3. The argillaceous, or clay rocks, including muds, clays, and

shales. These three classes pass by mixture into one another. Thus

there are limy and clayey sandstones, sandy and clayey limestones,

and sandy and limy shales.

GRANITE. This familiar rock may be studied as an example of the

second great group of rocks,--the unstratified, or igneous rocks.

These are not made of cemented sedimentary grains, but of

interlocking crystals which have crystallized from a molten mass.

Examining a piece of granite, the most conspicuous crystals which

meet the eye are those of feldspar. They are commonly pink, white,

or yellow, and break along smooth cleavage planes which reflect

the light like tiny panes of glass. Mica may be recognized by its

glittering plates, which split into thin elastic scales. A third

mineral, harder than steel, breaking along irregular surfaces like

broken glass, we identify as quartz.

How granite alters under the action of the weather may be seen in

outcrops where it forms the bed rock, or country rock, underlying

the loose formations of the surface, and in many parts of the

northern states where granite bowlders and pebbles more or less

decayed may be found in a surface sheet of stony clay called the

drift. Of the different minerals composing granite, quartz alone

remains unaltered. Mica weathers to detached flakes which have

lost their elasticity. The feldspar crystals have lost their

luster and hardness, and even have decayed to clay. Where long-

weathered granite forms the country rock, it often may be cut with

spade or trowel for several feet from the surface, so rotten is

the feldspar, and here the rock is seen to break down to a clayey

soil containing grains of quartz and flakes of mica.

These are a few simple illustrations of the surface changes which

some of the common kinds of rocks undergo. The agencies by which

these changes are brought about we will now take up under two

divisions,--CHEMICAL AGENCIES producing rock decay and MECHANICAL

AGENCIES producing rock disintegration.

THE CHEMICAL WORK OF WATER

As water falls on the earth in rain it has already absorbed from

the air carbon dioxide (carbonic acid gas) and oxygen. As it sinks

into the ground and becomes what is termed ground water, it takes

into solution from the soil humus acids and carbon dioxide, both

of which are constantly being generated there by the decay of

organic matter. So both rain and ground water are charged with

active chemical agents, by the help of which they corrode and rust

and decompose all rocks to a greater or less degree. We notice now

three of the chief chemical processes concerned in weathering,--

solution, the formation of carbonates, and oxidation.

SOLUTION. Limestone, although so little affected by pure water

that five thousand gallons would be needed to dissolve a single

pound, is easily dissolved in water charged with carbon dioxide.

In limestone regions well water is therefore "hard." On boiling

the water for some time the carbon dioxide gas is expelled, the

whole of the lime carbonate can no longer be held in solution, and

much of it is thrown down to form a crust or "scale" in the kettle

or in the tubes of the steam boiler. All waters which flow over

limestone rocks or soak through them are constantly engaged in

dissolving them away, and in the course of time destroy beds of

vast extent and great thickness.

The upper surface of limestone rocks becomes deeply pitted, as we

saw in the limestone quarry, and where the mantle of waste has

been removed it may be found so intricately furrowed that it is

difficult to traverse.

Beds of rock salt buried among the strata are dissolved by seeping

water, which issues in salt springs. Gypsum, a mineral composed of

hydrated sulphate of lime, and so soft that it may be scratched

with the finger nail, is readily taken up by water, giving to the

water of wells and springs a peculiar hardness difficult to

remove.

The dissolving action of moisture may be noted on marble

tombstones of some age, marble being a limestone altered by heat

and pressure and composed of crystalline grains. By assuming that

the date on each monument marks the year of its erection, one may

estimate how many years on the average it has taken for weathering

to loosen fine grains on the polished surface, so that they may be

rubbed off with the finger, to destroy the polish, to round the

sharp edges of tool marks in the lettering, and at last to open

cracks and seams and break down the stone. We may notice also

whether the gravestones weather more rapidly on the sunny or the

shady side, and on the sides or on the top.

The weathered surface of granular limestone containing shells

shows them standing in relief. As the shells are made of

crystalline carbonate of lime, we may infer whether the carbonate

of lime is less soluble in its granular or in its crystalline

condition.

THE FORMATION OF CARBONATES. In attacking minerals water does more

than merely take them into solution. It decomposes them, forming

new chemical compounds of which the carbonates are among the most

important. Thus feldspar consists of the insoluble silicate of

alumina, together with certain alkaline silicates which are broken

up by the action of water containing carbon dioxide, forming

alkaline carbonates. These carbonates are freely soluble and

contribute potash and soda to soils and river waters. By the

removal of the soluble ingredients of feldspar there is left the

silicate of alumina, united with water or hydrated, in the

condition of a fine plastic clay which, when white and pure, is

known as KAOLIN and is used in the manufacture of porcelain.

Feldspathic rocks which contain no iron compounds thus weather to

whitish crusts, and even apparently sound crystals of feldspar,

when ground to thin slices and placed under the microscope, may be

seen to be milky in color throughout because an internal change to

kaolin has begun.

OXIDATION. Rocks containing compounds of iron weather to reddish

crusts, and the seams of these rocks are often lined with rusty

films. Oxygen and water have here united with the iron, forming

hydrated iron oxide. The effects of oxidation may be seen in the

alteration of many kinds of rocks and in red and yellow colors of

soils and subsoils.

Pyrite is a very hard mineral of a pale brass color, found in

scattered crystals in many rocks, and is composed of iron and

sulphur (iron sulphide). Under the attack of the weather it takes

up oxygen, forming iron sulphate (green vitriol), a soluble

compound, and insoluble hydrated iron oxide, which as a mineral is

known as limonite. Several large masses of iron sulphide were

placed some years ago on the lawn in front of the National Museum

at Washington. The mineral changed so rapidly to green vitriol

that enough of this poisonous compound was washed into the ground

to kill the roots of the surrounding grass.

AGENTS OF MECHANICAL DISINTEGRATION

HEAT AND COLD. Rocks exposed to the direct rays of the sun become

strongly heated by day and expand. After sunset they rapidly cool

and contract. When the difference in temperature between day and

night is considerable, the repeated strains of sudden expansion

and contraction at last become greater than the rocks can bear,

and they break, for the same reason that a glass cracks when

plunged into boiling water (Fig. 5).

Rocks are poor conductors of heat, and hence their surfaces may

become painfully hot under the full blaze of the sun, while the

interior remains comparatively cool. By day the surface shell

expands and tends to break loose from the mass of the stone. In

cooling in the evening the surface shell suddenly contracts on the

unyielding interior and in time is forced off in scales.

Many rocks, such as granite, are made up of grains of various

minerals which differ in color and in their capacity to absorb

heat, and which therefore contract and expand in different ratios.

In heating and cooling these grains crowd against their neighbors

and tear loose from them, so that finally the rock disintegrates

into sand.

The conditions for the destructive action of heat and cold are

most fully met in arid regions when vegetation is wanting for lack

of sufficient rain. The soil not being held together by the roots

of plants is blown away over large areas, leaving the rocks bare

to the blazing sun in a cloudless sky. The air is dry, and the

heat received by the earth by day is therefore rapidly radiated at

night into space. There is a sharp and sudden fall of temperature

after sunset, and the rocks, strongly heated by day, are now

chilled perhaps even to the freezing point.

In the Sahara the thermometer has been known to fall 131 degrees

F. within a few hours. In the light air of the Pamir plateau in

central Asia a rise of 90 degrees F. has been recorded from seven

o'clock in the morning to one o'clock in the afternoon. On the

mountains of southwestern Texas there are frequently heard

crackling noises as the rocks of that arid region throw off scales

from a fraction of an inch to four inches in thickness, and loud

reports are made as huge bowlders split apart. Desert pebbles

weakened by long exposure to heat and cold have been shivered to

fine sharp-pointed fragments on being placed in sand heated to 180

degrees F. Beds half a foot thick, forming the floor of limestone

quarries in Wisconsin, have been known to buckle and arch and

break to fragments under the heat of the summer sun.

FROST. By this term is meant the freezing and thawing of water

contained in the pores and crevices of rocks. All rocks are more

or less porous and all contain more or less water in their pores.

Workers in stone call this "quarry water," and speak of a stone as

"green" before the quarry water has dried out. Water also seeps

along joints and bedding planes and gathers in all seams and

crevices. Water expands in freezing, ten cubic inches of water

freezing to about eleven cubic inches of ice. As water freezes in

the rifts and pores of rocks it expands with the irresistible

force illustrated in the freezing and breaking of water pipes in

winter. The first rift in the rock, perhaps too narrow to be seen,

is widened little by little by the wedges of successive frosts,

and finally the rock is broken into detached blocks, and these

into angular chip-stone by the same process.

It is on mountain tops and in high latitudes that the effects of

frost are most plainly seen. "Every summit" says Whymper, "amongst

the rock summits upon which I have stood has been nothing but a

piled-up heap of fragments" (Fig. 7). In Iceland, in Spitsbergen,

in Kamchatka, and in other frigid lands large areas are thickly

strewn with sharp-edged fragments into which the rock has been

shattered by frost.

ORGANIC AGENTS

We must reckon the roots of plants and trees among the agents

which break rocks into pieces. The tiny rootlet in its search for

food and moisture inserts itself into some minute rift, and as it

grows slowly wedges the rock apart. Moreover, the acids of the

root corrode the rocks with which they are in contact. One may

sometimes find in the soil a block of limestone wrapped in a mesh

of roots, each of which lies in a little furrow where it has eaten

into the stone.

Rootless plants called lichens often cover and corrode rocks as

yet bare of soil; but where lichens are destroying the rock less

rapidly than does the weather, they serve in a way as a

protection.

CONDITIONS FAVORING DISINTEGRATION AND DECAY. The

disintegration of rocks under frost and temperature changes

goes on most rapidly in cold and arid climates, and where

vegetation is scant or absent. On the contrary, the decay of rocks

under the chemical action of water is favored by a warm, moist

climate and abundant vegetation. Frost and heat and cold can only

act within the few feet from the surface to which the necessary

temperature changes are limited, while water penetrates and alters

the rocks to great depths.

The pupil may explain.

In what ways the presence of joints and bedding planes assists in

the breaking up and decay of rocks under the action of the

weather.

Why it is a good rule of stone masons never to lay stones on edge,

but always on their natural bedding planes.

Why stones fresh from the quarry sometimes go to pieces in early

winter, when stones which have been quarried for some months

remain uninjured.

Why quarrymen in the northern states often keep their quarry

floors flooded during winter.

Why laminated limestone should not be used for curbstone.

Why rocks composed of layers differing in fineness of grain and in

ratios of expansion do not make good building stone.

Fine-grained rocks with pores so small that capillary attraction

keeps the water which they contain from readily draining away are

more apt to hold their pores ten elevenths full of water than are

rocks whose pores are larger. Which, therefore, are more likely to

be injured by frost?

Which is subject to greater temperature changes, a dark rock or

one of a light color? the north side or the south side of a

valley?

THE MANTLE OF ROCK WASTE

We have seen that rocks are everywhere slowly wasting away. They

are broken in pieces by frost, by tree roots, and by heat and

cold. They dissolve and decompose under the chemical action of

water and the various corrosive substances which it contains,

leaving their insoluble residues as residual clays and sands upon

the surface. As a result there is everywhere forming a mantle of

rock waste which covers the land. It is well to imagine how the

country would appear were this mantle with its soil and vegetation

all scraped away or had it never been formed. The surface of the

land would then be everywhere of bare rock as unbroken as a quarry

floor.

THE THICKNESS OF THE MANTLE. In any locality the thickness of the

mantle of rock waste depends as much on the rate at which it is

constantly being removed as on the rate at which it is forming. On

the face of cliffs it is absent, for here waste is removed as fast

as it is made. Where waste is carried away more slowly than it is

produced, it accumulates in time to great depth.

The granite of Pikes Peak is disintegrated to a depth of twenty

feet. In the city of Washington granite rock is so softened to a

depth of eighty feet that it can be removed with pick and shovel.

About Atlanta, Georgia, the rocks are completely rotted for one

hundred feet from the surface, while the beginnings of decay may

be noticed at thrice that depth. In places in southern Brazil the

rock is decomposed to a depth of four hundred feet.

In southwestern Wisconsin a reddish residual clay has an average

depth of thirteen feet on broad uplands, where it has been removed

to the least extent. The country rock on which it rests is a

limestone with about ten per cent of insoluble impurities. At

least how thick, then, was that portion of the limestone which has

rotted down to the clay?

DISTINGUISHING CHARACTERISTICS OF RESIDUAL WASTE. We must learn to

distinguish waste formed in place by the action of the weather

from the products of other geological agencies. Residual waste is

unstratified. It contains no substances which have not been

derived from the weathering of the parent rock. There is a gradual

transition from residual waste into the unweathered rock beneath.

Waste resting on sound rock evidently has been shifted and was not

formed in place.

In certain regions of southern Missouri the land is covered with a

layer of broken flints and red clay, while the country rock is

limestone. The limestone contains nodules of flint, and we may

infer that it has been by the decay and removal of thick masses of

limestone that the residual layer of clay and flints has been left

upon the surface. Flint is a form of quartz, dull-lustered,

usually gray or blackish in color, and opaque except on thinnest

edges, where it is translucent.

Over much of the northern states there is spread an unstratified

stony clay called the drift. It often rests on sound rocks. It

contains grains of sand, pebbles, and bowlders composed of many

different minerals and rocks that the country rock cannot furnish.

Hence the drift cannot have been formed by the decay of the rock

of the region. A shale or limestone, for example, cannot waste to

a clay containing granite pebbles. The origin of the drift will be

explained in subsequent chapters.

The differences in rocks are due more to their soluble than to

their insoluble constituents. The latter are few in number and are

much the same in rocks of widely different nature, being chiefly

quartz, silicate of alumina, and iron oxide. By the removal of

their soluble parts very many and widely different rocks rot down

to a residual clay gritty with particles of quartz and colored red

or yellow with iron oxide.

In a broad way the changes which rocks undergo in weathering are

an adaptation to the environment in which they find themselves at

the earth's surface,--an environment different from that in which

they were formed under sea or under ground. In open air, where

they are attacked by various destructive agents, few of the rock-

making minerals are stable compounds except quartz, the iron

oxides, and the silicate of alumina; and so it is to one or more

of these comparatively insoluble substances that most rocks are

reduced by long decay.

Which produces a mantle of finer waste, frost or chemical decay?

which a thicker mantle? In what respects would you expect that the

mantle of waste would differ in warm humid lands like India, in

frozen countries like Alaska, and in deserts such as the Sahara?

THE SOIL. The same agencies which produce the mantle of waste are

continually at work upon it, breaking it up into finer and finer

particles and causing its more complete decay. Thus on the

surface, where the waste has weathered longest, it is gradually

made fine enough to support the growth of plants, and is then

known as soil. The coarser waste beneath is sometimes spoken of as

subsoil. Soil usually contains more or less dark, carbonaceous,

decaying organic matter, called humus, and is then often termed

the humus layer. Soil forms not only on waste produced in place

from the rock beneath, but also on materials which have been

transported, such as sheets of glacial drift and river deposits.

Until rocks are reduced to residual clays the work of the weather

is more rapid and effective on the fragments of the mantle of

waste than on the rocks from which waste is being formed. Why?

Any fresh excavation of cellar or cistern, or cut for road or

railway, will show the characteristics of the humus layer. It may

form only a gray film on the surface, or we may find it a layer a

foot or more thick, dark, or even black, above, and growing

gradually lighter in color as it passes by insensible gradations

into the subsoil. In some way the decaying vegetable matter

continually forming on the surface has become mingled with the

material beneath it.

HOW HUMUS AND THE SUBSOIL ARE MINGLED. The mingling of humus and

the subsoil is brought about by several means. The roots of plants

penetrate the waste, and when they die leave their decaying

substance to fertilize it. Leaves and stems falling on the surface

are turned under by several agents. Earthworms and other animals

whose home is in the waste drag them into their burrows either for

food or to line their nests. Trees overthrown by the wind, roots

and all, turn over the soil and subsoil and mingle them together.

Bacteria also work in the waste and contribute to its enrichment.

The animals living in the mantle do much in other ways toward the

making of soil. They bring the coarser fragments from beneath to

the surface, where the waste weathers more rapidly. Their burrows

allow air and water to penetrate the waste more freely and to

affect it to greater depths.

ANTS. In the tropics the mantle of waste is worked over chiefly by

ants. They excavate underground galleries and chambers, extending

sometimes as much as fourteen feet below the surface, and build

mounds which may reach as high above it. In some parts of Paraguay

and southern Brazil these mounds, like gigantic potato hills,

cover tracts of considerable area.

In search for its food--the dead wood of trees--the so-called

white ant constructs runways of earth about the size of gas pipes,

reaching from the base of the tree to the topmost branches. On the

plateaus of central Africa explorers have walked for miles through

forests every tree of which was plastered with these galleries of

mud. Each grain of earth used in their construction is moistened

and cemented by slime as it is laid in place by the ant, and is

thus acted on by organic chemical agents. Sooner or later these

galleries are beaten down by heavy rains, and their fertilizing

substances are scattered widely by the winds.

EARTHWORMS. In temperate regions the waste is worked over largely

by earthworms. In making their burrows worms swallow earth in

order to extract from it any nutritive organic matter which it may

contain. They treat it with their digestive acids, grind it in

their stony gizzards, and void it in castings on the surface of

the ground. It was estimated by Darwin that in many parts of

England each year, on every acre, more than ten tons of earth pass

through the bodies of earthworms and are brought to the surface,

and that every few years the entire soil layer is thus worked over

by them.

In all these ways the waste is made fine and stirred and enriched.

Grain by grain the subsoil with its fresh mineral ingredients is

brought to the surface, and the rich organic matter which plants

and animals have taken from the atmosphere is plowed under. Thus

Nature plows and harrows on "the great world's farm" to make ready

and ever to renew a soil fit for the endless succession of her

crops.

The world processes by which rocks are continually wasting away

are thus indispensable to the life of plants and animals. The

organic world is built on the ruins of the inorganic, and because

the solid rocks have been broken down into soil men are able to

live upon the earth.

SOLAR ENERGY. The source of the energy which accomplishes all this

necessary work is the sun. It is the radiant energy of the sun

which causes the disintegration of rocks, which lifts vapor into

the atmosphere to fall as rain, which gives life to plants and

animals. Considering the earth in a broad way, we may view it as a

globe of solid rock,--the lithosphere,--surrounded by two mobile

envelopes: the envelope of air,--THE ATMOSPHERE, and the envelope

of water,--THE HYDROSPHERE. Under the action of solar energy these

envelopes are in constant motion. Water from the hydrosphere is

continually rising in vapor into the atmosphere, the air of the

atmosphere penetrates the hydrosphere,--for its gases are

dissolved in all waters,--and both air and water enter and work

upon the solid earth. By their action upon the lithosphere they

have produced a third envelope,--the mantle of rock waste.

This envelope also is in movement, not indeed as a whole, but

particle by particle. The causes which set its particles in

motion, and the different forms which the mantle comes to assume,

we will now proceed to study.

MOVEMENTS OF THE MANTLE OF ROCK WASTE

At the sandstone ledges which we first visited we saw not only

that the rocks were crumbling away, but also that grains and

fragments of them were creeping down the slopes of the valley to

the stream and were carried by it onward toward the sea. This

process is going on everywhere. Slowly it may be, and with many

interruptions, but surely, the waste of the land moves downward to

the sea. We may divide its course into two parts,--the path to the

stream, which we will now consider, and its carriage onward by the

stream, which we will defer to a later chapter.

GRAVITY. The chief agent concerned in the movement of waste is

gravity. Each particle of waste feels the unceasing downward pull

of the earth's mass and follows it when free to do so. All

agencies which produce waste tend to set its particles free and in

motion, and therefore cooperate with gravity. On cliffs, rocks

fall when wedged off by frost or by roots of trees, and when

detached by any other agency. On slopes of waste, water freezes in

chinks between stones, and in pores between particles of soil, and

wedges them apart. Animals and plants stir the waste, heat expands

it, cold contracts it, the strokes of the raindrops drive loose

particles down the slope and the wind lifts and lets them fall. Of

all these movements, gravity assists those which are downhill and

retards those which are uphill. On the whole, therefore, the

downhill movements prevail, and the mantle of waste, block by

block and grain by grain, creeps along the downhill path.

A slab of sandstone laid on another of the same kind at an angle

of 17 degrees and left in the open air was found to creep down the

slope at the rate of a little more than a millimeter a month.

Explain why it did so.

RAIN. The most efficient agent in the carriage of waste to the

streams is the rain. It moves particles of soil by the force of

the blows of the falling drops, and washes them down all slopes to

within reach of permanent streams. On surfaces unprotected by

vegetation, as on plowed fields and in arid regions, the rain

wears furrows and gullies both in the mantle of waste and in

exposures of unaltered rock (Fig. 17).

At the foot of a hill we may find that the soil has accumulated by

creep and wash to the depth of several feet; while where the

hillside is steepest the soil may be exceedingly thin, or quite

absent, because removed about as fast as formed. Against the walls

of an abbey built on a slope in Wales seven hundred years ago, the

creeping waste has gathered on the uphill side to a depth of seven

feet. The slow-flowing sheet of waste is often dammed by fences

and walls, whose uphill side gathers waste in a few years so as to

show a distinctly higher surface than the downhill side,

especially in plowed fields where the movement is least checked by

vegetation.

TALUS. At the foot of cliffs there is usually to be found a slope

of rock fragments which clearly have fallen from above. Such a

heap of waste is known as talus. The amount of talus in any place

depends both on the rate of its formation and the rate of its

removal. Talus forms rapidly in climates where mechanical

disintegration is most effective, where rocks are readily broken

into blocks because closely jointed and thinly bedded rather than

massive, and where they are firm enough to be detached in

fragments of some size instead of in fine grains. Talus is removed

slowly where it decays slowly, either because of the climate or

the resistance of the rock. It may be rapidly removed by a stream

flowing along its base.

In a moist climate a soluble rock, such as massive limestone, may

form talus little if any faster than the talus weathers away. A

loose-textured sandstone breaks down into incoherent sand grains,

which in dry climates, where unprotected by vegetation, may be

blown away as fast as they fall, leaving the cliff bare to the

base. Cliffs of such slow-decaying rocks as quartzite and granite

when closely jointed accumulate talus in large amounts.

Talus slopes may be so steep as to reach THE ANGLE OF REPOSE, i.e.

the steepest angle at which the material will lie. This angle

varies with different materials, being greater with coarse and

angular fragments than with fine rounded grains. Sooner or later a

talus reaches that equilibrium where the amount removed from its

surface just equals that supplied from the cliff above. As the

talus is removed and weathers away its slope retreats together

with the retreat of the cliff, as seen in Figure 9.

GRADED SLOPES. Where rocks weather faster than their waste is

carried away, the waste comes at last to cover all rocky ledges.

On the steeper slopes it is coarser and in more rapid movement

than on slopes more gentle, but mountain sides and hills and

plains alike come to be mantled with sheets of waste which

everywhere is creeping toward the streams. Such unbroken slopes,

worn or built to the least inclination at which the waste supplied

by weathering can be urged onward, are known as GRADED SLOPES.

Of far less importance than the silent, gradual creep of waste,

which is going on at all times everywhere about us, are the

startling local and spasmodic movements which we are now to

describe.

AVALANCHES. On steep mountain sides the accumulated snows of

winter often slip and slide in avalanches to the valleys below.

These rushing torrents of snow sweep their tracks clean of waste

and are one of Nature's normal methods of moving it along the

downhill path.

LANDSLIDES. Another common and abrupt method of delivering waste

to streams is by slips of the waste mantle in large masses. After

long rains and after winter frosts the cohesion between the waste

and the sound rock beneath is loosened by seeping water

underground. The waste slips on the rock surface thus lubricated

and plunges down the mountain side in a swift roaring torrent of

mud and stones.

We may conveniently mention here a second type of landslide, where

masses of solid rock as well as the mantle of waste are involved

in the sudden movement. Such slips occur when valleys have been

rapidly deepened by streams or glaciers and their sides have not

yet been graded. A favorable condition is where the strata dip

(i.e. incline downwards) towards the valley (Fig. 11), or are

broken by joint planes dipping in the same direction. The upper

layers, including perhaps the entire mountain side, have been cut

across by the valley trench and are left supported only on the

inclined surface of the underlying rocks. Water may percolate

underground along this surface and loosen the cohesion between the

upper and the underlying strata by converting the upper surface of

a shale to soft wet clay, by dissolving layers of a limestone, or

by removing the cement of a sandstone and converting it into loose

sand. When the inclined surface is thus lubricated the overlying

masses may be launched into the valley below. The solid rocks are

broken and crushed in sliding and converted into waste consisting,

like that of talus, of angular unsorted fragments, blocks of all

sizes being mingled pellmell with rock meal and dust. The

principal effects of landslides may be gathered from the following

examples.

At Gohna, India, in 1893, the face of a spur four thousand feet

high, of the lower ranges of the Himalayas, slipped into the gorge

of the headwaters of the Ganges River in successive rock falls

which lasted for three days. Blocks of stone were projected for a

mile, and clouds of limestone dust were spread over the

surrounding country. The debris formed a dam one thousand feet

high, extending for two miles along the valley. A lake gathered

behind this barrier, gradually rising until it overtopped it in a

little less than a year. The upper portion of the dam then broke,

and a terrific rush of water swept down the valley in a wave

which, twenty miles away, rose one hundred and sixty feet in

height. A narrow lake is still held by the strong base of the dam.

In 1896, after forty days of incessant rain, a cliff of sandstone

slipped into the Yangtse River in China, reducing the width of the

channel to eighty yards and causing formidable rapids.

At Flims, in Switzerland, a prehistoric landslip flung a dam

eighteen hundred feet high across the headwaters of the Rhine. If

spread evenly over a surface of twenty-eight square miles, the

material would cover it to a depth of six hundred and sixty feet.

The barrier is not yet entirely cut away, and several lakes are

held in shallow basins on its hummocky surface.

A slide from the precipitous river front of the citadel hill of

Quebec, in 1889, dashed across Champlain Street, wrecking a number

of houses and causing the death of forty-five persons. The strata

here are composed of steeply dipping slate.

In lofty mountain ranges there may not be a single valley without

its traces of landslides, so common there is this method of the

movement of waste, and of building to grade over-steepened slopes.

ROCK SCULPTURE BY WEATHERING

We are now to consider a few of the forms into which rock masses

are carved by the weather.

BOWLDERS OF WEATHERING. In many quarries and outcrops we may see

that the blocks into which one or more of the uppermost layers

have been broken along their joints and bedding planes are no

longer angular, as are those of the layers below. The edges and

corners of these blocks have been worn away by the weather. Such

rounded cores, known as bowlders of weathering, are often left to

strew the surface.

DIFFERENTIAL WEATHERING. This term covers all cases in which a

rock mass weathers differently in different portions. Any weaker

spots or layers are etched out on the surface, leaving the more

resistant in relief. Thus massive limestones become pitted where

the weather drills out the weaker portions. In these pits, when

once they are formed, moisture gathers, a little soil collects,

vegetation takes root, and thus they are further enlarged until

the limestone may be deeply honeycombed.

On the sides of canyons, and elsewhere where the edges of strata

are exposed, the harder layers project as cliffs, while the softer

weather back to slopes covered with the talus of the harder layers

above them. It is convenient to call the former cliff makers and

the latter slope makers.

Differential weathering plays a large part in the sculpture of the

land. Areas of weak rock are wasted to plains, while areas of hard

rock adjacent are still left as hills and mountain ridges, as in

the valleys and mountains of eastern Pennsylvania. But in such

instances the lowering of the surface of the weaker rock is also

due to the wear of streams, and especially to the removal by them

from the land of the waste which covers and protects the rocks

beneath.

Rocks owe their weakness to several different causes. Some, such

as beds of loose sand, are soft and easily worn by rains; some, as

limestone and gypsum for example, are soluble. Even hard insoluble

rocks are weak under the attack of the weather when they are

closely divided by joints and bedding planes and are thus readily

broken up into blocks by mechanical agencies.

OUTLIERS AND MONUMENTS. As cliffs retreat under the attack of the

weather, portions are left behind where the rock is more resistant

or where the attack for any reason is less severe. Such remnant

masses, if large, are known as outliers. When

Note the rain furrows on the slope at the foot of the monuments.

In the foreground are seen fragments of petrified trunks of trees,

composed of silica and extremely resistant to the weather. On the

removal of the rock layers in which these fragments were imbedded

they are left to strew the surface in the same way as are the

residual flints of southern Missouri. flat-topped, because of the

protection of a resistant horizontal capping layer, they are

termed mesas,--a term applied also to the flat-topped portions of

dissected plateaus (Fig. 129). Retreating cliffs may fall back a

number of miles behind their outliers before the latter are

finally consumed.

Monuments are smaller masses and may be but partially detached

from the cliff face. In the breaking down of sheets of horizontal

strata, outliers grow smaller and smaller and are reduced to

massive rectangular monuments resembling castles (Fig. 17). The

rock castle falls into ruin, leaving here and there an isolated

tower; the tower crumbles to a lonely pillar, soon to be

overthrown. The various and often picturesque shapes of monuments

depend on the kind of rock, the attitude of the strata, and the

agent by which they are chiefly carved. Thus pillars may have a

capital formed of a resistant stratum. Monuments may be undercut

and come to rest on narrow pedestals, wherever they weather more

rapidly near the ground, either because of the greater moisture

there, or--in arid climates--because worn at their base by

drifting sands.

Stony clays disintegrating under the rain often contain bowlders

which protect the softer material beneath from the vertical blows

of raindrops, and thus come to stand on pedestals of some height.

One may sometimes see on the ground beneath dripping eaves pebbles

left in the same way, protecting tiny pedestals of sand.

MOUNTAIN PEAKS AND RIDGES. Most mountains have been carved out of

great broadly uplifted folds and blocks of the earth's crust.

Running water and glacier ice have cut these folds and blocks into

masses divided by deep valleys; but it is by the weather, for the

most part, that the masses thus separated have been sculptured to

the present forms of the individual peaks and ridges.

Frost and heat and cold sculpture high mountains to sharp,

tusklike peaks and ragged, serrate crests, where their waste is

readily removed.

The Matterhorn of the Alps is a famous example of a mountain peak

whose carving by the frost and other agents is in active progress.

On its face "scarcely a rock anywhere is firmly attached," and the

fall of loosened stones is incessant. Mountain climbers who have

camped at its base tell how huge rocks from time to time come

leaping down its precipices, followed by trains of dislodged

smaller fragments and rock dust; and how at night one may trace

the course of the bowlders by the sparks which they strike from

the mountain walls. Mount Assiniboine, Canada (Fig. 20), resembles

the Matterhorn in form and has been carved by the same agencies.

"The Needles" of Arizona are examples of sharp mountain peaks in a

warm arid region sculptured chiefly by temperature changes.

Chemical decay, especially when carried on beneath a cover of

waste and vegetation, favors the production of rounded knobs and

dome-shaped mountains.

THE WEATHER CURVE. We have seen that weathering reduces the

angular block quarried by the frost to a rounded bowlder by

chipping off its corners and smoothing away its edges. In much the

same way weathering at last reduces to rounded hills the earth

blocks cut by streams or formed in any other way. High mountains

may at first be sculptured by the weather to savage peaks (Fig.

181), but toward the end of their life history they wear down to

rounded hills (Fig. 182). The weather curve, which may be seen on

the summits of low hills (Fig. 21), is convex upward.

In Figure 22, representing a cubic block of stone whose faces are

a yard square, how many square feet of surface are exposed to the

weather by a cubic foot at a corner a; by one situated in the

middle of an edge b; by one in the center of a side c? How much

faster will a and b weather than c, and what will be the effect on

the shape of the block?

THE COOPERATION OF VARIOUS AGENCIES IN ROCK SCULPTURE. For the

sake of clearness it is necessary to describe the work of each

geological agent separately. We must not forget, however, that in

Nature no agent works independently and alone; that every result

is the outcome of a long chain of causes. Thus, in order that the

mountain peak may be carved by the agents of disintegration, the

waste must be rapidly removed,--a work done by many agents,

including some which we are yet to study; and in order that the

waste may be removed as fast as formed, the region must first have

been raised well above the level of the sea, so that the agents of

transportation could do their work effectively. The sculpture of

the rocks is accomplished only by the cooperation of many forces.

The constant removal of waste from the surface by creep and wash

and carriage by streams is of the highest importance, because it

allows the destruction of the land by means of weathering to go on

as long as any land remains above sea level. If waste were not

removed, it would grow to be so thick as to protect the rock

beneath from further weathering, and the processes of destruction

which we have studied would be brought to an end. The very

presence of the mantle of waste over the land proves that on the

whole rocks weather more rapidly than their waste is removed. The

destruction of the land is going on as fast as the waste can be

carried away.

We have now learned to see in the mantle of waste the record of

the destructive action of the agencies of weathering on the rocks

of the land surface. Similar records we shall find buried deeply

among the rocks of the crust in old soils and in rocks pitted and

decayed, telling of old land surfaces long wasted by the weather.

Ever since the dry land appeared these agencies have been as now

quietly and unceasingly at work upon it, and have ever been the

chief means of the destruction of its rocks. The vast bulk of the

stratified rocks of the earth's crust is made up almost wholly of

the waste thus worn from ancient lands.

In studying the various geological agencies we must remember the

almost inconceivable times in which they work. The slowest process

when multiplied by the immense time in which it is carried on

produces great results. The geologist looks upon the land forms of

the earth's surface as monuments which record the slow action of

weathering and other agents during the ages of the past. The

mountain peak, the rounded hill, the wide plain which lies where

hills and mountains once stood, tell clearly of the great results

which slow processes will reach when given long time in which to

do their work. We should accustom ourselves also to think of the

results which weathering will sooner or later bring to pass. The

tombstone and the bowlder of the field, which each year lose from

their surfaces a few crystalline grains, must in time be wholly

destroyed. The hill whose rocks are slowly rotting underneath a

cover of waste must become lower and lower as the centuries and

millenniums come and go, and will finally disappear. Even the

mountains are crumbling away continually, and therefore are but

fleeting features of the landscape.

CHAPTER II

THE WORK OF GROUND WATER

LAND WATERS. We have seen how large is the part that water plays

at and near the surface of the land in the processes of weathering

and in the slow movement of waste down all slopes to the stream

ways. We now take up the work of water as it descends beneath the

ground,--a corrosive agent still, and carrying in solution as its

load the invisible waste of rocks derived from their soluble

parts.

Land waters have their immediate source in the rainfall. By the

heat of the sun water is evaporated from the reservoir of the

ocean and from moist surfaces everywhere. Mingled as vapor with

the air, it is carried by the winds over sea and land, and

condensed it returns to the earth as rain or snow. That part of

the rainfall which descends on the ocean does not concern us, but

that which falls on the land accomplishes, as it returns to the

sea, the most important work of all surface geological agencies.

The rainfall may be divided into three parts: the first DRIES UP,

being discharged into the air by evaporation either directly from

the soil or through vegetation; the second RUNS OFF over the

surface to flood the streams; the third SOAKS IN the ground and is

henceforth known as GROUND or UNDERGROUND WATER.

THE DESCENT OF GROUND WATER. Seeping through the mantle of waste,

ground water soaks into the pores and crevices of the underlying

rock. All rocks of the upper crust of the earth are more or less

porous, and all drink in water. IMPERVIOUS ROCKS, such as granite,

clay, and shale, have pores so minute that the water which they

take in is held fast within them by capillary attraction, and none

drains through. PERVIOUS ROCKS, on the other hand, such as many

sandstones, have pore spaces so large that water filters through

them more or less freely. Besides its seepage through the pores of

pervious rocks, water passes to lower levels through the joints

and cracks by which all rocks, near the surface are broken.

Even the closest-grained granite has a pore space of 1 in 400,

while sandstone may have a pore space of 1 in 4. Sand is so porous

that it may absorb a third of its volume of water, and a loose

loam even as much as one half.

THE GROUND-WATER SURFACE is the name given the upper surface of

ground water, the level below which all rocks are saturated. In

dry seasons the ground-water surface sinks. For ground water is

constantly seeping downward under gravity, it is evaporated in the

waste and its moisture is carried upward by capillarity and the

roots of plants to the surface to be evaporated in the air. In wet

seasons these constant losses are more than made good by fresh

supplies from that part of the rainfall which soaks into the

ground, and the ground-water surface rises.

In moist climates the ground-water surface (Fig. 24) lies, as a

rule, within a few feet of the land surface and conforms to it in

a general way, although with slopes of less inclination than those

of the hills and valleys. In dry climates permanent ground water

may be found only at depths of hundreds of feet. Ground water is

held at its height by the fact that its circulation is constantly

impeded by capillarity and friction. If it were as free to drain

away as are surface streams, it would sink soon after a rain to

the level of the deepest valleys of the region.

WELLS AND SPRINGS. Excavations made in permeable rocks below the

ground-water surface fill to its level and are known as wells.

Where valleys cut this surface permanent streams are formed, the

water either oozing forth along ill-defined areas or issuing at

definite points called springs, where it is concentrated by the

structure of the rocks. A level tract where the ground-water

surface coincides with the surface of the ground is a swamp or

marsh.

By studying a spring one may learn much of the ways and work of

ground water. Spring water differs from that of the stream into

which it flows in several respects. If we test the spring with a

thermometer during successive months, we shall find that its

temperature remains much the same the year round. In summer it is

markedly cooler than the stream; in winter it is warmer and

remains unfrozen while the latter perhaps is locked in ice. This

means that its underground path must lie at such a distance from

the surface that it is little affected by summer's heat and

winter's cold.

While the stream is often turbid with surface waste washed into it

by rains, the spring remains clear; its water has been filtered

during its slow movement through many small underground passages

and the pores of rocks. Commonly the spring differs from the

stream in that it carries a far larger load of dissolved rock.

Chemical analysis proves that streams contain various minerals in

solution, but these are usually in quantities so small that they

are not perceptible to the taste or feel. But the water of springs

is often well charged with soluble minerals; in its slow, long

journey underground it has searched out the soluble parts of the

rocks through which it seeps and has dissolved as much of them as

it could. When spring water is boiled away, the invisible load

which it has carried is left behind, and in composition is found

to be practically identical with that of the soluble ingredients

of the country rock. Although to some extent the soluble waste of

rocks is washed down surface slopes by the rain, by far the larger

part is carried downward by ground water and is delivered to

streams by springs.

In limestone regions springs are charged with calcium carbonate

(the carbonate of lime), and where the limestone is magnesian they

contain magnesium carbonate also. Such waters are "hard"; when

used in washing, the minerals which they contain combine with the

fatty acids of soap to form insoluble curdy compounds. When

springs rise from rocks containing gypsum they are hard with

calcium sulphate. In granite regions they contain more or less

soda and potash from the decay of feldspar.

The flow of springs varies much less during the different seasons

of the year than does that of surface streams. So slow is the

movement of ground water through the rocks that even during long

droughts large amounts remain stored above the levels of surface

drainage.

MOVEMENTS OF GROUND WATER. Ground water is in constant movement

toward its outlets. Its rate varies according to many conditions,

but always is extremely slow. Even through loose sands beneath the

beds of rivers it sometimes does not exceed a fifth of a mile a

year.

In any region two zones of flow may be distinguished. The UPPER

ZONE OF FLOW extends from the ground-water surface downward

through the waste mantle and any permeable rocks on which the

mantle rests, as far as the first impermeable layer, where the

descending movement of the water is stopped. The DEEP ZONES OF

FLOW occupy any pervious rocks which may be found below the

impervious layer which lies nearest to the surface. The upper zone

is a vast sheet of water saturating the soil and rocks and slowly

seeping downward through their pores and interstices along the

slopes to the valleys, where in part it discharges in springs and

often unites also in a wide underflowing stream which supports and

feeds the river (Fig. 24).

A city in a region of copious rains, built on the narrow flood

plain of a river, overlooked by hills, depends for its water

supply on driven wells, within the city limits, sunk in the sand a

few yards from the edge of the stream. Are these wells fed by

water from the river percolating through the sand, or by ground

water on its way to the stream and possibly contaminated with the

sewage of the town?

At what height does underground water stand in the wells of your

region? Does it vary with the season? Have you ever known wells to

go dry? It may be possible to get data from different wells and to

draw a diagram showing the ground-water surface as compared with

the surface of the ground.

FISSURE SPRINGS AND ARTESIAN WELLS. The DEEPER ZONES OF FLOW lie in

pervious strata which are overlain by some impervious stratum.

Such layers are often carried by their dip to great depths, and

water may circulate in them to far below the level of the surface

streams and even of the sea. When a fissure crosses a water-

bearing stratum, or AQUIFIER, water is forced upward by the

pressure of the weight of the water contained in the higher parts

of the stratum, and may reach the surface as a fissure spring. A

boring which taps such an aquifer is known as an artesian well, a

name derived from a province in France where wells of this kind

have been long in use. The rise of the water in artesian wells,

and in fissure springs also, depends on the following conditions

illustrated in Figure 29. The aquifer dips toward the region of

the wells from higher ground, where it outcrops and receives its

water. It is inclosed between an impervious layer above and water-

tight or water-logged layers beneath. The weight of the column of

water thus inclosed in the aquifer causes water to rise in the

well, precisely as the weight of the water in a standpipe forces

it in connected pipes to the upper stories of buildings.

Which will supply the larger region with artesian wells, an

aquifer whose dip is steep or one whose dip is gentle? Which of

the two aquifers, their thickness being equal, will have the

larger outcrop and therefore be able to draw upon the larger

amount of water from the rainfall? Illustrate with diagrams.

THE ZONE OF SOLUTION. Near the surface, where the circulation of

ground water is most active, it oxidizes, corrodes, and dissolves

the rocks through which it passes. It leaches soils and subsoils

of their lime and other soluble minerals upon which plants depend

for their food. It takes away the soluble cements of rocks; it

widens fissures and joints and opens winding passages along the

bedding planes; it may even remove whole beds of soluble rocks,

such as rock salt, limestone, or gypsum. The work of ground water

in producing landslides has already been noticed. The zone in

which the work of ground water is thus for the most part

destructive we may call the zone of solution.

CAVES. In massive limestone rocks, ground water dissolves channels

which sometimes form large caves (Fig. 30). The necessary

conditions for the excavation of caves of great size are well

shown in central Kentucky, where an upland is built throughout of

thick horizontal beds of limestone. The absence of layers of

insoluble or impervious rock in its structure allows a free

circulation of ground water within it by the way of all natural

openings in the rock. These water ways have been gradually

enlarged by solution and wear until the upland is honeycombed with

caves. Five hundred open caverns are known in one county.

Mammoth Cave, the largest of these caverns, consists of a

labyrinth of chambers and winding galleries whose total length is

said to be as much as thirty miles. One passage four miles long

has an average width of about sixty feet and an average height of

forty feet. One of the great halls is three hundred feet in width

and is overhung by a solid arch of limestone one hundred feet

above the floor. Galleries at different levels are connected by

well-like pits, some of which measure two hundred and twenty-five

feet from top to bottom. Through some of the lowest of these

tunnels flows Echo River, still at work dissolving and wearing

away the rock while on its dark way to appear at the surface as a

great spring.

NATURAL BRIDGES. As a cavern enlarges and the surface of the land

above it is lowered by weathering, the roof at last breaks down

and the cave becomes an open ravine. A portion of the roof may for

a while remain, forming a "natural bridge."

SINK HOLES. In limestone regions channels under ground may become

so well developed that the water of rains rapidly drains away

through them. Ground water stands low and wells must be sunk deep

to find it. Little or no surface water is left to form brooks.

Thus across the limestone upland of central Kentucky one meets but

three surface streams in a hundred miles. Between their valleys

surface water finds its way underground by means of sink holes.

These are pits, commonly funnel shaped, formed by the enlargement

of crevice or joint by percolating water, or by the breakdown of

some portion of the roof of a cave. By clogging of the outlet a

sink hole may come to be filled by a pond.

Central Florida is a limestone region with its drainage largely

subterranean and in part below the level even of the sea. Sink

holes are common, and many of them are occupied by lakelets. Great

springs mark the point of issue of underground streams, while some

rise from beneath the sea. Silver Spring, one of the largest,

discharges from a basin eight hundred feet wide and thirty feet

deep a little river navigable for small steamers to its source.

About the spring there are no surface streams for sixty miles.

THE KARST. Along the eastern coast of the Adriatic, as far south

as Montenegro, lies a belt of limestone mountains singularly worn

and honeycombed by the solvent action of water. Where forests have

been cut from the mountain sides and the red soil has washed away,

the surface of the white limestone forms a pathless desert of rock

where each square rod has been corroded into an intricate branch

work of shallow furrows and sharp ridges. Great sink holes, some

of them six hundred feet deep and more, pockmark the surface of

the land. The drainage is chiefly subterranean. Surface streams

are rare and a portion of their courses is often under ground.

Fragmentary valleys come suddenly to an end at walls of rock where

the rivers which occupy the valleys plunge into dark tunnels to

reappear some miles away. Ground water stands so far below the

surface that it cannot be reached by wells, and the inhabitants

depend on rain water stored for household uses. The finest cavern

of Europe, the Adelsberg Grotto, is in this region. Karst, the

name of a part of this country, is now used to designate any

region or landscape thus sculptured by the chemical action of

surface and ground water. We must remember that Karst regions are

rare, and striking as is the work of their subterranean streams,

it is far less important than the work done by the sheets of

underground water slowly seeping through all subsoils and porous

rocks in other regions.

Even when gathered into definite channels, ground water does not

have the erosive power of surface streams, since it carries with

it little or no rock waste. Regions whose underground drainage is

so perfect that the development of surface streams has been

retarded or prevented escape to a large extent the leveling action

of surface running waters, and may therefore stand higher than the

surrounding country. The hill honeycombed by Luray Cavern,

Virginia, has been attributed to this cause.

CAVERN DEPOSITS. Even in the zone of solution water may under

certain circumstances deposit as well as erode. As it trickles

from the roof of caverns, the lime carbonate which it has taken

into solution from the layers of limestone above is deposited by

evaporation in the air in icicle-like pendants called STALACTITES.

As the drops splash on the floor there are built up in the same

way thicker masses called STALAGMITES, which may grow to join the

stalactites above, forming pillars. A stalagmitic crust often

seals with rock the earth which accumulates in caverns, together

with whatever relics of cave dwellers, either animals or men, it

may contain.

Can you explain why slender stalactites formed by the drip of

single drops are often hollow pipes?

THE ZONE OF CEMENTATION. With increasing depth subterranean water

becomes more and more sluggish in its movements and more and more

highly charged with minerals dissolved from the rocks above. At

such depths it deposits these minerals in the pores of rocks,

cementing their grains together, and in crevices and fissures,

forming mineral veins. Thus below the zone of solution where the

work of water is to dissolve, lies the zone of cementation where

its work is chemical deposit. A part of the invisible load of

waste is thus transferred from rocks near the surface to those at

greater depths.

As the land surface is gradually lowered by weathering and the

work of rain and streams, rocks which have lain deep within the

zone of cementation are brought within the zone of solution. Thus

there are exposed to view limestones, whose cracks were filled

with calcite (crystallized carbonate of lime), with quartz or

other minerals, and sandstones whose grains were well cemented

many feet below the surface.

CAVITY FILLING. Small cavities in the rocks are often found more

or less completely filled with minerals deposited from solution by

water in its constant circulation underground. The process may be

illustrated by the deposit of salt crystals in a cup of

evaporating brine, but in the latter instance the solution is not

renewed as in the case of cavities in the rocks. A cavity thus

lined with inward-pointing crystals is called a GEODE.

CONCRETIONS. Ground water seeping through the pores of rocks may

gather minerals disseminated throughout them into nodular masses

called concretions. Thus silica disseminated through limestone is

gathered into nodules of flint. While geodes grow from the outside

inwards, concretions grow outwards from the center. Nor are they

formed in already existing cavities as are geodes. In soft clays

concretions may, as they grow, press the clay aside. In many other

rocks concretions are made by the process of REPLACEMENT. Molecule

by molecule the rock is removed and the mineral of the concretion

substituted in its place. The concretion may in this way preserve

intact the lamination lines or other structures of the rock. Clays

and shales often contain concretions of lime carbonate, of iron

carbonate, or of iron sulphide. Some fossil, such as a leaf or

shell, frequently forms the nucleus around which the concretion

grows.

Why are building stones more easily worked when "green" than after

their quarry water has dried out?

DEPOSITS OF GROUND WATER IN ARID REGIONS. In arid lands where

ground water is drawn by capillarity to the surface and there

evaporates, it leaves as surface incrustations the minerals held

in solution. White limy incrustations of this nature cover

considerable tracts in northern Mexico. Evaporating beneath the

surface, ground water may deposit a limy cement in beds of loose

sand and gravel. Such firmly cemented layers are not uncommon in

western Kansas and Nebraska, where they are known as "mortar

beds."

THERMAL SPRINGS. While the lower limit of surface drainage is sea

level, subterranean water circulates much below that depth, and is

brought again to the surface by hydrostatic pressure. In many

instances springs have a higher temperature than the average

annual temperature of the region, and are then known as thermal

springs. In regions of present or recent volcanic activity, such

as the Yellowstone National Park, we may believe that the heat of

thermal springs is derived from uncooled lavas, perhaps not far

below the surface. But when hot springs occur at a distance of

hundreds of miles from any volcano, as in the case of the hot

springs of Bath, England, it is probable that their waters have

risen from the heated rocks of the earth's interior. The springs

of Bath have a temperature of 120 degrees F., 70 degrees above the

average annual temperature of the place. If we assume that the

rate of increase in the earth's internal heat is here the average

rate, 1 degree F. to every sixty feet of descent, we may conclude

that the springs of Bath rise from at least a depth of forty-two

hundred feet.

Water may descend to depths from which it can never be brought

back by hydrostatic pressure. It is absorbed by highly heated

rocks deep below the surface. From time to time some of this deep-

seated water may be returned to open air in the steam of volcanic

eruptions.

SURFACE DEPOSITS OF SPRINGS. Where subterranean water returns to

the surface highly charged with minerals in solution, on exposure

to the air it is commonly compelled to lay down much of its

invisible load in chemical deposits about the spring. These are

thrown down from solution either because of cooling, evaporation,

the loss of carbon dioxide, or the work of algae.

Many springs have been charged under pressure with carbon dioxide

from subterranean sources and are able therefore to take up large

quantities of lime carbonate from the limestone rocks through

which they pass. On reaching the surface the pressure is relieved,

the gas escapes, and the lime carbonate is thrown down in deposits

called TRAVERTINE. The gas is sometimes withdrawn and the deposit

produced in large part by the action of algae and other humble

forms of plant life.

At the Mammoth Hot Springs in the valley of the Gardiner River,

Yellowstone National Park, beautiful terraces and basins of

travertine are now building, chiefly by means of algae which cover

the bottoms, rims, and sides of the basins and deposit lime

carbonate upon them in successive sheets. The rock, snow-white

where dry, is coated with red and orange gelatinous mats where the

algae thrive in the over-flowing waters.

Similar terraces of travertine are found to a height of fourteen

hundred feet up the valley side. We may infer that the springs

which formed these ancient deposits discharged near what was then

the bottom of the valley, and that as the valley has been deepened

by the river the ground water of the region has found lower and

lower points of issue.

In many parts of the country calcareous springs occur which coat

with lime carbonate mosses, twigs, and other objects over which

their waters flow. Such are popularly known as petrifying springs,

although they merely incrust the objects and do not convert them

into stone.

Silica is soluble in alkaline waters, especially when these are

hot. Hot springs rising through alkaline siliceous rocks, such as

lavas, often deposit silica in a white spongy formation known as

SILICEOUS SINTER, both by evaporation and by the action of algae

which secrete silica from the waters. It is in this way that the

cones and mounds of the geysers in the Yellowstone National Park

and in Iceland have been formed.

Where water oozes from the earth one may sometimes see a rusty

deposit on the ground, and perhaps an iridescent scum upon the

water. The scum is often mistaken for oil, but at a touch it

cracks and breaks, as oil would not do. It is a film of hydrated

iron oxide, or LIMONITE, and the spring is an iron, or chalybeate,

spring. Compounds of iron have been taken into solution by ground

water from soil and rocks, and are now changed to the insoluble

oxide on exposure to the oxygen of the air.

In wet ground iron compounds leached by ground water from the soil

often collect in reddish deposits a few feet below the surface,

where their downward progress is arrested by some impervious clay.

At the bottom of bogs and shallow lakes iron ores sometimes

accumulate to a depth of several feet.

Decaying organic matter plays a large part in these changes. In

its presence the insoluble iron oxides which give color to most

red and yellow rocks are decomposed, leaving the rocks of a gray

or bluish color, and the soluble iron compounds which result are

readily leached out,--effects seen where red or yellow clays have

been bleached about some decaying tree root.

The iron thus dissolved is laid down as limonite when oxidized, as

about a chalybeate spring; but out of contact with the air and in

the presence of carbon dioxide supplied by decaying vegetation, as

in a peat bog, it may be deposited as iron carbonate, or SIDERITE.

TOTAL AMOUNT OF UNDERGROUND WATERS. In order to realize the vast

work in solution and cementation which underground waters are now

doing and have done in all geological ages, we must gain some

conception of their amount. At a certain depth, estimated at about

six miles, the weight of the crust becomes greater than the rocks

can bear, and all cavities and pores in them must be completely

closed by the enormous pressure which they sustain. Below a depth

of even three or four miles it is believed that ground water

cannot circulate. Estimating the average pore spaces of the

different rocks of the earth's crust above this depth, and the

average per cents of their pore spaces occupied by water, it has

been recently computed that the total amount of ground water is

equal to a sheet of water one hundred feet deep, covering the

entire surface of the earth.

CHAPTER III

RIVERS AND VALLEYS

THE RUN-OFF. We have traced the history of that portion of the

rainfall which soaks into the ground; let us now return to that

part which washes along the surface and is known as the RUN-OFF.

Fed by rains and melting snows, the run-off gathers into courses,

perhaps but faintly marked at first, which join more definite and

deeply cut channels, as twigs their stems. In a humid climate the

larger ravines through which the run-off flows soon descend below

the ground-water surface. Here springs discharge along the sides

of the little valleys and permanent streams begin. The water

supplied by the run-off here joins that part of the rainfall which

had soaked into the soil, and both now proceed together by way of

the stream to the sea.

RIVER FLOODS. Streams vary greatly in volume during the year. At

stages of flood they fill their immediate banks, or overrun them

and inundate any low lands adjacent to the channel; at stages of

low water they diminish to but a fraction of their volume when at

flood.

At times of flood, rivers are fed chiefly by the run-off; at times

of low water, largely or even wholly by springs.

How, then, will the water of streams differ at these times in

turbidity and in the relative amount of solids carried in

solution?

In parts of England streams have been known to continue flowing

after eighteen months of local drought, so great is the volume of

water which in humid climates is stored in the rocks above the

drainage level, and so slowly is it given off in springs.

In Illinois and the states adjacent, rivers remain low in winter

and a "spring freshet" follows the melting of the winter's snows.

A "June rise" is produced by the heavy rains of early summer. Low

water follows in July and August, and streams are again swollen to

a moderate degree under the rains of autumn.

THE DISCHARGE OF STREAMS. The per cent of rainfall discharged by

rivers varies with the amount of rainfall, the slope of the

drainage area, the texture of the rocks, and other factors. With

an annual rainfall of fifty inches in an open country, about fifty

per cent is discharged; while with a rainfall of twenty inches

only fifteen per cent is discharged, part of the remainder being

evaporated and part passing underground beyond the drainage area.

Thus the Ohio discharges thirty per cent of the rainfall of its

basin, while the Missouri carries away but fifteen per cent. A

number of the streams of the semi-arid lands of the West do not

discharge more than five per cent of the rainfall.

Other things being equal, which will afford the larger proportion

of run-off, a region underlain with granite rock or with coarse

sandstone? grass land or forest? steep slopes or level land? a

well-drained region or one abounding in marshes and ponds? frozen

or unfrozen ground? Will there be a larger proportion of run-off

after long rains or after a season of drought? after long and

gentle rains, or after the same amount of precipitation in a

violent rain? during the months of growing vegetation, from June

to August, or during the autumn months?

DESERT STREAMS. In arid regions the ground-water surface lies so

low that for the most part stream ways do not intersect it.

Streams therefore are not fed by springs, but instead lose volume

as their waters soak into the thirsty rocks over which they flow.

They contribute to the ground water of the region instead of being

increased by it. Being supplied chiefly by the run-off, they

wither at times of drought to a mere trickle of water, to a chain

of pools, or go wholly dry, while at long intervals rains fill

their dusty beds with sudden raging torrents. Desert rivers

therefore periodically shorten and lengthen their courses,

withering back at times of drought for scores of miles, or even

for a hundred miles from the point reached by their waters during

seasons of rain.

THE GEOLOGICAL WORK OF STREAMS. The work of streams is of three

kinds,--transportation, erosion, and deposition. Streams TRANSPORT

the waste of the land; they wear, or ERODE, their channels both on

bed and banks; and they DEPOSIT portions of their load from time

to time along their courses, finally laying it down in the sea.

Most of the work of streams is done at times of flood.

TRANSPORTATION

THE INVISIBLE LOAD OF STREAMS. Of the waste which a river

transports we may consider first the invisible load which it

carries in solution, supplied chiefly by springs but also in part

by the run-off and from the solution of the rocks of its bed. More

than half the dissolved solids in the water of the average river

consists of the carbonates of lime and magnesia; other substances

are gypsum, sodium sulphate (Glauber's salts), magnesium sulphate

(Epsom salts), sodium chloride (common salt), and even silica, the

least soluble of the common rock-making minerals. The amount of

this invisible load is surprisingly large. The Mississippi, for

example, transports each year 113,000,000 tons of dissolved rock

to the Gulf.

THE VISIBLE LOAD OF STREAMS. This consists of the silt which the

stream carries in suspension, and the sand and gravel and larger

stones which it pushes along its bed. Especially in times of flood

one may note the muddy water, its silt being kept from settling by

the rolling, eddying currents; and often by placing his ear close

to the bottom of a boat one may hear the clatter of pebbles as

they are hurried along. In mountain torrents the rumble of

bowlders as they clash together may be heard some distance away.

The amount of the load which a stream can transport depends on its

velocity. A current of two thirds of a mile per hour can move fine

sand, while one of four miles per hour sweeps along pebbles as

large as hen's eggs. The transporting power of a stream varies as

the sixth power of its velocity. If its velocity is multiplied by

two, its transporting power is multiplied by the sixth power of

two: it can now move stones sixty-four times as large as it could

before.

Stones weigh from two to three times as much as water, and in

water lose the weight of the volume of water which they displace.

What proportion, then, of their weight in air do stones lose when

submerged?

MEASUREMENT OF STREAM LOADS. To obtain the total amount of waste

transported by a river is an important but difficult matter. The

amount of water discharged must first be found by multiplying the

number of square feet in the average cross section of the stream

by its velocity per second, giving the discharge per second in

cubic feet. The amount of silt to a cubic foot of water is found

by filtering samples of the water taken from different parts of

the stream and at different times in the year, and drying and

weighing the residues. The average amount of silt to the cubic

foot of water, multiplied by the number of cubic feet of water

discharged per year, gives the total load carried in suspension

during that time. Adding to this the estimated amount of sand and

gravel rolled along the bed, which in many swift rivers greatly

exceeds the lighter material held in suspension, and adding also

the total amount of dissolved solids, we reach the exceedingly

important result of the total load of waste discharged by the

river. Dividing the volume of this load by the area of the river

basin gives another result of the greatest geological interest,--

the rate at which the region is being lowered by the combined

action of weathering and erosion, or the rate of denudation.

THE RATE OF DENUDATION OF RIVER BASINS. This rate varies widely.

The Mississippi basin may be taken as a representative land

surface because of the varieties of surface, altitude and slope,

climate, and underlying rocks which are included in its great

extent. Careful measurements show that the Mississippi basin is

now being lowered at a rate of one four-thousandth of a foot a

year, or one foot in four thousand years. Taking this as the

average rate of denudation for the land surfaces of the globe,

estimates have been made of the length of time required at this

rate to wash and wear the continents to the level of the sea. As

the average elevation of the lands of the globe is reckoned at

2411 feet, this result would occur in nine or ten million years,

if the present rate of denudation should remain unchanged. But

even if no movements of the earth's crust should lift or depress

the continents, the rate of wear and the removal of waste from

their surfaces will not remain the same. It must constantly

decrease as the lands are worn nearer to sea level and their

slopes become more gentle. The length of time required to wear

them away is therefore far in excess of that just stated.

The drainage area of the Potomac is 11,000 square miles. The silt

brought down in suspension in a year would cover a square mile to

the depth of four feet. At what rate is the Potomac basin being

lowered from this cause alone?

It is estimated that the Upper Ganges is lowering its basin at the

rate of one foot in 823 years, and the Po one foot in 720 years.

Why so much faster than the Potomac and the Mississippi?

HOW STREAMS GET THEIR LOADS. The load of streams is derived from a

number of sources, the larger part being supplied by the

weathering of valley slopes. We have noticed how the mantle of

waste creeps and washes to the stream ways. Watching the run-off

during a rain, as it hurries muddy with waste along the gutter or

washes down the hillside, we may see the beginning of the route by

which the larger part of their load is delivered to rivers.

Streams also secure some of their load by wearing it from their

beds and banks,--a process called erosion.

EROSION

Streams erode their beds chiefly by means of their bottom load,--

the stones of various sizes and the sand and even the fine mud

which they sweep along. With these tools they smooth, grind, and

rasp the rock of their beds, using them in much the fashion of

sandpaper or a file.

WEATHERING OF RIVER BEDS. The erosion of stream beds is greatly

helped by the work of the weather. Especially at low water more or

less of the bed is exposed to the action of frost and heat and

cold, joints are opened, rocks are pried loose and broken up and

made ready to be swept away by the stream at time of flood.

POTHOLES. In rapids streams also drill out their rocky beds. Where

some slight depression gives rise to an eddy, the pebbles which

gather in it are whirled round and round, and, acting like the bit

of an auger, bore out a cylindrical pit called a pothole. Potholes

sometimes reach a depth of a score of feet. Where they are

numerous they aid materially in deepening the channel, as the

walls between them are worn away and they coalesce.

WATERFALLS. One of the most effective means of erosion which the

river possesses is the waterfall. The plunging water dislodges

stones from the face of the ledge over which it pours, and often

undermines it by excavating a deep pit at its base. Slice after

slice is thus thrown down from the front of the cliff, and the

cataract cuts its way upstream leaving a gorge behind it.

NIAGARA FALLS. The Niagara River flows from Lake Erie at Buffalo in

a broad channel which it has cut but a few feet below the level of

the region. Some thirteen miles from the outlet it plunges over a

ledge one hundred and seventy feet high into the head of a narrow

gorge which extends for seven miles to the escarpment of the

upland in which the gorge is cut. The strata which compose the

upland dip gently upstream and consist at top of a massive

limestone, at the Falls about eighty feet thick, and below of soft

and easily weathered shale. Beneath the Falls the underlying shale

is cut and washed away by the descending water and retreats also

because of weathering, while the overhanging limestone breaks down

in huge blocks from time to time.

Niagara is divided by Goat Island into the Horseshoe Falls and the

American Falls. The former is supplied by the main current of the

river, and from the semicircular sweep of its rim a sheet of water

in places at least fifteen or twenty feet deep plunges into a pool

a little less than two hundred feet in depth. Here the force of

the falling water is sufficient to move about the fallen blocks of

limestone and use them in the excavation of the shale of the bed.

At the American Falls the lesser branch of the river, which flows

along the American side of Goat Island, pours over the side of the

gorge and breaks upon a high talus of limestone blocks which its

smaller volume of water is unable to grind to pieces and remove.

A series of surveys have determined that from 1842 to 1890 the

Horseshoe Falls retreated at the rate of 2.18 feet per year, while

the American Falls retreated at the rate of 0.64 feet in the same

period. We cannot doubt that the same agency which is now

lengthening the gorge at this rapid rate has cut it back its

entire length of seven miles.

While Niagara Falls have been cutting back a gorge seven miles

long and from two hundred to three hundred feet deep, the river

above the Falls has eroded its bed scarcely below the level of the

upland on which it flows. Like all streams which are the outlets

of lakes, the Niagara flows out of Lake Erie clear of sediment, as

from a settling basin, and carries no tools with which to abrade

its bed. We may infer from this instance how slight is the erosive

power of clear water on hard rock.

Assuming that the rate of recession of the combined volumes of the

American and Horseshoe Falls was three feet a year below Goat

Island, and ASSUMING THAT THIS RATE HAS BEEN UNIFORM IN THE PAST,

how long is it since the Niagara River fell over the edge of the

escarpment where now is the mouth of the present gorge?

The profile of the bed of the Niagara along the gorge (Fig. 39)

shows alternating deeps and shallows which cannot be accounted

for, except in a single instance, by the relative hardness of the

rocks of the river bed. The deeps do not exceed that at the foot

of the Horseshoe Falls at the present time. When the gorge was

being cut along the shallows, how did the Falls compare in

excavating power, in force, and volume with the Niagara of to-day?

How did the rate of recession at those times compare with the

present rate? Is the assumption made above that the rate of

recession has been uniform correct?

The first stretch of shallows below the Falls causes a tumultuous

rapid impossible to sound. Its depth has been estimated at thirty-

five feet. From what data could such an estimate be made?

Suggest a reason why the Horseshoe Falls are convex upstream.

At the present rate of recession which will reach the head of Goat

Island the sooner, the American or the Horseshoe Falls? What will

be the fate of the Falls left behind when the other has passed

beyond the head of the island?

The rate at which a stream erodes its bed depends in part upon the

nature of the rocks over which it flows. Will a stream deepen its

channel more rapidly on massive or on thin-bedded and close-

jointed rocks? on horizontal strata or on strata steeply inclined?

DEPOSITION

While the river carries its invisible load of dissolved rock on

without stop to the sea, its load of visible waste is subject to

many delays en route. Now and again it is laid aside, to be picked

up later and carried some distance farther on its way. One of the

most striking features of the river therefore is the waste

accumulated along its course, in bars and islands in the channel,

beneath its bed, and in flood plains along its banks. All this

alluvium, to use a general term for river deposits, with which the

valley is cumbered is really en route to the sea; it is only

temporarily laid aside to resume its journey later on. Constantly

the river is destroying and rebuilding its alluvial deposits, here

cutting and there depositing along its banks, here eroding and

there building a bar, here excavating its bed and there filling it

up, and at all times carrying the material picked up at one point

some distance on downstream before depositing it at another.

These deposits are laid down by slackening currents where the

velocity of the stream is checked, as on the inner side of curves,

and where the slope of the bed is diminished, and in the lee of

islands, bridge piers and projecting points of land. How slight is

the check required to cause a current to drop a large part of its

load may be inferred from the law of the relation of the

transporting power to the velocity. If the velocity is decreased

one half, the current can move fragments but one sixty-fourth the

size of those which it could move before, and must drop all those

of larger size.

Will a river deposit more at low water or at flood? when rising or

when falling?

STRATIFICATION. River deposits are stratified, as may be seen in

any fresh cut in banks or bars. The waste of which they are built

has been sorted and deposited in layers, one above another; some

of finer and some of coarser material. The sorting action of

running water depends on the fact that its transporting power

varies with the velocity. A current whose diminishing velocity

compels it to drop coarse gravel, for example, is still able to

move all the finer waste of its load, and separating it from the

gravel, carries it on downstream; while at a later time slower

currents may deposit on the gravel bed layers of sand, and, still

later, slack water may leave on these a layer of mud. In case of

materials lighter than water the transporting power does not

depend on the velocity, and logs of wood, for instance, are

floated on to the sea on the slowest as well as on the most rapid

currents.

CROSS BEDDING. A section of a bar exposed at low water may show

that it is formed of layers of sand, or coarser stuff, inclined

downstream as steeply often as the angle of repose of the

material. From a boat anchored over the lower end of a submerged

sand bar we may observe the way in which this structure, called

cross bedding, is produced. Sand is continually pushed over the

edge of the bar at b (Fig. 42) and comes to rest in successive

layers on the sloping surface. At the same time the bar may be

worn away at the upper end, a, and thus slowly advance down

stream. While the deposit is thus cross bedded, it constitutes as

a whole a stratum whose upper and lower surfaces are about

horizontal. In sections of river banks one may often see a

vertical succession of cross-bedded strata, each built in the way

described.

WATER WEAR. The coarser material of river deposits, such as

cobblestones, gravel, and the larger grains of sand, are WATER

WORN, or rounded, except when near their source. Rolling along the

bottom they have been worn round by impact and friction as they

rubbed against one another and the rocky bed of the stream.

Experiments have shown that angular fragments of granite lose

nearly half their weight and become well rounded after traveling

fifteen miles in rotating cylinders partly filled with water.

Marbles are cheaply made in Germany out of small limestone cubes

set revolving in a current of water between a rotating bed of

stone and a block of oak, the process requiring but about fifteen

minutes. It has been found that in the upper reaches of mountain

streams a descent of less than a mile is sufficient to round

pebbles of granite.

LAND FORMS DUE TO RIVER EROSION

RIVER VALLEYS. In their courses to the sea, rivers follow valleys

of various forms, some shallow and some deep, some narrow and some

wide. Since rivers are known to erode their beds and banks, it is

a fair presumption that, aided by the weather, they have excavated

the valleys in which they flow.

Moreover, a bird's-eye view or a map of a region shows the

significant fact that the valleys of a system unite with one

another in a branch work, as twigs meet their stems and the

branches of a tree its trunk. Each valley, from that of the

smallest rivulet to that of the master stream, is proportionate to

the size of the stream which occupies it. With a few explainable

exceptions the valleys of tributaries join that of the trunk

stream at a level; there is no sudden descent or break in the bed

at the point of juncture. These are the natural consequences which

must follow if the land has long been worked upon by streams, and

no other process has ever been suggested which is competent to

produce them. We must conclude that valley systems have been

formed by the river systems which drain them, aided by the work of

the weather; they are not gaping fissures in the earth's crust, as

early observers imagined, but are the furrows which running water

has drawn upon the land.

As valleys are made by the slow wear of streams and the action of

the weather, they pass in their development through successive

stages, each of which has its own characteristic features. We may

therefore classify rivers and valleys according to the stage which

they have reached in their life history from infancy to old age.

YOUNG RIVER VALLEYS

INFANCY. The Red River of the North. A region in northwestern

Minnesota and the adjacent portions of North Dakota and Manitoba

was so recently covered by the waters of an extinct lake, known as

Lake Agassiz, that the surface remains much as it was left when

the lake was drained away. The flat floor, spread smooth with

lake-laid silts, is still a plain, to the eye as level as the sea.

Across it the Red River of the North and its branches run in

narrow, ditch-like channels, steep-sided and shallow, not

exceeding sixty feet in depth, their gradients differing little

from the general slopes of the region. The trunk streams have but

few tributaries; the river system, like a sapling with few limbs,

is still undeveloped. Along the banks of the trunk streams short

gullies are slowly lengthening headwards, like growing twigs which

are sometime to become large branches.

The flat interstream areas are as yet but little scored by

drainage lines, and in wet weather water lingers in ponds in any

initial depressions on the plain.

CONTOURS. In order to read the topographic maps of the text-book

and the laboratory the student should know that contours are lines

drawn on maps to represent relief, all points on any given contour

being of equal height above sea level. The CONTOUR INTERVAL is the

uniform vertical distance between two adjacent contours and varies

on different maps.

To express regions of faint relief a contour interval of ten or

twenty feet is commonly selected; while in mountainous regions a

contour interval of two hundred and fifty, five hundred, or even

one thousand feet may be necessary in order that the contours may

not be too crowded for easy reading.

Whether a river begins its life on a lake plain, as in the example

just cited, or upon a coastal plain lifted from beneath the sea or

on a spread of glacial drift left by the retreat of continental

ice sheets, such as covers much of Canada and the northeastern

parts of the United States, its infantile stage presents the same

characteristic features,--a narrow and shallow valley, with

undeveloped tributaries and undrained interstream areas. Ground

water stands high, and, exuding in the undrained initial

depressions, forms marshes and lakes.

LAKES. Lakes are perhaps the most obvious of these fleeting

features of infancy. They are short-lived, for their destruction

is soon accomplished by several means. As a river system advances

toward maturity the deepening and extending valleys of the

tributaries lower the ground-water surface and invade the

undrained depressions of the region. Lakes having outlets are

drained away as their basin rims are cut down by the outflowing

streams,--a slow process where the rim is of hard rock, but a

rapid one where it is of soft material such as glacial drift.

Lakes are effaced also by the filling of their basins. Inflowing

streams and the wash of rains bring in waste. Waves abrade the

shore and strew the debris worn from it over the lake bed. Shallow

lakes are often filled with organic matter from decaying

vegetation.

Does the outflowing stream, from a lake carry sediment? How does

this fact affect its erosive power on hard rock? on loose

material?

Lake Geneva is a well-known example of a lake in process of

obliteration. The inflowing Rhone has already displaced the waters

of the lake for a length of twenty miles with the waste brought

down from the high Alps. For this distance there extends up the

Rhone Valley an alluvial plain, which has grown lakeward at the

rate of a mile and a half since Roman times, as proved by the

distance inland at which a Roman port now stands.

How rapidly a lake may be silted up under exceptionally favorable

conditions is illustrated by the fact that over the bottom of the

artificial lake, of thirty-five square miles, formed behind the

great dam across the Colorado River at Austin, Texas, sediments

thirty-nine feet deep gathered in seven years.

Lake Mendota, one of the many beautiful lakes of southern

Wisconsin, is rapidly cutting back the soft glacial drift of its

shores by means of the abrasion of its waves. While the shallow

basin is thus broadened, it is also being filled with the waste;

and the time is brought nearer when it will be so shoaled that

vegetation can complete the work of its effacement.

Along the margin of a shallow lake mosses, water lilies, grasses,

and other water-loving plants grow luxuriantly. As their decaying

remains accumulate on the bottom, the ring of marsh broadens

inwards, the lake narrows gradually to a small pond set in the

midst of a wide bog, and finally disappears. All stages in this

process of extinction may be seen among the countless lakelets

which occupy sags in the recent sheets of glacial drift in the

northern states; and more numerous than the lakes which still

remain are those already thus filled with carbonaceous matter

derived from the carbon dioxide of the atmosphere. Such fossil

lakes are marked by swamps or level meadows underlain with muck.

THE ADVANCE TO MATURITY. The infantile stage is brief. As a river

advances toward maturity the initial depressions, the lake basins

of its area, are gradually effaced. By the furrowing action of the

rain wash and the head ward lengthening, of tributaries a

branchwork of drainage channels grows until it covers the entire

area, and not an acre is left on which the fallen raindrop does

not find already cut for it an uninterrupted downward path which

leads it on by way of gully, brook, and river to the sea. The

initial surface of the land, by whatever agency it was modeled, is

now wholly destroyed; the region is all reduced to valley slopes.

THE LONGITUDINAL PROFILE OF A STREAM. This at first corresponds

with the initial surface of the region on which the stream begins

to flow, although its way may lead through basins and down steep

descents. The successive profiles to which it reduces its bed are

illustrated in Figure 51. As the gradient, or rate of descent of

its bed, is lowered, the velocity of the river is decreased until

its lessening energy is wholly consumed in carrying its load and

it can no longer erode its bed. The river is now AT GRADE, and its

capacity is just equal to its load. If now its load is increased

the stream deposits, and thus builds up, or AGGRADES, its bed. On

the other hand, if its load is diminished it has energy to spare,

and resuming its work of erosion, DEGRADES its bed. In either case

the stream continues aggrading or degrading until a new gradient

is found where the velocity is just sufficient to move the load,

and here again it reaches grade.

V-VALLEYS. Vigorous rivers well armed with waste make short work

of cutting their beds to grade, and thus erode narrow, steep-sided

gorges only wide enough at the base to accommodate the stream. The

steepness of the valley slopes depends on the relative rates at

which the bed is cut down by the stream and the sides are worn

back by the weather. In resistant rock a swift, well-laden stream

may saw out a gorge whose sides are nearly or even quite vertical,

but as a rule young valleys whose streams have not yet reached

grade are V-shaped; their sides flare at the top because here the

rocks have longest been opened up to the action of the weather.

Some of the deepest canyons may be found where a rising land mass,

either mountain range or plateau, has long maintained by its

continued uplift the rivers of the region above grade.

In the northern hemisphere the north sides of river valleys are

sometimes of more gentle slope than the south sides. Can you

suggest a reason?

THE GRAND CANYON OF THE COLORADO RIVER IN ARIZONA. The Colorado

River trenches the high plateau of northern Arizona with a

colossal canyon two hundred and eighteen miles long and more than

a mile in greatest depth. The rocks in which the canyon is cut are

for the most part flat-lying, massive beds of limestones and

sandstones, with some shales, beneath which in places harder

crystalline rocks are disclosed. Where the canyon is deepest its

walls have been profoundly dissected. Lateral ravines have widened

into immense amphitheaters, leaving between them long ridges of

mountain height, buttressed and rebuttressed with flanking spurs

and carved into majestic architectural forms. From the extremity

of one of these promontories it is two miles or more across the

gulf to the point of the one opposite, and the heads of the

amphitheaters are thirteen miles apart.

The lower portion of the canyon is much narrower (Fig. 54) and its

walls of dark crystalline rock sink steeply to the edge of the

river, a swift, powerful stream a few hundred feet wide, turbid

with reddish silt, by means of which it continually rasps its

rocky bed as it hurries on. The Colorado is still deepening its

gorge. In the Grand Canyon its gradient is seven and one half feet

to the mile, but, as in all ungraded rivers, the descent is far

from uniform. Graded reaches in soft rock alternate with steeper

declivities in hard rock, forming rapids such as, for example, a

stretch of ten miles where the fall averages twenty-one feet to

the mile. Because of these dangerous rapids the few exploring

parties who have traversed the Colorado canyon have done so at the

hazard of their lives.

The canyon has been shaped by several agencies. Its depth is due

to the river which has sawed its way far toward the base of a

lofty rising plateau. Acting alone this would have produced a

slitlike gorge little wider than the breadth of the stream. The

impressive width of the canyon and the magnificent architectural

masses which fill it are owing to two causes.: Running water has

gulched the walls and weathering has everywhere attacked and

driven them back. The horizontal harder beds stand out in long

lines of vertical cliffs, often hundreds of feet in height, at

whose feet talus slopes conceal the outcrop of the weaker strata.

As the upper cliffs have been sapped and driven back by the

weather, broad platforms are left at their bases and the sides of

the canyon descend to the river by gigantic steps. Far up and down

the canyon the eye traces these horizontal layers, like the

flutings of an elaborate molding, distinguishing each by its

contour as well as by its color and thickness.

The Grand Canyon of the Colorado is often and rightly cited as an

example of the stupendous erosion which may be accomplished by a

river. And yet the Colorado is a young stream and its work is no

more than well begun. It has not yet wholly reached grade, and the

great task of the river and its tributaries--the task of leveling

the lofty plateau to a low plain and of transporting it grain by

grain to the sea--still lies almost entirely in the future.

WATERFALLS AND RAPIDS. Before the bed of a stream is reduced to

grade it may be broken by abrupt descents which give rise to

waterfalls and rapids. Such breaks in a river's bed may belong to

the initial surface over which it began its course; still more

commonly are they developed in the rock mass through which it is

cutting its valley. Thus, wherever a stream leaves harder rocks to

flow over softer ones the latter are quickly worn below the level

of the former, and a sharp change in slope, with a waterfall or

rapid, results.

At time of flood young tributaries with steeper courses than that

of the trunk stream may bring down stones and finer waste, which

the gentler current cannot move along, and throw them as a dam

across its way. The rapids thus formed are also ephemeral, for as

the gradient of the tributaries is lowered the main stream becomes

able to handle the smaller and finer load which they discharge.

A rare class of falls is produced where the minor tributaries of a

young river are not able to keep pace with their master stream in

the erosion of their beds because of their smaller volume, and

thus join it by plunging over the side of its gorge. But as the

river approaches grade and slackens its down cutting, the

tributaries sooner or later overtake it, and effacing their falls,

unite with it on a level.

Waterfalls and rapids of all kinds are evanescent features of a

river's youth. Like lakes they are soon destroyed, and if any long

time had already elapsed since their formation they would have

been obliterated already.

LOCAL BASELEVELS. That balanced condition called grade, where a

river neither degrades its bed by erosion nor aggrades it by

deposition, is first attained along reaches of soft rocks,

ungraded outcrops of hard rocks remaining as barriers which give

rise to rapids or falls. Until these barriers are worn away they

constitute local baselevels, below which level the stream, up

valley from them, cannot cut. They are eroded to grade one after

another, beginning with the least strong, or the one nearest the

mouth of the stream. In a similar way the surface of a lake in a

river's course constitutes for all inflowing streams a local

baselevel, which disappears when the basin is filled or drained.

MATURE AND OLD RIVERS

Maturity is the stage of a river's complete development and most

effective work. The river system now has well under way its great

task of wearing down the land mass which it drains and carrying it

particle by particle to the sea. The relief of the land is now at

its greatest; for the main channels have been sunk to grade, while

the divides remain but little worn below their initial altitudes.

Ground water now stands low. The run-off washes directly to the

streams, with the least delay and loss by evaporation in ponds and

marches; the discharge of the river is therefore at its height.

The entire region is dissected by stream ways. The area of valley

slopes is now largest and sheds to the streams a heavier load of

waste than ever before. At maturity the river system is doing its

greatest amount of work both in erosion and in the carriage of

water and of waste to the sea.

LATERAL EROSION. On reaching grade a river ceases to scour its

bed, and it does not again begin to do so until some change in

load or volume enables it to find grade at a lower level. On the

other hand, a stream erodes its banks at all stages in its

history, and with graded rivers this process, called lateral

erosion, or PLANATION, is specially important. The current of a

stream follows the outer side of all curves or bends in the

channel, and on this side it excavates its bed the deepest and

continually wears and saps its banks. On the inner side deposition

takes place in the more shallow and slower-moving water. The inner

bank of bends is thus built out while the outer bank is worn away.

By swinging its curves against the valley sides a graded river

continually cuts a wider and wider floor. The V-valley of youth is

thus changed by planation to a flat-floored valley with flaring

sides which gradually become subdued by the weather to gentle

slopes. While widening their valleys streams maintain a constant

width of channel, so that a wide-floored valley does not signify

that it ever was occupied by a river of equal width.

THE GRADIENT. The gradients of graded rivers differ widely. A

large river with a light load reaches grade on a faint slope,

while a smaller stream heavily burdened with waste requires a

steep slope to give it velocity sufficient to move the load.

The Platte, a graded river of Nebraska with its headwaters in the

Rocky Mountains, is enfeebled by the semi-arid climate of the

Great Plains and surcharged with the waste brought down both by

its branches in the mountains and by those whose tracks lie over

the soft rocks of the plains. It is compelled to maintain a

gradient of eight feet to the mile in western Nebraska. The Ohio

reaches grade with a slope of less than four inches to the mile

from Cincinnati to its mouth, and the powerful Mississippi washes

along its load with a fall of but three inches per mile from Cairo

to the Gulf.

Other things being equal, which of graded streams will have the

steeper gradient, a trunk stream or its tributaries? a stream

supplied with gravel or one with silt?

Other factors remaining the same, what changes would occur if the

Platte should increase in volume? What changes would occur if the

load should be increased in amount or in coarseness?

THE OLD AGE OF RIVERS. As rivers pass their prime, as denudation

lowers the relief of the region, less waste and finer is washed

over the gentler slopes of the lowering hills. With smaller loads

to carry, the rivers now deepen their valleys and find grade with

fainter declivities nearer the level of the sea. This limit of the

level of the sea beneath which they cannot erode is known as

baselevel. [Footnote: The term "baselevel" is also used to

designate the close approximation to sea level to which streams

are able to subdue the land.] As streams grow old they approach

more and more closely to baselevel, although they are never able

to attain it. Some slight slope is needed that water may flow and

waste be transported over the land. Meanwhile the relief of the

land has ever lessened. The master streams and their main

tributaries now wander with sluggish currents over the broad

valley floors which they have planed away; while under the erosion

of their innumerable branches and the wear of the weather the

divides everywhere are lowered and subdued to more and more gentle

slopes. Mountains and high plateaus are thus reduced to rolling

hills, and at last to plains, surmounted only by such hills as may

still be unreduced to the common level, because of the harder

rocks of which they are composed or because of their distance from

the main erosion channels. Such regions of faint relief, worn down

to near base level by subaerial agencies, are known as PENEPLAINS

(almost plains). Any residual masses which rise above them are

called MONADNOCKS, from the name of a conical peak of New

Hampshire which overlooks the now uplifted peneplain of southern

New England.

In its old age a region becomes mantled with thick sheets of fine

and weathered waste, slowly moving over the faint slopes toward

the water ways and unbroken by ledges of bare rock. In other

words, the waste mantle also is now graded, and as waterfalls have

been effaced in the river beds, so now any ledges in the wide

streams of waste are worn away and covered beneath smooth slopes

of fine soil. Ground water stands high and may exude in areas of

swamp. In youth the land mass was roughhewn and cut deep by stream

erosion. In old age the faint reliefs of the land dissolve away,

chiefly under the action of the weather, beneath their cloak of

waste.

THE CYCLE OF EROSION. The successive stages through which a land

mass passes while it is being leveled to the sea constitute

together a cycle of erosion. Each stage of the cycle from infancy

to old age leaves, as we have seen, its characteristic records in

the forms sculptured on the land, such as the shapes of valleys

and the contours of hills and plains. The geologist is thus able

to determine by the land forms of any region the stage in the

erosion cycle to which it now belongs, and knowing what are the

earlier stages of the cycle, to read something of the geological

history of the region.

INTERRUPTED CYCLES. So long a time is needed to reduce a land mass

to baselevel that the process is seldom if ever completed during a

single uninterrupted cycle of erosion. Of all the various

interruptions which may occur the most important are gradual

movements of the earth's crust, by which a region is either

depressed or elevated relative to sea level.

The DEPRESSION of a region hastens its old age by decreasing the

gradient of streams, by destroying their power to excavate their

beds and carry their loads to a degree corresponding to the amount

of the depression, and by lessening the amount of work they have

to do. The slackened river currents deposit their waste in Hood

plains which increase in height as the subsidence continues. The

lower courses of the rivers are invaded by the sea and become

estuaries, while the lower tributaries are cut off from the trunk

stream.

ELEVATION, on the other hand, increases the activity of all

agencies of weathering, erosion, and transportation, restores the

region to its youth, and inaugurates a new cycle of erosion.

Streams are given a steeper gradient, greater velocity, and

increased energy to carry their loads and wear their beds. They

cut through the alluvium of their flood plains, leaving it on

either bank as successive terraces, and intrench themselves in the

underlying rock. In their older and wider valleys they cut narrow,

steep-walled inner gorges, in which they flow swiftly over rocky

floors, broken here and there by falls and rapids where a harder

layer of rock has been discovered. Winding streams on plains may

thus incise their meanders in solid rock as the plains are

gradually uplifted. Streams which are thus restored to their youth

are said to be REVIVED.

As streams cut deeper and the valley slopes are steepened, the

mantle of waste of the region undergoing elevation is set in more

rapid movement. It is now removed particle by particle faster than

it forms. As the waste mantle thins, weathering attacks the rocks

of the region more energetically until an equilibrium is reached

again; the rocks waste rapidly and their waste is as rapidly

removed.

DISSECTED PENEPLAINS. When a rise of the land brings one cycle to

an end and begins another, the characteristic land forms of each

cycle are found together and the topography of the region is

composite until the second cycle is so far advanced that the land

forms of the first cycle are entirely destroyed. The contrast

between the land surfaces of the later and the earlier cycles is

most striking when the earlier had advanced to age and the later

is still in youth. Thus many peneplains which have been elevated

and dissected have been recognized by the remnants of their

ancient erosion surfaces, and the length of time which has elapsed

since their uplift has been measured by the stage to which the new

cycle has advanced.

THE PIEDMONT BELT. As an example of an ancient peneplain uplifted

and dissected we may cite the Piedmont Belt, a broad upland lying

between the Appalachian Mountains and the Atlantic coastal plain.

The surface of the Piedmont is gently rolling. The divides, which

are often smooth areas of considerable width, rise to a common

plane, and from them one sees in every direction an even sky line

except where in places some lone hill or ridge may lift itself

above the general level (Fig. 62). The surface is an ancient one,

for the mantle of residual waste lies deep upon it, soils are

reddened by long oxidation, and the rocks are rotted to a depth of

scores of feet.

At present, however, the waste mantle is not forming so rapidly as

it is being removed. The streams of the upland are actively

engaged in its destruction. They flow swiftly in narrow, rock-

walled valleys over rocky beds. This contrast between the young

streams and the aged surface which they are now so vigorously

dissecting can only be explained by the theory that the region

once stood lower than at present and has recently been upraised.

If now we imagine the valleys refilled with the waste which the

streams have swept away, and the upland lowered, we restore the

Piedmont region to the condition in which it stood before its

uplift and dissection,--a gently rolling plain, surmounted here

and there by isolated hills and ridges.

The surface of the ancient Piedmont plain, as it may be restored

from the remnants of it found on the divides, is not in accordance

with the structures of the country rocks. Where these are exposed

to view they are seen to be far from horizontal. On the walls of

river gorges they dip steeply and in various directions and the

streams flow over their upturned edges. As shown in Figure 67, the

rocks of the Piedmont have been folded and broken and tilted.

It is not reasonable to believe that when the rocks of the

Piedmont were thus folded and otherwise deformed the surface of

the region was a plain. The upturned layers have not always

stopped abruptly at the even surface of the Piedmont plain which

now cuts across them. They are the bases of great folds and tilted

blocks which must once have risen high in air. The complex and

disorderly structures of the Piedmont rocks are those seen in

great mountain ranges, and there is every reason to believe that

these rocks after their deformation rose to mountain height.

The ancient Piedmont plain cuts across these upturned rocks as

independently of their structure as the even surface of the sawed

stump of some great tree is independent of the direction of its

fibers. Hence the Piedmont plain as it was before its uplift was

not a coastal plain formed of strata spread in horizontal sheets

beneath the sea and then uplifted; nor was it a structural plain,

due to the resistance to erosion of some hard, flat-lying layer of

rock. Even surfaces developed on rocks of discordant structure,

such as the Piedmont shows, are produced by long denudation, and

we may consider the Piedmont as a peneplain formed by the wearing

down of mountain ranges, and recently uplifted.

THE LAURENTIAN PENEPLAIN. This is the name given to a denuded

surface on very ancient rocks which extends from the Arctic Ocean

to the St. Lawrence River and Lake Superior, with small areas also

in northern Wisconsin and New York. Throughout this U-shaped area,

which incloses Hudson Bay within its arms, the country rocks have

the complicated and contorted structures which characterize

mountain ranges. But the surface of the area is by no means

mountainous. The sky line when viewed from the divides is unbroken

by mountain peaks or rugged hills. The surface of the arm west of

Hudson Bay is gently undulating and that of the eastern arm has

been roughened to low-rolling hills and dissected in places by

such deep river gorges as those of the Ottawa and Saguenay. This

immense area may be regarded as an ancient peneplain truncating

the bases of long-vanished mountains and dissected after

elevation.

In the examples cited the uplift has been a broad one and to

comparatively little height. Where peneplains have been uplifted

to great height and have since been well dissected, and where they

have been upfolded and broken and uptilted, their recognition

becomes more difficult. Yet recent observers have found evidences

of ancient lowland surfaces of erosion on the summits of the

Allegheny ridges, the Cascade Mountains (Fig. 69), and the western

slope of the Sierra Nevadas.

THE SOUTHERN APPALACHIAN REGION. We have here an example of an

area the latter part of whose geological history may be deciphered

by means of its land forms. The generalized section of Figure 70,

which passes from west to east across a portion of the region in

eastern Tennessee, shows on the west a part of the broad

Cumberland plateau. On the east is a roughened upland platform,

from which rise in the distance the peaks of the Great Smoky

Mountains. The plateau, consisting of strata but little changed

from their original flat-lying attitude, and the platform,

developed on rocks of disordered structure made crystalline by

heat and pressure, both stand at the common level of the line AB.

They are separated by the Appalachian valley, forty miles wide,

cut in strata which have been folded and broken into long narrow

blocks. The valley is traversed lengthwise by long, low ridges,

the outcropping edges of the harder strata, which rise to about

the same level,--that of the line cd. Between these ridges stretch

valley lowlands at the level ef excavated in the weaker rocks,

while somewhat below them lie the channels of the present streams

now busily engaged in deepening their beds.

THE VALLEY LOWLANDS. Were they planed by graded or ungraded

streams? Have the present streams reached grade? Why did the

streams cease widening the floors of the valley lowlands? How long

since? When will they begin anew the work of lateral planation?

What effect will this have on the ridges if the present cycle of

erosion continues long uninterrupted?

THE RIDGES OF THE APPALACHIAN VALLEY. Why do they stand above the

valley lowlands? Why do their summits lie in about the same plane?

Refilling the valleys intervening between these ridges with the

material removed by the streams, what is the nature of the surface

thus restored? Does this surface cd accord with the rock

structures on which' it has been developed? How may it have been

made? At what height did the land stand then, compared with its

present height? What elevations stood above the surface cd? Why?

What name may you use to designate them? How does the length of

time needed to develop the surface cd compare with that needed to

develop the valley lowlands?

THE PLATFORM AND PLATEAU. Why do they stand at a common level ab?

Of what surface may they be remnants? Is it accordant with the

rock structure? How was it produced? What unconsumed masses

overlooked it? Did the rocks of the Appalachian valley stand above

this surface when it was produced? Did they then stand below it?

Compare the time needed to develop this surface with that needed

to develop cd. Which surface is the older?

How many cycles of erosion are represented here? Give the erosion

history of the region by cycles, beginning with the oldest, the

work done in each and the work left undone, what brought each

cycle to a close, and how long relatively it continued.

CHAPTER IV

RIVER DEPOSITS

The characteristic features of river deposits and the forms which

they assume may be treated under three heads: (1) valley deposits,

(2) basin deposits, and (3) deltas.

VALLEY DEPOSITS

FLOOD PLAINS are the surfaces of the alluvial deposits which

streams build along their courses at times of flood. A swift

current then sweeps along the channel, while a shallow sheet of

water moves slowly over the flood plain, spreading upon it a thin

layer of sediment. It has been estimated that each inundation of

the Nile leaves a layer of fertilizing silt three hundredths of an

inch thick over the flood plain of Egypt.

Flood plains may consist of a thin spread of alluvium over the

flat rock floor of a valley which is being widened by the lateral

erosion of a graded stream (Fig. 60). Flood-plain deposits of

great thickness may be built by aggrading rivers even in valleys

whose rock floors have never been thus widened.

A cross section of a flood plain shows that it is highest next the

river, sloping gradually thence to the valley sides. These wide

natural embankments are due to the fact that the river deposit is

heavier near the bank, where the velocity of the silt-laden

channel current is first checked by contact with the slower-moving

overflow.

Thus banked off from the stream, the outer portions of a flood

plain are often ill-drained and swampy, and here vegetal deposits,

such as peat, may be interbedded with river silts.

A map of a wide flood plain, such as that of the Mississippi or

the Missouri (Fig. 77), shows that the courses of the tributaries

on entering it are deflected downstream. Why?

The aggrading streams by which flood plains are constructed

gradually build their immediate banks and beds to higher and

higher levels, and therefore find it easy at times of great floods

to break their natural embankments and take new courses over the

plain. In this way they aggrade each portion of it in turn by

means of their shifting channels,

BRAIDED CHANNELS. A river actively engaged in aggrading its valley

with coarse waste builds a flood plain of comparatively steep

gradient and often flows down it in a fairly direct course and

through a network of braided channels. From time to time a channel

becomes choked with waste, and the water no longer finding room in

it breaks out and cuts and builds itself a new way which reunites

down valley with the other channels. Thus there becomes

established a network of ever-changing channels inclosing low

islands of sand and gravel.

TERRACES. While aggrading streams thus tend to shift their

channels, degrading streams, on the contrary, become more and more

deeply intrenched in their valleys. It often occurs that a stream,

after having built a flood plain, ceases to aggrade its bed

because of a lessened load or for other reasons, such as an uplift

of the region, and begins instead to degrade it. It leaves the

original flood plain out of reach of even the highest floods. When

again it reaches grade at a lower level it produces a new flood

plain by lateral erosion in the older deposits, remnants of which

stand as terraces on one or both sides of the valley. In this way

a valley may be lined with a succession of terraces at different

levels, each level representing an abandoned flood plain.

MEANDERS. Valleys aggraded with fine waste form well-nigh level

plains over which streams wind from side to side of a direct

course in symmetric bends known as meanders, from the name of a

winding river of Asia Minor. The giant Mississippi has developed

meanders with a radius of one and one half miles, but a little

creek may display on its meadow as perfect curves only a rod or so

in radius. On the flood plain of either river or creek we may find

examples of the successive stages in the development of the

meander, from its beginning in the slight initial bend sufficient

to deflect the current against the outer side. Eroding here and

depositing on the inner side of the bend, it gradually reaches

first the open bend whose width and length are not far from equal,

and later that of the horseshoe meander whose diameter transverse

to the course of the stream is much greater than that parallel

with it. Little by little the neck of land projecting into the

bend is narrowed, until at last it is cut through and a "cut-off"

is established. The old channel is now silted up at both ends and

becomes a crescentic lagoon, or oxbow lake, which fills gradually

to an arc-shaped shallow depression.

FLOOD PLAINS CHARACTERISTIC OF MATURE RIVERS. On reaching grade a

stream planes a flat floor for its continually widening valley.

Ever cutting on the outer bank of its curves, it deposits on the

inner bank scroll-like flood-plain patches. For a while the valley

bluffs do not give its growing meanders room to develop to their

normal size, but as planation goes on, the bluffs are driven back

to the full width of the meander belt and still later to a width

which gives room for broad stretches of flood plain on either

side.

Usually a river first attains grade near its mouth, and here first

sinks its bed to near baselevel. Extending its graded course

upstream by cutting away barrier after barrier, it comes to have a

widened and mature valley over its lower course, while its young

headwaters are still busily eroding their beds. Its ungraded

branches may thus bring down to its lower course more waste than

it is competent to carry on to the sea, and here it aggrades its

bed and builds a flood plain in order to gain a steeper gradient

and velocity enough to transport its load.

As maturity is past and the relief of the land is lessened, a

smaller and smaller load of waste is delivered to the river. It

now has energy to spare and again degrades its valley, excavating

its former flood plains and leaving them in terraces on either

side, and at last in its old age sweeping them away.

ALLUVIAL CONES AND FANS. In hilly and mountainous countries one

often sees on a valley side a conical or fan-shaped deposit of

waste at the mouth of a lateral stream. The cause is obvious: the

young branch has not been able as yet to wear its bed to accordant

level with the already deepened valley of the master stream. It

therefore builds its bed to grade at the point of juncture by

depositing here its load of waste,--a load too heavy to be carried

along the more gentle profile of the trunk valley.

Where rivers descend from a mountainous region upon the plain they

may build alluvial fans of exceedingly gentle slope. Thus the

rivers of the western side of the Sierra Nevada Mountains have

spread fans with a radius of as much as forty miles and a slope

too slight to be detected without instruments, where they leave

the rock-cut canyons in the mountains and descend upon the broad

central valley of California.

As a river flows over its fan it commonly divides into a

branchwork of shifting channels called DISTRIBUTARIES, since they

lead off the water from the main stream. In this way each part of

the fan is aggraded and its symmetric form is preserved.

PIEDMONT PLAINS. Mountain streams may build their confluent fans

into widespread piedmont (foot of the mountain) alluvial plains.

These are especially characteristic of arid lands, where the

streams wither as they flow out upon the thirsty lowlands and are

therefore compelled to lay down a large portion of their load. In

humid climates mountain-born streams are usually competent to

carry their loads of waste on to the sea, and have energy to spare

to cut the lower mountain slopes into foothills. In arid regions

foothills are commonly absent and the ranges rise, as from

pedestals, above broad, sloping plains of stream-laid waste.

THE HIGH PLAINS. The rivers which flow eastward from the Rocky

Mountains have united their fans in a continuous sheet of waste

which stretches forward from the base of the mountains for

hundreds of miles and in places is five hundred feet thick (Fig.

80). That the deposit was made in ancient times on land and not in

the sea is proved by the remains which it contains of land animals

and plants of species now extinct. That it was laid by rivers and

not by fresh-water lakes is shown by its structure. Wide stretches

of flat-lying, clays and sands are interrupted by long, narrow

belts of gravel which mark the channels of the ancient streams.

Gravels, and sands are often cross bedded, and their well worn

pebbles may be identified with the rocks of the mountains. After

building this sheet of waste the streams ceased to aggrade and

began the work of destruction. Large uneroded remnants, their

surfaces flat as a floor, remain as the High Plains of western

Kansas and Nebraska.

RIVER DEPOSITS IN SUBSIDING TROUGHS. To a geologist the most

important river deposits are those which gather in areas of

gradual subsidence; they are often of vast extent and immense

thickness, and such deposits of past geological ages have not

infrequently been preserved, with all their records of the times

in which they were built, by being carried below the level of the

sea, to be brought to light by a later uplift. On the other hand,

river deposits which remain above baselevels of erosion are swept

away comparatively soon.

THE GREAT VALLEY OF CALIFORNIA is a monotonously level plain of

great fertility, four hundred miles in length and fifty miles in

average width, built of waste swept down by streams from the

mountain ranges which inclose it,--the Sierra Nevada on the east

and the Coast Range on the west. On the waste slopes at the foot

of the bordering hills coarse gravels and even bowlders are left,

while over the interior the slow-flowing streams at times of

flood spread wide sheets of silt. Organic deposits are now forming

by the decay of vegetation in swampy tule (reed) lands and in

shallow lakes which occupy depressions left by the aggrading

streams.

Deep borings show that this great trough is filled to a depth of

at least two thousand feet below sea level with recent

unconsolidated sands and silts containing logs of wood and fresh-

water shells. These are land deposits, and the absence of any

marine deposits among them proves that the region has not been

invaded by the sea since the accumulation began. It has therefore

been slowly subsiding and its streams, although continually

carried below grade, have yet been able to aggrade the surface as

rapidly as the region sank, and have maintained it, as at present,

slightly above sea level.

THE INDO-GANGETIC PLAIN, spread by the Brahmaputra, the Ganges,

and the Indus river systems, stretches for sixteen hundred miles

along the southern base of the Himalaya Mountains and occupies an

area of three hundred thousand square miles (Fig.342). It consists

of the flood plains of the master streams and the confluent fans

of the tributaries which issue from the mountains on the north.

Large areas are subject to overflow each season of flood, and

still larger tracts mark abandoned flood plains below which the

rivers have now cut their beds. The plain is built of far-

stretching beds of clay, penetrated by streaks of sand, and also

of gravel near the mountains. Beds of impure peat occur in it, and

it contains fresh-water shells and the bones of land animals of

species now living in northern India. At Lucknow an artesian well

was sunk to one thousand feet below sea level without reaching the

bottom of these river-laid sands and silts, proving a slow

subsidence with which the aggrading rivers have kept pace.

WARPED VALLEYS. It is not necessary that an area should sink below

sea level in order to be filled with stream-swept waste. High

valleys among growing mountain ranges may suffer warping, or may

be blockaded by rising mountain folds athwart them. Where the

deformation is rapid enough, the river may be ponded and the

valley filled with lake-laid sediments. Even when the river is

able to maintain its right of way it may yet have its declivity so

lessened that it is compelled to aggrade its course continually,

filling the valley with river deposits which may grow to an

enormous thickness.

Behind the outer ranges of the Himalaya Mountains lie several

waste-filled valleys, the largest of which are Kashmir and Nepal,

the former being an alluvial plain about as large as the state of

Delaware. The rivers which drain these plains have already cut

down their outlet gorges sufficiently to begin the task of the

removal of the broad accumulations which they have brought in from

the surrounding mountains. Their present flood plains lie as much

as some hundreds of feet below wide alluvial terraces which mark

their former levels. Indeed, the horizontal beds of the Hundes

Valley have been trenched to the depth of nearly three thousand

feet by the Sutlej River. These deposits are recent or subrecent,

for there have been found at various levels the remains of land

plants and land and fresh-water shells, and in some the bones of

such animals as the hyena and the goat, of species or of genera

now living. Such soft deposits cannot be expected to endure

through any considerable length of future time the rapid erosion

to which their great height above the level of the sea will

subject them.

CHARACTERISTICS OF RIVER DEPOSITS. The examples just cited teach

clearly the characteristic features of extensive river deposits.

These deposits consist of broad, flat-lying sheets of clay and

fine sand left by the overflow at time of flood, and traversed

here and there by long, narrow strips of coarse, cross-bedded

sands and gravels thrown down by the swifter currents of the

shifting channels. Occasional beds of muck mark the sites of

shallow lakelets or fresh-water swamps. The various strata also

contain some remains of the countless myriads of animals and

plants which live upon the surface of the plain as it is in

process of building. River shells such as the mussel, land shells

such as those of snails, the bones of fishes and of such land

animals as suffer drowning at times of flood or are mired in

swampy places, logs of wood, and the stems and leaves of plants

are examples of the variety of the remains of land and fresh-water

organisms which are entombed in river deposits and sealed away as

a record of the life of the time, and as proof that the deposits

were laid by streams and not beneath the sea.

BASIN DEPOSITS

DEPOSITS IN DRY BASINS. On desert areas without outlet to the sea,

as on the Great Basin of the United States and the deserts of

central Asia, stream-swept waste accumulates indefinitely. The

rivers of the surrounding mountains, fed by the rains and melting

snows of these comparatively moist elevations, dry and soak away

as they come down upon the arid plains. They are compelled to lay

aside their entire load of waste eroded from the mountain valleys,

in fans which grow to enormous size, reaching in some instances

thousands of feet in thickness.

The monotonous levels of Turkestan include vast alluvial tracts

now in process of building by the floods of the frequently

shifting channels of the Oxus and other rivers of the region. For

about seven hundred miles from its mouth in Aral Lake the Oxus

receives no tributaries, since even the larger branches of its

system are lost in a network of distributaries and choked with

desert sands before they reach their master stream. These

aggrading rivers, which have channels but no valleys, spread their

muddy floods--which in the case of the Oxus sometimes equal the

average volume of the Mississippi--far and wide over the plain,

washing the bases of the desert dunes.

PLAYAS. In arid interior basins the central depressions may be

occupied by playas,--plains of fine mud washed forward from the

margins. In the wet season the playa is covered with a thin sheet

of muddy water, a playa lake, supplied usually by some stream at

flood. In the dry season the lake evaporates, the river which fed

it retreats, and there is left to view a hard, smooth, level floor

of sun-baked and sun-cracked yellow clay utterly devoid of

vegetation.

In the Black Rock desert of Nevada a playa lake spreads over an

area fifty miles long and twenty miles wide. In summer it

disappears; the Quinn River, which feeds it, shrinks back one

hundred miles toward its source, leaving an absolutely barren

floor of clay, level as the sea.

LAKE DEPOSITS. Regarding lakes as parts of river systems, we may

now notice the characteristic features of the deposits in lake

basins. Soundings in lakes of considerable size and depth show

that their bottoms are being covered with tine clays. Sand and

gravel are found along; their margins, being brought in by streams

and worn by waves from the shore, but there are no tidal or other

strong currents to sweep coarse waste out from shore to any

considerable distance. Where fine clays are now found on the land

in even, horizontal layers containing the remains of fresh-water

animals and plants, uncut by channels tilled with cross-bedded

gravels and sands and bordered by beach deposits of coarse waste,

we may safely infer the existence of ancient lakes.

MARL. Marl is a soft, whitish deposit of carbonate of lime,

mingled often with more or less of clay, accumulated in small

lakes whose feeding springs are charged with carbonate of lime and

into which little waste is washed from the land. Such lakelets are

not infrequent on the surface of the younger drift sheets of

Michigan and northern Indiana, where their beds of marl--sometimes

as much as forty feet thick--are utilized in the manufacture of

Portland cement. The deposit results from the decay of certain

aquatic plants which secrete lime carbonate from the water, from

the decomposition of the calcareous shells of tiny mollusks which

live in countless numbers on the lake floor, and in some cases

apparently from chemical precipitation.

PEAT. We have seen how lakelets are extinguished by the decaying

remains of the vegetation which they support. A section of such a

fossil lake shows that below the growing mosses and other plants

of the surface of the bog lies a spongy mass composed of dead

vegetable tissue, which passes downward gradually into PEAT,--a

dense, dark brown carbonaceous deposit in which, to the unaided

eye, little or no trace of vegetable structure remains. When

dried, peat forms a fuel of some value and is used either cut into

slabs and dried or pressed into bricks by machinery.

When vegetation decays in open air the carbon of its tissues,

taken from the atmosphere by the leaves, is oxidized and returned

to it in its original form of carbon dioxide. But decomposing in

the presence of water, as in a bog, where the oxygen of the air is

excluded, the carbonaceous matter of plants accumulates in

deposits of peat.

Peat bogs are numerous in regions lately abandoned by glacier ice,

where river systems are so immature that the initial depressions

left in the sheet of drift spread over the country have not yet

been drained. One tenth of the surface of Ireland is said to be

covered with peat, and small bogs abound in the drift-covered area

of New England and the states lying as far west as the Missouri

River. In Massachusetts alone it has been reckoned that there are

fifteen billion cubic feet of peat, the largest bog occupying

several thousand acres.

Much larger swamps occur on the young coastal plain of the

Atlantic from New Jersey to Florida. The Dismal Swamp, for

example, in Virginia and North Carolina is forty miles across. It

is covered with a dense growth of water-loving trees such as the

cypress and black gum. The center of the swamp is occupied by Lake

Drummond, a shallow lake seven miles in diameter, with banks of

pure-peat, and still narrowing from the encroachment of vegetation

along its borders.

SALT LAKES. In arid climates a lake rarely receives sufficient

inflow to enable it to rise to the basin rim and find an outlet.

Before this height is reached its surface becomes large enough to

discharge by evaporation into the dry air the amount of water that

is supplied by streams. As such a lake has no outlet, the minerals

in solution brought into it by its streams cannot escape from the

basin. The lake water becomes more and more heavily charged with

such substances as common salt and the sulphates and carbonates of

lime, of soda, and of potash, and these are thrown down from

solution one after another as the point of saturation for each

mineral is reached. Carbonate of lime, the least soluble and often

the most abundant mineral brought in, is the first to be

precipitated. As concentration goes on, gypsum, which is insoluble

in a strong brine, is deposited, and afterwards common salt. As

the saltness of the lake varies with the seasons and with climatic

changes, gypsum and salt are laid in alternate beds and are

interleaved with sedimentary clays spread from the waste brought

in by streams at times of flood. Few forms of life can live in

bodies of salt water so concentrated that chemical deposits take

place, and hence the beds of salt, gypsum, and silt of such lakes

are quite barren of the remains of life. Similar deposits are

precipitated by the concentration of sea water in lagoons and arms

of the sea cut off from the ocean.

LAKES BONNEVILLE AND LAHONTAN. These names are given to extinct

lakes which once occupied large areas in the Great Basin, the

former in Utah, the latter in northwestern Nevada. Their records

remain in old horizontal beach lines which they drew along their

mountainous shores at the different levels at which they stood,

and in the deposits of their beds. At its highest stage Lake

Bonneville, then one thousand feet deep, overflowed to the north

and was a fresh-water lake. As it shrank below the outlet it

became more and more salty, and the Great Salt Lake, its withered

residue, is now depositing salt along its shores. In its strong

brine lime carbonate is insoluble, and that brought in by streams

is thrown down at once in the form of travertine.

Lake Lahontan never had an outlet. The first chemical deposits to

be made along its shores were deposits of travertine, in places

eighty feet thick. Its floor is spread with fine clays, which must

have been laid in deep, still water, and which are charged with

the salts absorbed by them as the briny water of the lake dried

away. These sedimentary clays are in two divisions, the upper and

lower, each being about one hundred feet thick. They are separated

by heavy deposits of well-rounded, cross-bedded gravels and sands,

similar to those spread at the present time by the intermittent

streams of arid regions. A similar record is shown in the old

floors of Lake Bonneville. What conclusions do you draw from these

facts as to the history of these ancient lakes?

DELTAS

In the river deposits which are left above sea level particles of

waste are allowed to linger only for a time. From alluvial fans

and flood plains they are constantly being taken up and swept

farther on downstream. Although these land forms may long persist,

the particles which compose them are ever changing. We may

therefore think of the alluvial deposits of a valley as a stream

of waste fed by the waste mantle as it creeps and washes down the

valley sides, and slowly moving onwards to the sea.

In basins waste finds a longer rest, but sooner or later lakes and

dry basins are drained or filled, and their deposits, if above sea

level, resume their journey to their final goal. It is only when

carried below the level of the sea that they are indefinitely

preserved.

On reaching this terminus, rivers deliver their load to the ocean.

In some cases the ocean is able to take it up by means of strong

tidal and other currents, and to dispose of it in ways which we

shall study later. But often the load is so large, or the tides

are so weak, that much of the waste which the river brings in

settles at its mouth, there building up a deposit called the

DELTA, from the Greek letter of that name, whose shape it

sometimes resembles.

Deltas and alluvial fans have many common characteristics. Both

owe their origin to a sudden check in the velocity of the river,

compelling a deposit of the load; both are triangular in outline,

the apex pointing upstream; and both are traversed by

distributaries which build up all parts in turn.

In a delta we may distinguish deposits of two distinct kinds,--

the submarine and the subaerial. In part a delta is built of waste

brought down by the river and redistributed and spread by waves

and tides over the sea bottom adjacent to the river's mouth. The

origin of these deposits is recorded in the remains of marine

animals and plants which they contain.

As the submarine delta grows near to the level of the sea the

distributaries of the river cover it with subaerial deposits

altogether similar to those of the flood plain, of which indeed

the subaerial delta is the prolongation. Here extended deposits of

peat may accumulate in swamps, and the remains of land and fresh-

water animals and plants swept down by the stream are imbedded in

the silts laid at times of flood.

Borings made in the deltas of great rivers such as the

Mississippi, the Ganges, and the Nile, show that the subaerial

portion often reaches a surprising thickness. Layers of peat, old

soils, and forest grounds with the stumps of trees are discovered

hundreds of feet below sea level. In the Nile delta some eight

layers of coarse gravel were found interbedded with river silts,

and in the Ganges delta at Calcutta a boring nearly five hundred

feet in depth stopped in such a layer.

The Mississippi has built a delta of twelve thousand three hundred

square miles, and is pushing the natural embankments of its chief

distributaries into the Gulf at a maximum rate of a mile in

sixteen years. Muddy shoals surround its front, shallow lakes,

e.g. lakes Pontchartrain and Borgne, are formed between the

growing delta and the old shore line, and elongate lakes and

swamps are inclosed between the natural embankments of the

distributaries.

The delta of the Indus River, India, lies so low along shore that

a broad tract of country is overflowed by the highest tides. The

submarine portion of the delta has been built to near sea level

over so wide a belt offshore that in many places large vessels

cannot come even within sight of land because of the shallow

water.

A former arm of the sea, the Rann of Cutch, adjoining the delta on

the east has been silted up and is now an immense barren flat of

sandy mud two hundred miles in length and one hundred miles in

greatest breadth. Each summer it is flooded with salt water when

the sea is brought in by strong southwesterly monsoon winds, and

the climate during the remainder of the year is hot and dry. By

the evaporation of sea water the soil is thus left so salty that

no vegetation can grow upon it, and in places beds of salt several

feet in thickness have accumulated. Under like conditions salt

beds of great thickness have been formed in the past and are now

found buried among the deposits of ancient deltas.

SUBSIDENCE OF GREAT DELTAS. As a rule great deltas are slowly

sinking. In some instances upbuilding by river deposits has gone

on as rapidly as the region has subsided. The entire thickness of

the Ganges delta, for example, so far as it has been sounded,

consists of deposits laid in open air. In other cases interbedded

limestones and other sedimentary rocks containing marine fossils

prove that at times subsidence has gained on the upbuilding and

the delta has been covered with the sea.

It is by gradual depression that delta deposits attain enormous

thickness, and, being lowered beneath the level of the sea, are

safely preserved from erosion until a movement of the earth's

crust in the opposite direction lifts them to form part of the

land. We shall read later in the hard rocks of our continent the

records of such ancient deltas, and we shall not be surprised to

find them as thick as are those now building at the mouths of

great rivers.

LAKE DELTAS. Deltas are also formed where streams lose their

velocity on entering the still waters of lakes. The shore lines of

extinct lakes, such as Lake Agassiz and Lakes Bonneville and

Lahontan, may be traced by the heavy deposits at the mouths of

their tributary streams.

We have seen that the work of streams is to drain the lands of the

water poured upon them by the rainfall, to wear them down, and to

carry their waste away to the sea, there to be rebuilt by other

agents into sedimentary rocks. The ancient strata of which the

continents are largely made are composed chiefly of material thus

worn from still more ancient lands--lands with their hills and

valleys like those of to-day--and carried by their rivers to the

ocean. In all geological times, as at the present, the work of

streams has been to destroy the lands, and in so doing to furnish

to the ocean the materials from which the lands of future ages

were to be made. Before we consider how the waste of the land

brought in by streams is rebuilt upon the ocean floor, we must

proceed to study the work of two agents, glacier ice and the wind,

which cooperate with rivers in the denudation of the land.

CHAPTER V

THE WORK OF GLACIERS

THE DRIFT. The surface of northeastern North America, as far south

as the Ohio and Missouri rivers, is generally covered by the

drift,--a formation which is quite unlike any which we have so far

studied. A section of it, such as that illustrated in Figure 87,

shows that for the most part it is unstratified, consisting of

clay, sand, pebbles, and even large bowlders, all mingled pell-

mell together. The agent which laid the drift is one which can

carry a load of material of all sizes, from the largest bowlder to

the finest clay, and deposit it without sorting.

The stones of the drift are of many kinds. The region from which

it was gathered may well have been large in order to supply these

many different varieties of rocks. Pebbles and bowlders have been

left far from their original homes, as may be seen in southern

Iowa, where the drift contains nuggets of copper brought from the

region about Lake Superior. The agent which laid the drift is one

able to gather its load over a large area and carry it a long way.

The pebbles of the drift are unlike those rounded by running water

or by waves. They are marked with scratches. Some are angular,

many have had their edges blunted, while others have been ground

flat and smooth on one or more sides, like gems which have been

faceted by being held firmly against the lapidary's wheel. In many

places the upper surface of the country rock beneath the drift has

been swept clean of residual clays and other waste. All rock

rotten has been planed away, and the ledges of sound rock to which

the surface has been cut down have been rubbed smooth and

scratched with long, straight, parallel lines. The agent which

laid the drift can hold sand and pebbles firmly in its grasp and

can grind them against the rock beneath, thus planing it down and

scoring it, while faceting the pebbles also.

Neither water nor wind can do these things. Indeed, nothing like

the drift is being formed by any process now at work anywhere in

the eastern United States. To find the agent which has laid this

extensive formation we must go to a region of different climatic

conditions.

THE INLAND ICE OF GREENLAND. Greenland is about fifteen hundred

miles long and nearly seven hundred miles in greatest width. With

the exception of a narrow fringe of mountainous coast land, it is

completely buried beneath a sheet of ice, in shape like a vast

white shield, whose convex surface rises to a height of nine

thousand feet above the sea. The few explorers who have crossed

the ice cap found it a trackless desert destitute of all life save

such lowly forms as the microscopic plant which produces the so-

called "red snow." On the smooth plain of the interior no rock

waste relieves the snow's dazzling whiteness; no streams of

running water are seen; the silence is broken only by howling

storm winds and the rustle of the surface snow which they drive

before them. Sounding with long poles, explorers find that below

the powdery snow of the latest snowfall lie successive layers of

earlier snows, which grow more and more compact downward, and at

last have altered to impenetrable ice. The ice cap formed by the

accumulated snows of uncounted centuries may well be more than a

mile in depth. Ice thus formed by the compacting of snow is

distinguished when in motion as GLACIER ICE.

The inland ice of Greenland moves. It flows with imperceptible

slowness under its own weight, like, a mass of some viscous or

plastic substance, such as pitch or molasses candy, in all

directions outward toward the sea. Near the edge it has so thinned

that mountain peaks are laid bare, these islands in the sea of ice

being known as NUNATAKS. Down the valleys of the coastal belt it

drains in separate streams of ice, or GLACIERS. The largest of

these reach the sea at the head of inlets, and are therefore

called TIDE GLACIERS. Their fronts stand so deep in sea water that

there is visible seldom more than three hundred feet of the wall

of ice, which in many glaciers must be two thousand and more feet

high. From the sea walls of tide glaciers great fragments break

off and float away as icebergs. Thus snows which fell in the

interior of this northern land, perhaps many thousands of years

ago, are carried in the form of icebergs to melt at last in the

North Atlantic.

Greenland, then, is being modeled over the vast extent of its

interior not by streams of running water, as are regions in warm

and humid climates, nor by currents of air, as are deserts to a

large extent, but by a sheet of flowing ice. What the ice sheet is

doing in the interior we may infer from a study of the separate

glaciers into which it breaks at its edge.

THE SMALLER GREENLAND GLACIERS. Many of the smaller glaciers of

Greenland do not reach the sea, but deploy on plains of sand and

gravel. The edges of these ice tongues are often as abrupt as if

sliced away with a knife (Fig. 92), and their structure is thus

readily seen. They are stratified, their layers representing in

part the successive snowfalls of the interior of the country. The

upper layers are commonly white and free from stones; but the

lower layers, to the height of a hundred feet or more, are dark

with debris which is being slowly carried on. So thickly studded

with stones is the base of the ice that it is sometimes difficult

to distinguish it from the rock waste which has been slowly

dragged beneath the glacier or left about its edges. The waste

beneath and about the glacier is unsorted. The stones are of many

kinds, and numbers of them have been ground to flat faces. Where

the front of the ice has retreated the rock surface is seen to be

planed and scored in places by the stones frozen fast in the sole

of the glacier.

We have now found in glacier ice an agent able to produce the

drift of North America. The ice sheet of Greenland is now doing

what we have seen was done in the recent past in our own land. It

is carrying for long distances rocks of many kinds gathered, we

may infer, over a large extent of country. It is laying down its

load without assortment in unstratified deposits. It grinds down

and scores the rock over which it moves, and in the process many

of the pebbles of its load are themselves also ground smooth and

scratched. Since this work can be done by no other agent, we must

conclude that the northeastern part of our own continent was

covered in the recent past by glacier ice, as Greenland is to-day.

VALLEY GLACIERS

The work of glacier ice can be most conveniently studied in the

separate ice streams which creep down mountain valleys in many

regions such as Alaska, the western mountains of the United States

and Canada, the Himalayas, and the Alps. As the glaciers of the

Alps have been studied longer and more thoroughly than any others,

we shall describe them in some detail as examples of valley

glaciers in all parts of the world.

CONDITIONS OF GLACIER FORMATION. The condition of the great

accumulation of snow to which glaciers are due--that more or less

of each winter's snow should be left over unmelted and

unevaporated to the next--is fully met in the Alps. There is

abundant moisture brought by the winds from neighboring seas. The

currents of moist air driven up the mountain slopes are cooled by

their own expansion as they rise, and the moisture which they

contain is condensed at a temperature at or below 32 degrees F.,

and therefore is precipitated in the form of snow. The summers are

cool and their heat does not suffice to completely melt the heavy

snow of the preceding winter. On the Alps the SNOW LINE--the lower

limit of permanent snow--is drawn at about eight thousand five

hundred feet above sea level. Above the snow line on the slopes

and crests, where these are not too steep, the snow lies the year

round and gathers in valley heads to a depth of hundreds of feet.

This is but a small fraction of the thickness to which snow would

be piled on the Alps were it not constantly being drained away.

Below the snow fields which mantle the heights the mountain

valleys are occupied by glaciers which extend as much as a

vertical mile below the snow line. The presence in the midst of

forests and meadows and cultivated fields of these tongues of ice,

ever melting and yet from year to year losing none of their bulk,

proves that their loss is made good in the only possible way. They

are fed by snow fields above, whose surplus of snow they drain

away in the form of ice. The presence of glaciers below the snow

line is a clear proof that, rigid and motionless as they appear,

glaciers really are in constant motion down valley.

THE NEVE FIELD. The head of an Alpine valley occupied by a glacier

is commonly a broad amphitheater deeply filled with snow. Great

peaks tower above it, and snowy slopes rise on either side on the

flanks of mountain spurs. From these heights fierce winds drift

the snows into the amphitheater, and avalanches pour in their

torrents of snow and waste. The snow of the amphitheater is like

that of drifts in late winter after many successive thaws and

freezings. It is made of hard grains and pellets and is called

NEVE. Beneath the surface of the neve field and at its outlet the

granular neve has been compacted to a mass of porous crystalline

ice. Snow has been changed to neve, and neve to glacial ice, both

by pressure, which drives the air from the interspaces of the

snowflakes, and also by successive meltings and freezings, much as

a snowball is packed in the warm hand and becomes frozen to a ball

of ice.

THE BERGSCHRUND. The neve is in slow motion. It breaks itself

loose from the thinner snows about it, too shallow to share its

motion, and from the rock rim which surrounds it, forming a deep

fissure called the bergschrund, sometimes a score and more feet

wide.

SIZE OF GLACIERS. The ice streams of the Alps vary in size

according to the amount of precipitation and the area of the neve

fields which they drain. The largest of Alpine glaciers, the

Aletsch, is nearly ten miles long and has an average width of

about a mile. The thickness of some of the glaciers of the Alps is

as much as a thousand feet. Giant glaciers more than twice the

length of the longest in the Alps occur on the south slope of the

Himalaya Mountains, which receive frequent precipitations of snow

from moist winds from the Indian Ocean. The best known of the many

immense glaciers of Alaska, the Muir, has an area of about eight

hundred square miles (Fig. 95).

GLACIER MOTION. The motion of the glaciers of the Alps seldom

exceeds one or two feet a day. Large glaciers, because of the

enormous pressure of their weight and because of less marginal

resistance, move faster than small ones. The Muir advances at the

rate of seven feet a day, and some of the larger tide glaciers of

Greenland are reported to move at the exceptional rate of fifty

feet and more in the same time. Glaciers move faster by day than

by night, and in summer than in winter. Other laws of glacier

motion may be discovered by a study of Figures 96 and 97. It is

important to remember that glaciers do not slide bodily over their

beds, but urged by gravity move slowly down valley in somewhat the

same way as would a stream of thick mud. Although small pieces of

ice are brittle, the large mass of granular ice which composes a

glacier acts as a viscous substance.

CREVASSES. Slight changes of slope in the glacier bed, and the

different rates of motion in different parts, produce tensions

under which the ice cracks and opens in great fissures called

crevasses. At an abrupt descent in the bed the ice is shattered

into great fragments, which unite again below the icefall.

Crevasses are opened on lines at right angles to the direction of

the tension. TRANSVERSE CREVASSES are due to a convexity in the

bed which stretches the ice lengthwise (Fig. 99). MARGINAL

CREVASSES are directed upstream and inwards; RADIAL CREVASSES are

found where the ice stream deploys from some narrow valley and

spreads upon some more open space. What is the direction of the

tension which causes each and to what is it due?

LATERAL AND MEDIAL MORAINES. The surface of a glacier is striped

lengthwise by long dark bands of rock debris. Those in the center

are called the medial moraines. The one on either margin is a

lateral moraine, and is clearly formed of waste which has fallen

on the edge of the ice from the valley slopes. A medial moraine

cannot be formed in this way, since no rock fragments can fall so

far out from the sides. But following it up the glacial stream,

one finds that a medial moraine takes its beginning at the

junction of the glacier and some tributary and is formed by the

union of their two adjacent lateral moraines. Each branch thus

adds a medial moraine, and by counting the number of medial

moraines of a trunk stream one may learn of how many branches it

is composed.

Surface moraines appear in the lower course of the glacier as

ridges, which may reach the exceptional height of one hundred

feet. The bulk of such a ridge is ice. It has been protected from

the sun by the veneer of moraine stuff; while the glacier surface

on either side has melted down at least the distance of the height

of the ridge. In summer the lowering of the glacial surface by

melting goes on rapidly. In Swiss glaciers it has been estimated

that the average lowering of the surface by melting and

evaporation amounts to ten feet a year. As a moraine ridge grows

higher and more steep by the lowering of the surface of the

surrounding ice, the stones of its cover tend to slip down its

sides. Thus moraines broaden, until near the terminus of a glacier

they may coalesce in a wide field of stony waste.

ENGLACIAL DRIFT. This name is applied to whatever debris is

carried within the glacier. It consists of rock waste fallen on

the neve and there buried by accumulations of snow, and of that

engulfed in the glacier where crevasses have opened beneath a

surface moraine. As the surface of the glacier is lowered by

melting, more or less englacial drift is brought again to open

air, and near the terminus it may help to bury the ice from view

beneath a sheet of debris.

THE GROUND MORAINE. The drift dragged along at the glacier's base

and lodged beneath it is known as the ground moraine. Part of the

material of it has fallen down deep crevasses and part has been

torn and worn from the glacier's bed and banks. While the stones

of the surface moraines remain as angular as when they lodged on

the ice, many of those of the ground moraine have been blunted on

the edges and faceted and scratched by being ground against one

another and the rocky bed.

In glaciers such as those of Greenland, whose basal layers are

well loaded with drift and whose surface layers are nearly clean,

different layers have different rates of motion, according to the

amount of drift with which they are clogged. One layer glides over

another, and the stones inset in each are ground and smoothed and

scratched. Usually the sides of glaciated pebbles are more worn

than the ends, and the scratches upon them run with the longer

axis of the stone. Why?

THE TERMINAL MORAINE. As a glacier is in constant motion, it

brings to its end all of its load except such parts of the ground

moraine as may find permanent lodgment beneath the ice. Where the

glacier front remains for some time at one place, there is formed

an accumulation of drift known as the terminal moraine. In valley

glaciers it is shaped by the ice front to a crescent whose convex

side is downstream. Some of the pebbles of the terminal moraine

are angular, and some are faceted and scored, the latter having

come by the hard road of the ground moraine. The material of the

dump is for the most part unsorted, though the water of the

melting ice may find opportunity to leave patches of stratified

sands and gravels in the midst of the unstratified mass of drift,

and the finer material is in places washed away.

GLACIER DRAINAGE. The terminal moraine is commonly breached by a

considerable stream, which issues from beneath the ice by a tunnel

whose portal has been enlarged to a beautiful archway by melting

in the sun and the warm air (Fig. 107). The stream is gray with

silt and loaded with sand and gravel washed from the ground

moraine. "Glacier milk" the Swiss call this muddy water, the gray

color of whose silt proves it rock flour freshly ground by the ice

from the unoxidized sound rock of its bed, the mud of streams

being yellowish when it is washed from the oxidized mantle of

waste. Since glacial streams are well loaded with waste due to

vigorous ice erosion, the valley in front of the glacier is

commonly aggraded to a broad, flat floor. These outwash deposits

are known as VALLEY DRIFT.

The sand brought out by streams from beneath a glacier differs

from river sand in that it consists of freshly broken angular

grains. Why?

The stream derives its water chiefly from the surface melting of

the glacier. As the ice is touched by the rays of the morning sun

in summer, water gathers in pools, and rills trickle and unite in

brooklets which melt and cut shallow channels in the blue ice. The

course of these streams is short. Soon they plunge into deep wells

cut by their whirling waters where some crevasse has begun to open

across their path. These wells lead into chambers and tunnels by

which sooner or later their waters find way to the rock floor of

the valley and there unite in a subglacial stream.

THE LOWER LIMIT OF GLACIERS. The glaciers of a region do not by

any means end at a uniform height above sea level. Each terminates

where its supply is balanced by melting. Those therefore which are

fed by the largest and deepest neves and those also which are best

protected from the sun by a northward exposure or by the depth of

their inclosing valleys flow to lower levels than those whose

supply is less and whose exposure to the sun is greater.

A series of cold, moist years, with an abundant snowfall, causes

glaciers to thicken and advance; a series of warm, dry years

causes them to wither and melt back. The variation in glaciers is

now carefully observed in many parts of the world. The Muir

glacier has retreated two miles in twenty years. The glaciers of

the Swiss Alps are now for the most part melting back, although a

well-known glacier of the eastern Alps, the Vernagt, advanced five

hundred feet in the year 1900, and was then plowing up its

terminal moraine.

How soon would you expect a glacier to advance after its neve

fields have been swollen with unusually heavy snows, as compared

with the time needed for the flood of a large river to reach its

mouth after heavy rains upon its headwaters?

On the surface of glaciers in summer time one may often see large

stones supported by pillars of ice several feet in height (Fig.

108). These "glacier tables" commonly slope more or less strongly

to the south, and thus may be used to indicate roughly the points

of the compass. Can you explain their formation and the direction

of their slope? On the other hand, a small and thin stone, or a

patch of dust, lying on the ice, tends to sink a few inches into

it. Why?

In what respects is a valley glacier like a mountain stream which

flows out upon desert plains?

Two confluent glaciers do not mingle their currents as do two

confluent rivers. What characteristics of surface moraines prove

this fact?

What effect would you expect the laws of glacier motion to have on

the slant of the sides of transverse crevasses?

A trunk glacier has four medial moraines. Of how many tributaries

is it composed? Illustrate by diagram.

State all the evidences which you have found that glaciers move.

If a glacier melts back with occasional pauses up a valley, what

records are left of its retreat?

PIEDMONT GLACIERS

THE MALASPINA GLACIER. Piedmont (foot of the mountain) glaciers

are, as the name implies, ice fields formed at the foot of

mountains by the confluence of valley glaciers. The Malaspina

glacier of Alaska, the typical glacier of this kind, is seventy

miles wide and stretches for thirty miles from the foot of the

Mount Saint Elias range to the shore of the Pacific Ocean. The

valley glaciers which unite and spread to form this lake of ice

lie above the snow line and their moraines are concealed beneath

neve. The central area of the Malaspina is also free from debris;

but on the outer edge large quantities of englacial drift are

exposed by surface melting and form a belt of morainic waste a few

feet thick and several miles wide, covered in part with a

luxuriant forest, beneath which the ice is in places one thousand

feet in depth. The glacier here is practically stagnant, and lakes

a few hundred yards across, which could not exist were the ice in

motion and broken with crevasses, gather on their beds sorted

waste from the moraine. The streams which drain the glacier have

cut their courses in englacial and subglacial tunnels; none flow

for any distance on the surface. The largest, the Yahtse River,

issues from a high archway in the ice,--a muddy torrent one

hundred feet wide and twenty feet deep, loaded with sand and

stones which it deposits in a broad outwash plain (Fig. 110).

Where the ice has retreated from the sea there is left a hummocky

drift sheet with hollows filled with lakelets. These deposits help

to explain similar hummocky regions of drift and similar plains of

coarse, water-laid material often found in the drift-covered area

of the northeastern United States.

THE GEOLOGICAL WORK OF GLACIER ICE

The sluggish glacier must do its work in a different way from the

agile river. The mountain stream is swift and small, and its

channel occupies but a small portion of the valley. The glacier is

slow and big; its rate of motion may be less than a millionth of

that of running water over the same declivity, and its bulk is

proportionately large and fills the valley to great depth.

Moreover, glacier ice is a solid body plastic under slowly applied

stresses, while the water of rivers is a nimble fluid.

TRANSPORTATION. Valley glaciers differ from rivers as carriers in

that they float the major part of their load upon their surface,

transporting the heaviest bowlder as easily as a grain of sand;

while streams push and roll much of their load along their beds,

and their power of transporting waste depends solely upon their

velocity. The amount of the surface load of glaciers is limited

only by the amount of waste received from the mountain slopes

above them. The moving floor of ice stretched high across a valley

sweeps along as lateral moraines much of the waste which a

mountain stream would let accumulate in talus and alluvial cones.

While a valley glacier carries much of its load on top, an ice

sheet, such as that of Greenland, is free from surface debris,

except where moraines trail away from some nunatak. If at its edge

it breaks into separate glaciers which drain down mountain

valleys, these tongues of ice will carry the selvages of waste

common to valley glaciers. Both ice sheets and valley glaciers

drag on large quantities of rock waste in their ground moraines.

Stones transported by glaciers are sometimes called erratics. Such

are the bowlders of the drift of our northern states. Erratics may

be set down in an insecure position on the melting of the ice.

DEPOSIT. Little need be added here to what has already been said

of ground and terminal moraines. All strictly glacial deposits are

unstratified. The load laid down at the end of a glacier in the

terminal moraine is loose in texture, while the drift lodged

beneath the glacier as ground moraine is often an extremely dense,

stony clay, having been compacted under the pressure of the

overriding ice.

EROSION. A glacier erodes its bed and banks in two ways,--by

abrasion and by plucking.

The rock bed over which a glacier has moved is seen in places to

have been abraded, or ground away, to smooth surfaces which are

marked by long, straight, parallel scorings aligned with the line

of movement of the ice and varying in size from hair lines and

coarse scratches to exceptional furrows several feet deep. Clearly

this work has been accomplished by means of the sharp sand, the

pebbles, and the larger stones with which the base of the glacier

is inset, and which it holds in a firm grasp as running water

cannot. Hard and fine-grained rocks, such as granite and

quartzite, are often not only ground down to a smooth surface but

are also highly polished by means of fine rock flour worn from the

glacier bed.

In other places the bed of the glacier is rough and torn. The

rocks have been disrupted and their fragments have been carried

away,--a process known as PLUCKING. Moving under immense pressure

the ice shatters the rock, breaks off projections, presses into

crevices and wedges the rocks apart, dislodges the blocks into

which the rock is divided by joints and bedding planes, and

freezing fast to the fragments drags them on. In this work the

freezing and thawing of subglacial waters in any cracks and

crevices of the rock no doubt play an important part. Plucking

occurs especially where the bed rock is weak because of close

jointing. The product of plucking is bowlders, while the product

of abrasion is fine rock flour and sand.

Is the ground moraine of Figure 87 due chiefly to abrasion or to

plucking?

ROCHES MOUTONNEES AND ROUNDED HILLS. The prominences left between

the hollows due to plucking are commonly ground down and rounded

on the stoss side,--the side from which the ice advances,--and

sometimes on the opposite, the lee side, as well. In this way the

bed rock often comes to have a billowy surface known as roches

moutonnees (sheep rocks). Hills overridden by an ice sheet often

have similarly rounded contours on the stoss side, while on the

lee side they may be craggy, either because of plucking or because

here they have been less worn from their initial profile.

THE DIRECTION OF GLACIER MOVEMENT. The direction of the flow of

vanished glaciers and ice sheets is recorded both in the

differences just mentioned in the profiles of overridden hills and

also in the minute details of the glacier trail.

Flint nodules or other small prominences in the bed rock are found

more worn on the stoss than on the lee side, where indeed they may

have a low cone of rock protected by them from abrasion. Cavities,

on the other hand, have their edges worn on the lee side and left

sharp upon the stoss.

Surfaces worn and torn in the ways which we have mentioned are

said to be glaciated. But it must not be supposed that a glacier

everywhere glaciates its bed. Although in places it acts as a rasp

or as a pick, in others, and especially where its pressure is

least, as near the terminus, it moves over its bed in the manner

of a sled. Instances are known where glaciers have advanced over

deposits of sand and gravel without disturbing them to any notable

degree. Like a river, a glacier does not everywhere erode. In

places it leaves its bed undisturbed and in places aggrades it by

deposits of the ground moraine.

CIRQUES. Valley glaciers commonly head as we have seen, in broad

amphitheaters deeply filled with snow and ice. On mountains now

destitute of glaciers, but whose glaciation shows that they have

supported glaciers in the past, there are found similar crescentic

hollows with high, precipitous walls and glaciated floors. Their

floors are often basined and hold lakelets whose deep and quiet

waters reflect the sheltering ramparts of rugged rock which tower

far above them. Such mountain hollows are termed CIRQUES. As a

powerful spring wears back a recess in the valley side where it

discharges, so the fountain head of a glacier gradually wears back

a cirque. In its slow movement the neve field broadly scours its

bed to a flat or basined floor. Meanwhile the sides of the valley

head are steepened and driven back to precipitous walls. For in

winter the crevasse of the bergschrund which surrounds the neve

field is filled with snow and the neve is frozen fast to the rocky

sides of the valley. In early summer the neve tears itself free,

dislodging and removing any loosened blocks, and the open fissure

of the bergschrund allows frost and other agencies of weathering

to attack the unprotected rock. As cirques are thus formed and

enlarged the peaks beneath which they lie are sharpened, and the

mountain crests are scalloped and cut back from either side to

knife-edged ridges.

In the western mountains of the United States many cirques, now

empty of neve and glacier ice, and known locally as "basins,"

testify to the fact that in recent times the snow line stood

beneath the levels of their floors, and thus far below its present

altitude.

GLACIER TROUGHS. The channel worn to accommodate the big and

clumsy glacier differs markedly from the river valley cut as with

a saw by the narrow and flexible stream and widened by the weather

and the wash of rains. The valley glacier may easily be from one

thousand to three thousand feet deep and from one to three miles

wide. Such a ponderous bulk of slowly moving ice does not readily

adapt itself to sharp turns and a narrow bed. By scouring and

plucking all resisting edges it develops a fitting channel with a

wide, flat floor, and steep, smooth sides, above which are seen

the weathered slopes of stream-worn mountain valleys. Since the

trunk glacier requires a deeper channel than do its branches, the

bed of a branch glacier enters the main trough at some distance

above the floor of the latter, although the surface of the two ice

streams may be accordant. Glacier troughs can be studied best

where large glaciers have recently melted completely away, as is

the case in many valleys of the mountains of the western United

States and of central and northern Europe (Fig. 114). The typical

glacier trough, as shown in such examples, is U-shaped, with a

broad, flat floor, and high, steep walls. Its walls are little

broken by projecting spurs and lateral ravines. It is as if a V-

valley cut by a river had afterwards been gouged deeper with a

gigantic chisel, widening the floor to the width of the chisel

blade, cutting back the spurs, and smoothing and steepening the

sides. A river valley could only be as wide-floored as this after

it had long been worn down to grade.

The floor of a glacier trough may not be graded; it is often

interrupted by irregular steps perhaps hundreds and even a

thousand feet in height, over which the stream that now drains the

valley tumbles in waterfalls. Reaches between the steps are often

basined. Lakelets may occupy hollows excavated in solid rock, and

other lakes may be held behind terminal moraines left as dams

across the valley at pauses in the retreat of the glacier.

FJORDS are glacier troughs now occupied in part or wholly by the

sea, either because they were excavated by a tide glacier to their

present depth below sea level, or because of a submergence of the

land. Their characteristic form is that of a long, deep, narrow

bay with steep rock walls and basined floor. Fjords are found only

in regions which have suffered glaciation, such as Norway and

Alaska.

HANGING VALLEYS. These are lateral valleys which open on their

main valley some distance above its floor. They are conspicuous

features of glacier troughs from which the ice has vanished; for

the trunk glacier in widening and deepening its channel cut its

bed below the bottoms of the lateral valleys.

Since the mouths of hanging valleys are suspended on the walls of

the glacier trough, their streams are compelled to plunge down its

steep, high sides in waterfalls. Some of the loftiest and most

beautiful waterfalls of the world leap from hanging valleys,--

among them the celebrated Staubbach of the Lauterbrunnen valley of

Switzerland, and those of the fjords of Norway and Alaska.

Hanging valleys are found also in river gorges where the smaller

tributaries have not been able to keep pace with a strong master

stream in cutting down their beds. In this case, however, they are

a mark of extreme youth; for, as the trunk stream approaches grade

and its velocity and power to erode its bed decrease, the side

streams soon cut back their falls and wear their beds at their

mouths to a common level with that of the main river. The Grand

Canyon of the Colorado must be reckoned a young valley. At its

base it narrows to scarcely more than the width of the river, and

yet its tributaries, except the very smallest, enter it at a

common level.

Why could not a wide-floored valley, such as a glacier trough,

with hanging valleys opening upon it, be produced in the normal

development of a river valley?

THE TROUGHS OF YOUNG AND OF MATURE GLACIERS. The features of a

glacier trough depend much on the length of time the preexisting

valley was occupied with ice. During the infancy of a glacier, we

may believe, the spurs of the valley which it fills are but little

blunted and its bed is but little broken by steps. In youth the

glacier develops icefalls, as a river in youth develops

waterfalls, and its bed becomes terraced with great stairs. The

mature glacier, like the mature river, has effaced its falls and

smoothed its bed to grade. It has also worn back the projecting

spurs of its valley, making itself a wide channel with smooth

sides. The bed of a mature glacier may form a long basin, since it

abrades most in its upper and middle course, where its weight and

motion are the greatest. Near the terminus, where weight and

motion are the least, it erodes least, and may instead deposit a

sheet of ground moraine, much as a river builds a flood plain in

the same part of its course as it approaches maturity. The bed of

a mature glacier thus tends to take the form of a long, relatively

narrow basin, across whose lower end may be stretched the dam of

the terminal moraine. On the disappearance of the ice the basin is

rilled with a long, narrow lake, such as Lake Chelan in Washington

and many of the lakes in the Highlands of Scotland.

Piedmont glaciers apparently erode but little. Beneath their lake-

like expanse of sluggish or stagnant ice a broad sheet of ground

moraine is probably being deposited.

Cirques and glaciated valleys rapidly lose their characteristic

forms after the ice has withdrawn. The weather destroys all

smoothed, polished, and scored surfaces which are not protected

beneath glacial deposits. The oversteepened sides of the trough

are graded by landslips, by talus slopes, and by alluvial cones.

Morainic heaps of drift are dissected and carried away. Hanging

valleys and the irregular bed of the trough are both worn down to

grade by the streams which now occupy them. The length of time

since the retreat of the ice from a mountain valley may thus be

estimated by the degree to which the destruction of the

characteristic features of the glacier trough has been carried.

In Figure 104 what characteristics of a glacier trough do you

notice? What inference do you draw as to the former thickness of

the glacier?

Name all the evidences you would expect to find to prove the fact

that in the recent geological past the valleys of the Alps

contained far larger glaciers than at present, and that on the

north of the Alps the ice streams united in a piedmont glacier

which extended across the plains of Switzerland to the sides of

the Jura Mountains.

THE RELATIVE IMPORTANCE OF GLACIERS AND OF RIVERS. Powerful as

glaciers are, and marked as are the land forms which they produce,

it is easy to exaggerate their geological importance as compared

with rivers. Under present climatic conditions they are confined

to lofty mountains or polar lands. Polar ice sheets are permanent

only so long as the lands remain on which they rest. Mountain

glaciers can stay only the brief time during which the ranges

continue young and high. As lofty mountains, such as the Selkirks

and the Alps, are lowered by frost and glacier ice, the snowfall

will decrease, the line of permanent snow will rise, and as the

mountain hollows in which snow may gather are worn beneath the

snow line, the glaciers must disappear. Under present climatic

conditions the work of glaciers is therefore both local and of

short duration.

Even the glacial epoch, during which vast ice sheets deposited

drift over northeastern North America, must have been brief as

well as recent, for many lofty mountains, such as the Rockies and

the Alps, still bear the marks of great glaciers which then filled

their valleys. Had the glacial epoch been long, as the earth

counts time, these mountains would have been worn low by ice; had

the epoch been remote, the marks of glaciation would already have

been largely destroyed by other agencies.

On the other hand, rivers are well-nigh universally at work over

the land surfaces of the globe, and ever since the dry land

appeared they have been constantly engaged in leveling the

continents and in delivering to the seas the waste which there is

built into the stratified rocks.

ICEBERGS. Tide glaciers, such as those of Greenland and Alaska,

are able to excavate their beds to a considerable distance below

sea level. From their fronts the buoyancy of sea water raises and

breaks away great masses of ice which float out to sea as

icebergs. Only about one seventh of a mass of glacier ice floats

above the surface, and a berg three hundred feet high may be

estimated to have been detached from a glacier not less than two

thousand feet thick where it met the sea.

Icebergs transport on their long journeys whatever drift they may

have carried when part of the glacier, and scatter it, as they

melt, over the ocean floor. In this way pebbles torn by the inland

ice from the rocks of the interior of Greenland and glaciated

during their carriage in the ground moraine are dropped at last

among the oozes of the bottom of the North Atlantic.

CHAPTER VI

THE WORK OF THE WIND

We are now to study the geological work of the currents of the

atmosphere, and to learn how they erode, and transport and deposit

waste as they sweep over the land. Illustrations of the wind's

work are at hand in dry weather on any windy day.

Clouds of dust are raised from the street and driven along by the

gale. Here the roadway is swept bare; and there, in sheltered

places, the dust settles in little windrows. The erosive power of

waste-laden currents of air is suggested as the sharp grains of

flying sand sting one's face or clatter against the window. In the

country one sometimes sees the dust whirled in clouds from dry,

plowed fields in spring and left in the lee of fences in small

drifts resembling in form those of snow in winter.

THE ESSENTIAL CONDITIONS for the wind's conspicuous work are

illustrated in these simple examples; they are aridity and the

absence of vegetation. In humid climates these conditions are only

rarely and locally met; for the most part a thick growth of

vegetation protects the moist soil from the wind with a cover of

leaves and stems and a mattress of interlacing roots. But in arid

regions either vegetation is wholly lacking, or scant growths are

found huddled in detached clumps, leaving interspaces of

unprotected ground (Fig. 119). Here, too, the mantle of waste,

which is formed chiefly under the action of temperature changes,

remains dry and loose for long periods. Little or no moisture is

present to cause its particles to cohere, and they are therefore

readily lifted and drifted by the wind.

TRANSPORTATION BY THE WIND

In the desert the finer waste is continually swept to and fro by

the ever-shifting wind. Even in quiet weather the air heated by

contact with the hot sands rises in whirls, and the dust is lifted

in stately columns, sometimes as much as one thousand feet in

height, which march slowly across the plain. In storms the sand is

driven along the ground in a continuous sheet, while the air is

tilled with dust. Explorers tell of sand storms in the deserts of

central Asia and Africa, in which the air grows murky and

suffocating. Even at midday it may become dark as night, and

nothing can be heard except the roar of the blast and the whir of

myriads of grains of sand as they fly past the ear.

Sand storms are by no means uncommon in the arid regions of the

western United States. In a recent year, six were reported from

Yuma, Arizona. Trains on transcontinental railways are

occasionally blockaded by drifting sand, and the dust sifts into

closed passenger coaches, covering the seats and floors. After

such a storm thirteen car loads of sand were removed from the

platform of a station on a western railway.

DUST FALLS. Dust launched by upward-whirling winds on the swift

currents of the upper air is often blown for hundreds of miles

beyond the arid region from which it was taken. Dust falls from

western storms are not unknown even as far east as the Great

Lakes. In 1896 a "black snow" fell in Chicago, and in another dust

storm in the same decade the amount of dust carried in the air

over Rock Island, Ill., was estimated at more than one thousand

tons to the cubic mile.

In March, 1901, a cyclonic storm carried vast quantities of dust

from the Sahara northward across the Mediterranean to fall over

southern and central Europe. On March 8th dust storms raged in

southern Algeria; two days later the dust fell in Italy; and on

the 11th it had reached central Germany and Denmark. It is

estimated that in these few days one million eight hundred

thousand tons of waste were carried from northern Africa and

deposited on European soil.

We may see from these examples the importance of the wind as an

agent of transportation, and how vast in the aggregate are the

loads which it carries. There are striking differences between air

and water as carriers of waste. Rivers flow in fixed and narrow

channels to definite goals. The channelless streams of the air

sweep across broad areas, and, shifting about continually, carry

their loads back and forth, now in one direction and now in

another.

WIND DEPOSITS

The mantle of waste of deserts is rapidly sorted by the wind. The

coarser rubbish, too heavy to be lifted into the air, is left to

strew wide tracts with residual gravels (Fig. 120). The sand

derived from the disintegration of desert rocks gathers in vast

fields. About one eighth of the surface of the Sahara is said to

be thus covered with drifting sand. In desert mountains, as those

of Sinai, it lies like fields of snow in the high valleys below

the sharp peaks. On more level tracts it accumulates in seas of

sand, sometimes, as in the deserts of Arabia, two hundred and more

feet deep.

DUNES. The sand thus accumulated by the wind is heaped in wavelike

hills called dunes. In the desert of northwestern India, where the

prevalent wind is of great strength, the sand is laid in

longitudinal dunes, i.e. in stripes running parallel with the

direction of the wind; but commonly dunes lie, like ripple marks,

transverse to the wind current. On the windward side they show a

long, gentle slope, up which grains of sand can readily be moved;

while to the lee their slope is frequently as great as the angle

of repose (Fig. 122). Dunes whose sands are not fixed by

vegetation travel slowly with the wind; for their material is ever

shifted forward as the grains are driven up the windward slope

and, falling over the crest, are deposited in slanting layers in

the quiet of the lee.

Like river deposits, wind-blown sands are stratified, since they

are laid by currents of air varying in intensity, and therefore

in transporting power, which carry now finer and now coarser

materials and lay them down where their velocity is checked (Fig.

123). Since the wind varies in direction, the strata dip in

various directions. They also dip at various angles, according to

the inclination of the surface on which they were laid.

Dunes occur not only in arid regions, but also wherever loose sand

lies unprotected by vegetation from the wind. From the beaches of

sea and lake shores the wind drives inland the surface sand left

dry between tides and after storms, piling it in dunes which may

invade forests and fields and bury villages beneath their slowly

advancing waves. On flood plains during summer droughts river

deposits are often worked over by the wind; the sand is heaped in

hummocks and much of the fine silt is caught and held by the

forests and grassy fields of the bordering hills.

The sand of shore dunes differs little in composition and the

shape of its grains from that of the beach from which it was

derived. But in deserts, by the long wear of grain on grain as

they are blown hither and thither by the wind, all soft minerals

are ground to powder and the sand comes to consist almost wholly

of smooth round grams of hard quartz.

Some marine sandstones, such as the St. Peter sandstone of the

upper Mississippi valley, are composed so entirely of polished

spherules of quartz that it has been believed by some that their

grains were long blown about in ancient deserts before they were

deposited in the sea.

DUST DEPOSITS. As desert sands are composed almost wholly of

quartz, we may ask what has become of the softer minerals of which

the rocks whose disintegration has supplied the sand were in part,

and often in large part, composed. The softer minerals have been

ground to powder, and little by little the quartz sand also is

worn by attrition to fine dust. Yet dust deposits are scant and

few in great deserts such as the Sahara. The finer waste is blown

beyond its limits and laid in adjacent oceans, where it adds to

the muds and oozes of their floors, and on bordering steppes and

forest lands, where it is bound fast by vegetation and slowly

accumulates in deposits of unstratified loose yellow earth. The

fine waste of the Sahara has been identified in dredgings from the

bottom of the Atlantic Ocean, taken hundreds of miles from the

coast of Africa.

LOESS. In northern China an area as large as France is deeply

covered with a yellow pulverulent earth called loess (German,

loose), which many consider a dust deposit blown from the great

Mongolian desert lying to the west. Loess mantles the recently

uplifted mountains to the height of eight thousand feet and

descends on the plains nearly to sea level. Its texture and lack

of stratification give it a vertical cleavage; hence it stands in

steep cliffs on the sides of the deep and narrow trenches which

have been cut in it by streams.

On loess hillsides in China are thousands of villages whose

eavelike dwellings have been excavated in this soft, yet firm, dry

loam. While dust falls are common at the present time in this

region, the loess is now being rapidly denuded by streams, and its

yellow silt gives name to the muddy Hwang-ho (Yellow River), and

to the Yellow Sea, whose waters it discolors for scores of miles

from shore.

Wind deposits both of dust and of sand may be expected to contain

the remains of land shells, bits of wood, and bones of land

animals, testifying to the fact that they were accumulated in open

air and not in the sea or in bodies of fresh water.

WIND EROSION

Sand-laden currents of air abrade and smooth and polish exposed

rock surfaces, acting in much the same way as does the jet of

steam fed with sharp sand, which is used in the manufacture of

ground glass. Indeed, in a single storm at Cape Cod a plate glass

of a lighthouse was so ground by flying sand that its transparency

was destroyed and its removal made necessary.

Telegraph poles and wires whetted by wind-blown sands are

destroyed within a few years. In rocks of unequal resistance the

harder parts are left in relief, while the softer are etched away.

Thus in the pass of San Bernardino, Cal., through which strong

winds stream from the west, crystals of garnet are left projecting

on delicate rock fingers from the softer rock in which they were

imbedded.

Wind-carved pebbles are characteristically planed, the facets

meeting along a summit ridge or at a point like that of a pyramid.

We may suppose that these facets were ground by prevalent winds

from certain directions, or that from time to time the stone was

undermined and rolled over as the sand beneath it was blown away

on the windward side, thus exposing fresh surfaces to the driving

sand. Such wind-carved pebbles are sometimes found in ancient

rocks and may be accepted as evidence that the sands of which the

rocks are composed were blown about by the wind.

DEFLATION. In the denudation of an arid region, wind erosion is

comparatively ineffective as compared with deflation (Latin, de,

from; flare, to blow),--a term by which is meant the constant

removal of waste by the wind, leaving the rocks bare to the

continuous attack of the weather. In moist climates denudation is

continually impeded by the mantle of waste and its cover of

vegetation, and the land surface can be lowered no faster than the

waste is removed by running water. Deep residual soils come to

protect all regions of moderate slope, concealing from view the

rock structure, and the various forms of the land are due more to

the agencies of erosion and transportation than to differences in

the resistance of the underlying rocks.

But in arid regions the mantle is rapidly removed, even from well-

nigh level plains and plateaus, by the sweep of the wind and the

wash of occasional rains. The geological structure of these

regions of naked rock can be read as far as the eye can see, and

it is to this structure that the forms of the land are there

largely due. In a land mass of horizontal strata, for example, any

softer surface rocks wear down to some underlying, resistant

stratum, and this for a while forms the surface of a level plateau

(Fig. 129). The edges of the capping layer, together with those of

any softer layers beneath it, wear back in steep cliffs, dissected

by the valleys of wet-weather streams and often swept bare to the

base by the wind. As they are little protected by talus, which

commonly is removed about as fast as formed, these escarpments and

the walls of the valleys retreat indefinitely, exposing some hard

stratum beneath which forms the floor of a widening terrace.

The high plateaus of northern Arizona and southern Utah, north of

the Grand Canyon of the Colorado River, are composed of stratified

rocks more than ten thousand feet thick and of very gentle

inclination northward. From the broad plat form in which the

canyon has been cut rises a series of gigantic stairs, which are

often more than one thousand feet high and a score or more of

miles in breadth. The retreating escarpments, the cliffs of the

mesas and buttes which they have left behind as outliers, and the

walls of the ravines are carved into noble architectural forms--

into cathedrals, pyramids, amphitheaters, towers, arches, and

colonnades--by the processes of weathering aided by deflation. It

is thus by the help of the action of the wind that great plateaus

in arid regions are dissected and at last are smoothed away to

waterless plains, either composed of naked rock, or strewed with

residual gravels, or covered with drifting residual sand.

The specific gravity of air is 1/823 that of water. How does this

fact affect the weight of the material which each can carry at the

same velocity?

If the rainfall should lessen in your own state to from five to

ten inches a year, what changes would take place in the vegetation

of the country? in the soil? in the streams? in the erosion of

valleys? in the agencies chiefly at work in denuding the land?

In what way can a wind-carved pebble be distinguished from a

river-worn pebble? from a glaciated pebble?

CHAPTER VII

THE SEA AND ITS SHORES

We have already seen that the ocean is the goal at which the waste

of the land arrives. The mantle of rock waste, creeping down

slopes, is washed to the sea by streams, together with the

material which the streams have worn from their beds and that

dissolved by underground waters. In arid regions the winds sweep

waste either into bordering oceans or into more humid regions

where rivers take it up and carry it on to the sea. Glaciers

deliver the load of their moraines either directly to the sea or

leave it for streams to transport to the same goal. All deposits

made on the land, such as the flood plains of rivers, the silts of

lake beds, dune sands, and sheets of glacial drift, mark but

pauses in the process which is to bring all the materials of the

land now above sea level to rest upon the ocean bed.

But the sea is also at work along all its shores as an agent of

destruction, and we must first take up its work in erosion before

we consider how it transports and deposits the waste of the land.

SEA EROSION

THE SEA CLIFF AND THE ROCK BENCH. On many coasts the land fronts

the ocean in a line of cliffs. To the edge of the cliffs there

lead down valleys and ridges, carved by running water, which, if

extended, would meet the water surface some way out from shore.

Evidently they are now abruptly cut short at the present shore

line because the land has been cut back.

Along the foot of the cliff lies a gently shelving bench of rock,

more or less thickly veneered with sand and shingle. At low tide

its inner margin is laid bare, but at high tide it is covered

wholly, and the sea washes the base of the cliffs. A notch, of

which the SEA CLIFF and the ROCK BENCH are the two sides, has been

cut along the shore.

WAVES. The position of the rock bench, with its inner margin

slightly above low tide, shows that it has been cut by some agent

which acts like a horizontal saw set at about sea level. This

agent is clearly the surface agitation of the water; it is the

wind-raised wave.

As a wave comes up the shelving bench the crest topples forward

and the wave "breaks," striking a blow whose force is measured by

the momentum of all its tons of falling water. On the coast of

Scotland the force of the blows struck by the waves of the

heaviest storms has sometimes exceeded three tons to the square

foot. But even a calm sea constantly chafes the shore. It heaves

in gentle undulations known as the ground swell, the result of

storms perhaps a thousand miles distant, and breaks on the shore

in surf.

The blows of the waves are not struck with clear water only, else

they would have little effect on cliffs of solid rock. Storm waves

arm themselves with the sand and gravel, the cobbles, and even the

large bowlders which lie at the base of the cliff, and beat

against it with these hammers of stone.

Where a precipice descends sheer into deep water, waves swash up

and down the face of the rocks but cannot break and strike

effective blows. They therefore erode but little until the talus

fallen from the cliff is gradually built up beneath the sea to the

level at which the waves drag bottom upon it and break.

Compare the ways in which different agents abrade. The wind

lightly brushes sand and dust over exposed surfaces of rock.

Running water sweeps fragments of various sizes along its

channels, holding them with a loose hand. Glacial ice grinds the

stones of its ground moraine against the underlying rock with the

pressure of its enormous weight. The wave hurls fragments of rock

against the sea cliff, bruising and battering it by the blow. It

also rasps the bench as it drags sand and gravel to and fro upon

it.

WEATHERING OF SEA CLIFFS. The sea cliff furnishes the weapons for

its own destruction. They are broken from it not only by the wave

but also by the weather. Indeed the sea cliff weathers more

rapidly, as a rule, than do rock ledges inland. It is abundantly

wet with spray. Along its base the ground water of the neighboring

land finds its natural outlet in springs which under mine it.

Moreover, it is unprotected by any shield of talus. Fragments of

rock as they fall from its face are battered to pieces by the

waves and swept out to sea. The cliff is thus left exposed to the

attack of the weather, and its retreat would be comparatively

rapid for this reason alone.

Sea cliffs seldom overhang, but commonly, as in Figure 134, slope

seaward, showing that the upper portion has retreated at a more

rapid rate than has the base. Which do you infer is on the whole

the more destructive agent, weathering or the wave?

Draw a section of a sea cliff cut in well jointed rocks whose

joints dip toward the land. Draw a diagram of a sea cliff where

the joints dip toward the sea.

SEA CAVES. The wave does not merely batter the face of the cliff.

Like a skillful quarryman it inserts wedges in all natural

fissures, such as joints, and uses explosive forces. As a wave

flaps against a crevice it compresses the air within with the

sudden stroke; as it falls back the air as suddenly expands. On

lighthouses heavily barred doors have been burst outward by the

explosive force of the air within, as it was released from

pressure when a partial vacuum was formed by the refluence of the

wave. Where a crevice is filled with water the entire force of the

blow of the wave is transmitted by hydraulic pressure to the sides

of the fissure. Thus storm waves little by little pry and suck the

rock loose, and in this way, and by the blows which they strike

with the stones of the beach, they quarry out about a joint, or

wherever the rock may be weak, a recess known as a SEA CAVE,

provided that the rock above is coherent enough to form a roof.

Otherwise an open chasm results.

BLOWHOLES AND SEA ARCHES. As a sea cave is drilled back into the

rock, it may encounter a joint or crevice opened to the surface by

percolating water. The shock of the waves soon enlarges this to a

blowhole, which one may find on the breezy upland, perhaps a

hundred yards and more back from the cliff's edge. In quiet

weather the blowhole is a deep well; in storm it plays a fountain

as the waves drive through the long tunnel below and spout their

spray high in air in successive jets. As the roof of the cave thus

breaks down in the rear, there may remain in front for a while a

sea arch, similar to the natural bridges of land caverns.

STACKS AND WAVE-CUT ISLANDS. As the sea drives its tunnels and

open drifts into the cliff, it breaks through behind the

intervening portions and leaves them isolated as stacks, much as

monuments are detached from inland escarpments by the weather; and

as the sea cliff retreats, these remnant masses may be left behind

as rocky islets. Thus the rock bench is often set with stacks,

islets in all stages of destruction, and sunken reefs, all wrecks

of the land testifying to its retreat before the incessant attack

of the waves.

COVES. Where zones of soft or closely jointed rock outcrop along a

shore, or where minor water courses conic down to the sea and aid

in erosion, the shore is worn back in curved reentrants called

coves; while the more resistant rocks on either hand are left

projecting as headlands (Fig. 139). After coves are cut back a

short distance by the waves, the headlands come to protect them,

as with breakwaters, and prevent their indefinite retreat. The

shore takes a curve of equilibrium, along which the hard rock of

the exposed headland and the weak rock of the protected cove wear

back at an equal rate.

RATE OF RECESSION. The rate at which a shore recedes depends on

several factors. In soft or incoherent rocks exposed to violent

storms the retreat is so rapid as to be easily measured. The coast

of Yorkshire, England, whose cliffs are cut in glacial drift,

loses seven feet a year on the average, and since the Norman

conquest a strip a mile wide, with farmsteads and villages and

historic seaports, has been devoured by the sea. The sandy south

shore of Martha's Vineyard wears back three feet a year. But hard

rocks retreat so slowly that their recession has seldom been

measured by the records of history.

SHORE DRIFT

BOWLDER AND PEBBLE BEACHES. About as fast as formed the waste of

the sea cliff is swept both along the shore and out to sea. The

road of waste along shore is the BEACH. We may also define the

beach as the exposed edge of the sheet of sediment formed by the

carriage of land waste out to sea. At the foot of sea cliffs,

where the waves are pounding hardest, one commonly finds the rock

bench strewn on its inner margin with large stones, dislodged by

the waves and by the weather and some-what worn on their corners

and edges. From this BOWLDER BEACH the smaller fragments of waste

from the cliff and the fragments into which the bowlders are at

last broken drift on to more sheltered places and there accumulate

in a PEBBLE BEACH, made of pebbles well rounded by the wear which

they have suffered. Such beaches form a mill whose raw material is

constantly supplied by the cliff. The breakers of storms set it in

motion to a depth of several feet, grinding the pebbles together

with a clatter to be heard above the roar of the surf. In such a

rock crusher the life of a pebble is short. Where ships have

stranded on our Atlantic coast with cargoes of hard-burned brick

or of coal, a year of time and a drift of five miles along the

shore have proved enough to wear brick and coal to powder. At no

great distance from their source, therefore, pebble beaches give

place to beaches of sand, which occupy the more sheltered reaches

of the shore.

SAND BEACHES. The angular sand grains of various minerals into

which pebbles are broken by the waves are ground together under

the beating surf and rounded, and those of the softer minerals are

crushed to powder. The process, however, is a slow one, and if we

study these sand grains under a lens we may be surprised to see

that, though their corners and edges have been blunted, they are

yet far from the spherical form of the pebbles from which they

were derived. The grains are small, and in water they have lost

about half their weiglit in air; the blows which they strike one

another are therefore weak. Besides, each grain of sand of the wet

beach is protected by a cushion of water from the blows of its

neighbors.

The shape and size of these grains and the relative proportion of

grains of the softer minerals which still remain give a rough

measure of the distance in space and time which they have traveled

from their source. The sand of many beaches, derived from the

rocks of adjacent cliffs or brought in by torrential streams from

neighboring highlands, is dark with grains of a number of minerals

softer than quartz. The white sand of other beaches, as those of

the east coast of Florida, is almost wholly composed of quartz

grains; for in its long travel down the Atlantic coast the weaker

minerals have been worn to powder and the hardest alone survive.

How does the absence of cleavage in quartz affect the durability

of quartz sand?

HOW SHORE DRIFT MIGRATES. It is under the action of waves and

currents that shore drift migrates slowly along a coast. Where

waves strike a coast obliquely they drive the waste before them

little by little along the shore. Thus on a north-south coast,

where the predominant storms are from the northeast, there will be

a migration of shore drift southwards.

All shores are swept also by currents produced by winds and tides.

These are usually far too gentle to transport of themselves the

coarse materials of which beaches are made. But while the wave

stirs the grains of sand and gravel, and for a moment lifts them

from the bottom, the current carries them a step forward on their

way. The current cannot lift and the wave cannot carry, but

together the two transport the waste along the shore. The road of

shore drift is therefore the zone of the breaking waves.

THE BAY-HEAD BEACH. As the waste derived from the wear of waves

and that brought in by streams is trailed along a coast it

assumes, under varying conditions, a number of distinct forms.

When swept into the head of a sheltered bay it constitutes the

bay-head beach. By the highest storm waves the beach is often

built higher than the ground immediately behind it, and forms a

dam inclosing a shallow pond or marsh.

THE BAY BAR. As the stream of shore drift reaches the mouth of a

bay of some size it often occurs that, instead of turning in, it

sets directly across toward the opposite headland. The waste is

carried out from shore into the deeper waters of the bay mouth;

where it is no longer supported by the breaking waves, and sinks

to the bottom. The dump is gradually built to the surface as a

stubby spur, pointing across the bay, and as it reaches the zone

of wave action current and wave can now combine to carry shore

drift along it, depositing their load continually at the point of

the spur. An embankment is thus constructed in much the same

manner as a railway fill, which, while it is building, serves as a

roadway along which the dirt from an adjacent cut is carted to be

dumped at the end. When the embankment is completed it bridges the

bay with a highway along which shore drift now moves without

interruption, and becomes a bay bar.

INCOMPLETE BAY BARS. Under certain conditions the sea cannot carry

out its intention to bridge a bay. Rivers discharging in bays

demand open way to the ocean. Strong tidal currents also are able

to keep open channels scoured by their ebb and flow. In such cases

the most that land waste can do is to build spits and shoals,

narrowing and shoaling the channel as much as possible. Incomplete

bay bars sometimes have their points recurved by currents setting

at right angles to the stream of shore drift and are then

classified as HOOKS (Fig. 142).

SAND REEFS. On low coasts where shallow water extends some

distance out, the highway of shore drift lies along a low, narrow

ridge, termed the sand reef, separated from the land by a narrow

stretch of shallow water called the LAGOON. At intervals the reef

is held open by INLETS,--gaps through which the tide flows and

ebbs, and by which the water of streams finds way to the sea.

No finer example of this kind of shore line is to be found in the

world than the coast of Texas. From near the mouth of the Rio

Grande a continuous sand reef draws its even curve for a hundred

miles to Corpus Christi Pass, and the reefs are but seldom

interrupted by inlets as far north as Galveston Harbor. On this

coast the tides are variable and exceptionally weak, being less

than one foot in height, while the amount of waste swept along the

shore is large. The lagoon is extremely shallow, and much of it is

a mud flat too shoal for even small boats. On the coast of New

Jersey strong tides are able to keep open inlets at intervals of

from two to twenty miles in spite of a heavy alongshore drift.

Sand reefs are formed where the water is so shallow near shore

that storm waves cannot run in it and therefore break some

distance out from land. Where storm waves first drag bottom they

erode and deepen the sea floor, and sweep in sediment as far as

the line where they break. Here, where they lose their force, they

drop their load and beat up the ridge which is known as the sand

reef when it reaches the surface.

SHORES OF ELEVATION AND DEPRESSION

Our studies have already brought to our notice two distinct forms

of strand lines,--one the high, rocky coast cut back to cliffs by

the attack of the waves, and the other the low, sandy coast where

the waves break usually upon the sand reef. To understand the

origin of these two types we must know that the meeting place of

sea and land is determined primarily by movements of the earth's

crust. Where a coast land emerges the--shore line moves seaward;

where it is being submerged the shore line advances on the land.

SHORES OF ELEVATION. The retreat of the sea, either because of a

local uplift of the land or for any other reason, such as the

lowering of any portion of ocean bottom, lays bare the inner

margin of the sea floor. Where the sea floor has long received the

waste of the land it has been built up to a smooth, subaqueous

plain, gently shelving from the land. Since the new shore line is

drawn across this even surface it is simple and regular, and is

bordered on the one side by shallow water gradually deepening

seaward, and on the other by low land composed of material which

has not yet thoroughly consolidated to firm rock. A sand reef is

soon beaten up by the waves, and for some time conditions will

favor its growth. The loss of sand driven into the lagoon beyond,

and of that ground to powder by the surf and carried out to sea,

is more than made up by the stream of alongshore drift, and

especially by the drag of sediments to the reef by the waves as

they deepen the sea floor on its seaward side.

Meanwhile the lagoon gradually fills with waste from the reef and

from the land. It is invaded by various grasses and reeds which

have learned to grow in salt and brackish water; the marsh, laid

bare only at low tide, is built above high tide by wind drift and

vegetable deposits, and becomes a meadow, soldering the sand reef

to the mainland.

While the lagoon has been filling, the waves have been so

deepening the sea floor off the sand reef that at last they are

able to attack it vigorously. They now wear it back, and, driving

the shore line across the lagoon or meadow, cut a line of low

cliffs on the mainland. Such a shore is that of Gascony in

southwestern France,--a low, straight, sandy shore, bordered by

dunes and unprotected by reefs from the attack of the waves of the

Bay of Biscay.

We may say, then, that on shores of elevation the presence of sand

reefs and lagoons indicates the stage of youth, while the absence

of these features and the vigorous and unimpeded attack by the sea

upon the mainland indicate the stage of maturity. Where much waste

is brought in by rivers the maturity of such a coast may be long

delayed. The waste from the land keeps the sea shallow offshore

and constantly renews the sand reef. The energy of the waves is

consumed in handling shore drift, and no energy is left for an

effective attack upon the land. Indeed, with an excessive amount

of waste brought down by streams the land may be built out and

encroach temporarily upon the sea; and not until long denudation

has lowered the land, and thus decreased the amount of waste from

it, may the waves be able to cut through the sand reef and thus

the coast reach maturity.

SHORES OF DEPRESSION

Where a coastal region is undergoing submergence the shore line

moves landward. The horizontal plane of the sea now intersects an

old land surface roughened by subaerial denudation. The shore line

is irregular and indented in proportion to the relief of the land

and the amount of the submergence which the land has suffered. It

follows up partially submerged valleys, forming bays, and bends

round the divides, leaving them to project as promontories and

peninsulas. The outlines of shores of depression are as varied as

are the forms of the land partially submerged. We give a few

typical illustrations.

The characteristics of the coast of Maine are due chiefly to the

fact that a mountainous region of hard rocks, once worn to a

peneplain, and after a subsequent elevation deeply dissected by

north-south valleys, has subsided, the depression amounting on its

southern margin to as much as six hundred feet below sea level.

Drowned valleys penetrate the land in long, narrow bays, and

rugged divides project in long, narrow land arms prolonged seaward

by islands representing the high portions of their extremities. Of

this exceedingly ragged shore there are said to be two thousand

miles from the New Brunswick boundary as far west as Portland,--a

straight-line distance of but two hundred miles. Since the time of

its greatest depression the land is known to have risen some three

hundred feet; for the bays have been shortened, and the waste with

which their floors were strewn is now in part laid bare as clay

plains about the bay heads and in narrow selvages about the

peninsulas and islands.

The coast of Dalmatia, on the Adriatic Sea, is characterized by

long land arms and chains of long and narrow islands, all parallel

to the trend of the coast. A region of parallel mountain ranges

has been depressed, and the longitudinal valleys which lie between

them are occupied by arms of the sea.

Chesapeake Bay is a branching bay due to the depression of an

ancient coastal plain which, after having emerged from the sea,

was channeled with broad, shallow valleys. The sea has invaded the

valley of the trunk stream and those of its tributaries, forming a

shallow bay whose many branches are all directed toward its axis

(Fig. 146).

Hudson Bay, and the North, the Baltic, and the Yellow seas are

examples where the sinking of the land has brought the sea in over

low plains of large extent, thus deeply indenting the continental

out-line. The rise of a few hundred feet would restore these

submerged plains to the land.

THE CYCLE OF SHORES OF DEPRESSION. In its infantile stage the

outline of a shore of depression depends almost wholly on the

previous relief of the land, and but little on erosion by the sea.

Sea cliffs and narrow benches appear where headlands and outlying

islands have been nipped by the waves. As yet, little shore waste

has been formed. The coast of Maine is an example of this stage.

In early youth all promontories have been strongly cliffed, and

under a vigorous attack of the sea the shore of open bays may be

cut back also. Sea stacks and rocky islets, caves and coves, make

the shore minutely ragged. The irregularity of the coast, due to

depression, is for a while increased by differential wave wear on

harder and softer rocks. The rock bench is still narrow. Shore

waste, though being produced in large amounts, is for the most

part swept into deeper water and buried out of sight. Examples of

this stage are the east coast of Scotland and the California coast

near San Francisco.

Later youth is characterized by a large accumulation of shore

waste. The rock bench has been cut back so that it now furnishes a

good roadway for shore drift. The stream of alongshore drift grows

larger and larger, filling the heads of the smaller bays with

beaches, building spits and hooks, and tying islands with sand

bars to the mainland. It bridges the larger bays with bay bars,

while their length is being reduced as their inclosing

promontories are cut back by the waves. Thus there comes to be a

straight, continuous, and easy road, no longer interrupted by

headlands and bays, for the transportation of waste alongshore.

The Baltic coast of Germany is in this stage.

All this while streams have been busy filling with delta deposits

the bays into which they empty. By these steps a coast gradually

advances to MATURITY, the stage when the irregularities due to

depression have been effaced, when outlying islands formed by

subsidence have been planed away, and when the shore line has been

driven back behind the former bay heads. The sea now attacks the

land most effectively along a continuous and fairly straight line

of cliffs. Although the first effect of wave wear was to increase

the irregularities of the shore, it sooner or later rectifies it,

making it simple and smooth. Northwestern France may be cited as

an upland plain, dissected and depressed, whose coast has reached

maturity.

In the OLD AGE of coasts the rock bench is cut back so far that

the waves can no longer exert their full effect upon the shore.

Their energy is dissipated in moving shore drift hither and

thither and in abrading the bench when they drag bottom upon it.

Little by little the bench is deepened by tidal currents and the

drag of waves; but this process is so slow that meanwhile the sea

cliffs melt down under the weather, and the bench becomes a broad

shoal where waves and tides gradually work over the waste from the

land to greater fineness and sweep it out to sea.

PLAINS OF MARINE ABRASION. While subaerial denudation reduces the

land to baselevel, the sea is sawing its edges to WAVE BASE, i.e.

the lowest limit of the wave's effective wear. The widened rock

bench forms when uplifted a plain of marine abrasion, which like

the peneplain bevels across strata regardless of their various

inclinations and various degrees of hardness.

How may a plain of marine abrasion be expected to differ from a

peneplain in its mantle of waste?

Compared with subaerial denudation, marine abrasion is a

comparatively feeble agent. At the rate of five feet per century--

a higher rate than obtains on the youthful rocky, coast of

Britain--it would require more than ten million years to pare a

strip one hundred miles wide from the margin of a continent, a

time sufficient, at the rate at which the Mississippi valley is

now being worn away, for subaerial denudation to lower the lands

of the globe to the level of the sea.

Slow submergence favors the cutting of a wide rock bench. The

water continually deepens upon the bench; storm waves can

therefore always ride in to the base of the cliffs and attack them

with full force; shore waste cannot impede the onset of the waves,

for it is continually washed out in deeper water below wave base.

BASAL CONGOLMERATES. As the sea marches across the land during a

slow submergence, the platform is covered with sheets of sea-laid

sediments. Lowest of these is a conglomerate,--the bowlder and

pebble beach, widened indefinitely by the retreat of the cliffs at

whose base it was formed, and preserved by the finer deposits laid

upon it in the constantly deepening water as the land subsides.

Such basal conglomerates are not uncommon among the ancient rocks

of the land, and we may know them by their rounded pebbles and

larger stones, composed of the same kind of rock as that of the

abraded and evened surface on which they lie.

CHAPTER VIII

OFFSHORE AND DEEP-SEA DEPOSITS

The alongshore deposits which we have now studied are the exposed

edge of a vast subaqueous sheet of waste which borders the

continents and extends often for as much as two or three hundred

miles from land. Soundings show that offshore deposits are laid in

belts parallel to the coast, the coarsest materials lying nearest

to the land and the finest farthest out. The pebbles and gravel

and the clean, coarse sand of beaches give place to broad

stretches of sand, which grows finer and finer until it is

succeeded by sheets of mud. Clearly there is an offshore movement

of waste by which it is sorted, the coarser being sooner dropped

and the finer being carried farther out.

OFFSHORE DEPOSITS

The debris torn by waves from rocky shores is far less in amount

than the waste of the land brought down to the sea by rivers,

being only one thirty-third as great, according to a conservative

estimate. Both mingle alongshore in all the forms of beach and bar

that have been described, and both are together slowly carried out

to sea. On the shelving ocean floor waste is agitated by various

movements of the unquiet water,--by the undertow (an outward-

running bottom current near the shore), by the ebb and flow of

tides, by ocean currents where they approach the land, and by

waves and ground swells, whose effects are sometimes felt to a

depth of six hundred feet. By all these means the waste is slowly

washed to and fro, and as it is thus ground finer and finer and

its soluble parts are more and more dissolved, it drifts farther

and farther out from land. It is by no steady and rapid movement

that waste is swept from the shore to its final resting place. Day

after day and century after century the grains of sand and

particles of mud are shifted to and fro, winnowed and spread in

layers, which are destroyed and rebuilt again and again before

they are buried safe from further disturbance.

These processes which are hidden from the eye are among the most

important of those with which our science has to do; for it is

they which have given shape to by far the largest part of the

stratified rocks of which the land is made.

THE CONTINENTAL DELTA. This fitting term has been recently

suggested for the sheet of waste slowly accumulating along the

borders of the continents. Within a narrow belt, which rarely

exceeds two or three hundred miles, except near the mouths of

muddy rivers such as the Amazon and Congo, nearly all the waste of

the continent, whether worn from its surface by the weather, by

streams, by glaciers, or by the wind, or from its edge by the

chafing of the waves, comes at last to its final resting place.

The agencies which spread the material of the continental delta

grow more and more feeble as they pass into deeper and more quiet

water away from shore. Coarse materials are therefore soon dropped

along narrow belts near land. Gravels and coarse sands lie in

thick, wedge-shaped masses which thin out seaward rapidly and give

place to sheets of finer sand.

SEA MUDS. Outermost of the sediments derived from the waste of the

continents is a wide belt of mud; for fine clays settle so slowly,

even in sea water,--whose saltness causes them to sink much faster

than they would in fresh water,--that they are wafted far before

they reach a bottom where they may remain undisturbed. Muds are

also found near shore, carpeting the floors of estuaries, and

among stretches of sandy deposits in hollows where the more quiet

water has permitted the finer silt to rest.

Sea muds are commonly bluish and consolidate to bluish shales; the

red coloring matter brought from land waste--iron oxide--is

altered to other iron compounds by decomposing organic matter in

the presence of sea water. Yellow and red muds occur where the

amount of iron oxide in the silt brought down to the sea by rivers

is too great to be reduced, or decomposed, by the organic matter

present.

Green muds and green sand owe their color to certain chemical

changes which take place where waste from the land accumulates on

the sea floor with extreme slowness. A greenish mineral called

GLAUCONITE--a silicate of iron and alumina--is then formed. Such

deposits, known as GREEN SAND, are now in process of making in

several patches off the Atlantic coast, and are found on the

coastal plain of New Jersey among the offshore deposits of earlier

geological ages.

ORGANIC DEPOSITS. Living creatures swarm along the shore and on

the shallows out from land as nowhere else in the ocean. Seaweed

often mantles the rock of the sea cliff between the levels of high

and low tide, protecting it to some degree from the blows of

waves. On the rock bench each little pool left by the ebbing tide

is an aquarium abounding in the lowly forms of marine life. Below

low-tide level occur beds of molluscous shells, such as the

oyster, with countless numbers of other humble organisms. Their

harder parts--the shells of mollusks, the white framework of

corals, the carapaces of crabs and other crustaceans, the shells

of sea urchins, the bones and teeth of fishes--are gradually

buried within the accumulating sheets of sediment, either whole

or, far more often, broken into fragments by the waves.

By means of these organic remains each layer of beach deposits and

those of the continental delta may contain a record of the life of

the time when it was laid. Such a record has been made ever since

living creatures with hard parts appeared upon the globe. We shall

find it sealed away in the stratified rocks of the continents,--

parts of ancient sea deposits now raised to form the dry land.

Thus we have in the traces of living creatures found in the rocks,

i.e. in fossils, a history of the progress of life upon the

planet.

MOLLUSCOUS SHELL DEPOSITS. The forms of marine life of importance

in rock making thrive best in clear water, where little sediment

is being laid, and where at the same time the depth is not so

great as to deprive them of needed light, heat, and of sufficient

oxygen absorbed by sea water from the air. In such clear and

comparatively shallow water there often grow countless myriads of

animals, such as mollusks and corals, whose shells and skeletons

of carbonate of lime gradually accumulate in beds of limestone.

A shell limestone made of broken fragments cemented together is

sometimes called COQUINA, a local term applied to such beds

recently uplifted from the sea along the coast of Florida (Fig.

149).

OOLITIC limestone (oon, an egg; lithos, a stone) is so named from

the likeness of the tiny spherules which compose it to the roe of

fish. Corals and shells have been pounded by the waves to

calcareous sand, and each grain has been covered with successive

concentric coatings of lime carbonate deposited about it from

solution.

The impalpable powder to which calcareous sand is ground by the

waves settles at some distance from shore in deeper and quieter

water as a limy silt, and hardens into a dense, fine-grained

limestone in which perhaps no trace of fossil is found to suggest

the fact that it is of organic origin.

From Florida Keys there extends south to the trough of Florida

Straits a limestone bank covered by from five hundred and forty to

eighteen hundred feet of water. The rocky bottom consists of

limestone now slowly building from the accumulation of the remains

of mollusks, small corals, sea urchins, worms with calcareous

tubes, and lime-secreting seaweed, which live upon its surface.

Where sponges and other silica-secreting organisms abound on

limestone banks, silica forms part of the accumulated deposit,

either in its original condition, as, for example, the spicules of

sponges, or gathered into concretions and layers of flint.

Where considerable mud is being deposited along with carbonate of

lime there is in process of making a clayey limestone or a limy

shale; where considerable sand, a sandy limestone or a limy

sandstone.

CONSOLIDATION OF OFFSHORE DEPOSITS. We cannot doubt that all these

loose sediments of the sea floor are being slowly consolidated to

solid rock. They are soaked with water which carries in solution

lime carbonate and other cementing substances. These cements are

deposited between the fragments of shells and corals, the grains

of sand and the particles of mud, binding them together into firm

rock. Where sediments have accumulated to great thickness the

lower portions tend also to consolidate under the weight of the

overlying beds. Except in the case of limestones, recent sea

deposits uplifted to form land are seldom so well cemented as are

the older strata, which have long been acted upon by underground

waters deep below the surface within the zone of cementation, and

have been exposed to view by great erosion.

RIPPLE MARKS, SUN CRACKS, ETC. The pulse of waves and tidal

currents agitates the loose material of offshore deposits,

throwing it into fine parallel ridges called ripple marks. One may

see this beautiful ribbing imprinted on beach sands uncovered by

the outgoing tide, and it is also produced where the water is of

considerable depth. While the tide is out the surface of shore

deposits may be marked by the footprints of birds and other

animals, or by the raindrops of a passing shower.

The mud of flats, thus exposed to the sun and dried, cracks in a

characteristic way. Such markings may be covered over with a thin

layer of sediment at the next flood tide and sealed away as a

lasting record of the manner and place in which the strata were

laid. In Figure 150 we have an illustration of a very ancient

ripple-marked sand consolidated to hard stone, uplifted and set on

edge by movements of the earth's crust, and exposed to open air

after long erosion.

STRATIFICATION. For the most part the sheet of sea-laid waste is

hidden from our sight. Where its edge is exposed along the shore

we may see the surface markings which have just been noticed.

Soundings also, and the observations made in shallow waters by

divers, tell something of its surface; but to learn more of its

structures we must study those ancient sediments which have been

lifted from the sea and dissected by subaerial agencies. From them

we ascertain that sea deposits are stratified. They lie in

distinct layers which often differ from one another in thickness,

in size of particles, and perhaps in color. They are parted by

bedding planes, each of which represents either a change in

material or a pause during which deposition ceased and the

material of one layer had time to settle and become somewhat

consolidated before the material of the next was laid upon

it. Stratification is thus due to intermittently acting forces,

such as the agitation of the water during storms, the flow and ebb

of the tide, and the shifting channels of tidal currents. Off the

mouths of rivers, stratification is also caused by the coarser and

more abundant material brought down at time of floods being laid

on the finer silt which is discharged during ordinary stages.

How stratified deposits are built up is well illustrated in the

flats which border estuaries, such as the Bay of Fundy. Each

advance of the tide spreads a film of mud, which dries and hardens

in the air during low water before another film is laid upon it by

the next incoming tidal flood. In this way the flats have been

covered by a clay which splits into leaves as thin as sheets of

paper.

It is in fine material, such as clays and shales and limestones,

that the thinnest and most uniform layers, as well as those of

widest extent, occur. On the other hand, coarse materials are

commonly laid in thick beds, which soon thin out seaward and give

place to deposits of finer stuff. In a general way strata are laid

in well-nigh horizontal sheets, for the surface on which they are

laid is generally of very gentle inclination. Each stratum,

however, is lenticular, or lenslike, in form, having an area where

it is thickest, and thinning out thence to its edges, where it is

overlapped by strata similar in shape.

CROSS BEDDING. There is an apparent exception to this rule where

strata whose upper and lower surfaces may be about horizontal are

made up of layers inclined at angles which may be as high as the

angle of repose. In this case each stratum grew by the addition

along its edge of successive layers of sediment, precisely as does

a sand bar in a river, the sand being pushed continuously over the

edge and coming to rest on a sloping surface. Shoals built by

strong and shifting tidal currents often show successive strata in

which the cross bedding is inclined in different directions.

THICKNESS OF SEA DEPOSITS. Remembering the vast amount of

material denuded from the land and deposited offshore, we should

expect that with the lapse of time sea deposits would have grown

to an enormous thickness. It is a suggestive fact that, as a rule,

the profile of the ocean bed is that of a soup plate,--a basin

surrounded by a flaring rim. On the CONTINENTAL SHELF, as the rim

is called, the water is seldom more than six hundred feet in depth

at the outer edge, and shallows gradually towards shore. Along the

eastern coast of the United States the continental shelf is from

fifty to one hundred and more miles in width; on the Pacific coast

it is much narrower. So far as it is due to upbuilding, a wide

continental shelf, such as that of the Atlantic coast, implies a

massive continental delta thousands of feet in thickness. The

coastal plain of the Atlantic states may be regarded as the

emerged inner margin of this shelf, and borings made along the

coast probe it to the depth of as much as three thousand feet

without finding the bottom of ancient offshore deposits.

Continental shelves may also be due in part to a submergence of

the outer margin of a continental plateau and to marine abrasion.

DEPOSITION OF SEDIMENTS AND SUBSIDENCE. The stratified rocks of

the land show in many places ancient sediments which reach a

thickness which is measured in miles, and which are yet the

product of well-nigh continuous deposition. Such strata may prove

by their fossils and by their composition and structure that they

were all laid offshore in shallow water. We must infer that,

during the vast length of time recorded by the enormous pile, the

floor of the sea along the coast was slowly sinking, and that the

trough was constantly being filled, foot by foot, as fast as it

was depressed. Such gradual, quiet movements of the earth's crust

not only modify the outline of coasts, as we have seen, but are of

far greater geological importance in that they permit the making

of immense deposits of stratified rock.

A slow subsidence continued during long time is recorded also in

the succession of the various kinds of rock that come to be

deposited in the same area. As the sea transgresses the land, i.e.

encroaches upon it, any given part of the sea bottom is brought

farther and farther from the shore. The basal conglomerate formed

by bowlder and pebble beaches comes to be covered with sheets of

sand, and these with layers of mud as the sea becomes deeper and

the shore more remote; while deposits of limestone are made when

at last no waste is brought to the place from the now distant

land, and the water is left clear for the growth of mollusks and

other lime-secreting organisms.

RATE OF DEPOSITION. As deposition in the sea corresponds to

denudation on the land, we are able to make a general estimate of

the rate at which the former process is going on. Leaving out of

account the soluble matter removed, the Mississippi is lowering

its basin at the rate of one foot in five thousand years, and we

may assume this as the average rate at which the earth's land

surface of fifty-seven million square miles is now being denuded

by the removal of its mechanical waste. But sediments from the

land are spread within a zone but two or three hundred miles in

width along the margin of the continents, a line one hundred

thousand miles long. As the area of deposition--about twenty-five

million square miles--is about one half the area of denudation,

the average rate of deposition must be twice the average rate of

denudation, i.e. about one foot in twenty-five hundred years. If

some deposits are made much more rapidly than this, others are

made much more slowly. If they were laid no faster than the

present average rate, the strata of ancient sea deposits exposed

in a quarry fifty feet deep represent a lapse of at least one

hundred and twenty-five thousand years, and those of a formation

five hundred feet thick required for their accumulation one

million two hundred and fifty thousand years.

THE SEDIMENTARY RECORD AND THE DENUDATION CYCLE. We have seen that

the successive stages in a cycle of denudation, such as that by

which a land mass of lofty mountains is worn to low plains, are

marked each by its own peculiar land forms, and that the forms of

the earlier stages are more or less completely effaced as the

cycle draws toward an end. Far more lasting records of each stage

are left in the sedimentary deposits of the continental delta.

Thus, in the youth of such a land mass as we have mentioned,

torrential streams flowing down the steep mountain sides deliver

to the adjacent sea their heavy loads of coarse waste, and thick

offshore deposits of sand and gravel (Fig. 156) record the high

elevation of the bordering land. As the land is worn to lower

levels, the amount and coarseness of the waste brought to the sea

diminishes, until the sluggish streams carry only a fine silt

which settles on the ocean floor near to land in wide sheets of

mud which harden into shale. At last, in the old age of the region

(Fig. 157), its low plains contribute little to the sea except the

soluble elements of the rocks, and in the clear waters near the

land lime-secreting organisms flourish and their remains

accumulate in beds of limestone. When long-weathered lands

mantled with deep, well-oxidized waste are uplifted by a gradual

movement of the earth's crust, and the mantle is rapidly stripped

off by the revived streams, the uprise is recorded in wide

deposits of red and yellow clays and sands upon the adjacent ocean

floor.

Where the waste brought in is more than the waves can easily

distribute, as off the mouths of turbid rivers which drain

highlands near the sea, deposits are little winnowed, and are laid

in rapidly alternating, shaly sandstones and sandy shales.

Where the highlands are of igneous rock, such as granite, and

mechanical disintegration is going on more rapidly than chemical

decay, these conditions are recorded in the nature of the deposits

laid offshore. The waste swept in by streams contains much

feldspar and other minerals softer and more soluble than quartz,

and where the waves have little opportunity to wear and winnow it,

it comes to rest in beds of sandstone in which grains of feldspar

and other soft minerals are abundant. Such feldspathic sandstones

are known as ARKOSE.

On the other hand, where the waste supplied to the sea comes

chiefly from wide, sandy, coastal plains, there are deposited off-

shore clean sandstones of well-worn grains of quartz alone. In

such coastal plains the waste of the land is stored for ages.

Again and again they are abandoned and invaded by the sea as from

time to time the land slowly emerges and is again submerged. Their

deposits are long exposed to the weather, and sorted over by the

streams, and winnowed and worked over again and again by the

waves. In the course of long ages such deposits thus become

thoroughly sorted, and the grains of all minerals softer than

quartz are ground to mud.

DEEP-SEA OOZES AND CLAYS

GLOBIGERINA OOZE. Beyond the reach of waste from the land the

bottom of the deep sea is carpeted for the most part with either

chalky ooze or a fine red clay. The surface waters of the warm

seas swarm with minute and lowly animals belonging to the order of

the Foraminifera, which secrete shells of carbonate of lime. At

death these tiny white shells fall through the sea water like

snowflakes in the air, and, slowly dissolving, seem to melt quite

away before they can reach depths greater than about three miles.

Near shore they reach bottom, but are masked by the rapid deposit

of waste derived from the land. At intermediate depths they mantle

the ocean floor with a white, soft lime deposit known as

Globigerina ooze, from a genus of the Foraminifera which

contributes largely to its formation.

RED CLAY. Below depths of from fifteen to eighteen thousand feet

the ocean bottom is sheeted with red or chocolate colored clay. It

is the insoluble residue of seashells, of the debris of submarine

volcanic eruptions, of volcanic dust wafted by the winds, and of

pieces of pumice drifted by ocean currents far from the volcanoes

from which they were hurled. The red clay builds up with such

inconceivable slowness that the teeth of sharks and the hard ear

bones of whales may be dredged in large numbers from the deep

ocean bed, where they have lain unburied for thousands of years;

and an appreciable part of the clay is also formed by the dust of

meteorites consumed in the atmosphere,--a dust which falls

everywhere on sea and land, but which elsewhere is wholly masked

by other deposits.

The dark, cold abysses of the ocean are far less affected by

change than any other portion of the surface of the lithosphere.

These vast, silent plains of ooze lie far below the reach of

storms. They know no succession of summer and winter, or of night

and day. A mantle of deep and quiet water protects them from the

agents of erosion which continually attack, furrow, and destroy

the surface of the land. While the land is the area of erosion,

the sea is the area of deposition. The sheets of sediment which

are slowly spread there tend to efface any inequalities, and to

form a smooth and featureless subaqueous plain.

With few exceptions, the stratified rocks of the land are proved

by their fossils and composition to have been laid in the sea; but

in the same way they are proved to be offshore, shallow-water

deposits, akin to those now making on continental shelves. Deep-

sea deposits are absent from the rocks of the land, and we may

therefore infer that the deep sea has never held sway where the

continents now are,--that the continents have ever been, as now,

the elevated portions of the lithosphere, and that the deep seas

of the present have ever been its most depressed portions.

THE REEF-BUILDING CORALS

In warm seas the most conspicuous of rock-making organisms are the

corals known as the reef builders. Floating in a boat over a coral

reef, as, for example, off the south coast of Florida or among the

Bahamas, one looks down through clear water on thickets of

branching coral shrubs perhaps as much as eight feet high, and

hemispherical masses three or four feet thick, all abloom with

countless minute flowerlike coral polyps, gorgeous in their colors

of yellow, orange, green, and red. In structure each tiny polyp is

little more than a fleshy sac whose mouth is surrounded with

petal-like tentacles, or feelers. From the sea water the polyps

secrete calcium carbonate and build it up into the stony framework

which supports their colonies. Boring mollusks, worms, and sponges

perforate and honeycomb this framework even while its surface is

covered with myriads of living polyps. It is thus easily broken by

the waves, and white fragments of coral trees strew the ground

beneath. Brilliantly colored fishes live in these coral groves,

and countless mollusks, sea urchins, and other forms of marine

life make here their home. With the debris from all these sources

the reef is constantly built up until it rises to low-tide level.

Higher than this the corals cannot grow, since they are killed by

a few hours' exposure to the air.

When the reef has risen to wave base, the waves abrade it on the

windward side and pile to leeward coral blocks torn from their

foundation, filling the interstices with finer fragments. Thus

they heap up along the reef low, narrow islands (Fig. 160).

Reef building is a comparatively rapid progress. It has been

estimated that off Florida a reef could be built up to the surface

from a depth of fifty feet in about fifteen hundred years.

CORAL LIMESTONES. Limestones of various kinds are due to the reef

builders. The reef rock is made of corals in place and broken

fragments of all sizes, cemented together with calcium carbonate

from solution by infiltrating waters. On the island beaches coral

sand is forming oolitic limestone, and the white coral mud with

which the sea is milky for miles about the reef in times of storm

settles and concretes into a compact limestone of finest grain.

Corals have been among the most important limestone builders of

the sea ever since they made their appearance in the early

geological ages.

The areas on which coral limestone is now forming are large. The

Great Barrier Reef of Australia, which lies off the north-eastern

coast, is twelve hundred and fifty miles long, and has a width of

from ten to ninety miles. Most of the islands of the tropics are

either skirted with coral reefs or are themselves of coral

formation.

CONDITIONS OF CORAL GROWTH. Reef-building corals cannot live

except in clear salt water less, as a rule, than one hundred and

fifty feet in depth, with a winter temperature not lower than 68

degrees F. An important condition also is an abundant food supply,

and this is best secured in the path of the warm oceanic currents.

Coral reefs may be grouped in three classes,--fringing reefs,

barrier reefs, and atolls.

FRINGING REEFS. These take their name from the fact that they are

attached as narrow fringes to the shore. An example is the reef

which forms a selvage about a mile wide along the northeastern

coast of Cuba. The outer margin, indicated by the line of white

surf, where the corals are in vigorous growth, rises from about

forty feet of water. Between this and the shore lies a stretch of

shoal across which one can wade at low water, composed of coral

sand with here and there a clump of growing coral.

BARRIER REEFS. Reefs separated from the shore by a ship channel of

quiet water, often several miles in width and sometimes as much as

three hundred feet in depth, are known as barrier reefs. The

seaward face rises abruptly from water too deep for coral growth.

Low islands are cast up by the waves upon the reef, and inlets

give place for the ebb and flow of the tides. Along the west coast

of the island of New Caledonia a barrier reef extends for four

hundred miles, and for a length of many leagues seldom approaches

within eight miles of the shore.

ATOLLS. These are ring-shaped or irregular coral islands, or

island-studded reefs, inclosing a central lagoon. The narrow zone

of land, like the rim of a great bowl sunken to the water's edge,

rises hardly more than twenty feet at most above the sea, and is

covered with a forest of trees such as the cocoanut, whose seeds

can be drifted to it uninjured from long distances. The white

beach of coral sand leads down to the growing reef, on whose outer

margin the surf is constantly breaking. The sea face of the reef

falls off abruptly, often to depths of thousands of feet, while

the lagoon varies in depth from a few feet to one hundred and

fifty or two hundred, and exceptionally measures as much as three

hundred and fifty feet.

THEORIES OF CORAL REEFS. Fringing reefs require no explanation,

since the depth of water about them is not greater than that at

which coral can grow; but barrier reefs and atolls, which may rise

from depths too great for coral growth demand a theory of their

origin.

Darwin's theory holds that barrier reefs and atolls are formed

from fringing reefs by SUBSIDENCE. The rate of sinking cannot be

greater than that of the upbuilding of the reef, since otherwise

the corals would be carried below their depth and drowned. The

process is illustrated in Figure 161, where v represents a

volcanic island in mid ocean undergoing slow depression, and ss

the sea level before the sinking began, when the island was

surrounded by a fringing reef. As the island slowly sinks, the

reef builds up with equal pace. It rears its seaward face more

steep than the island slope, and thus the intervening space

between the sinking, narrowing land and the outer margin of the

reef constantly widens. In this intervening space the corals are

more or less smothered with silt from the outer reef and from the

land, and are also deprived in large measure of the needful supply

of food and oxygen by the vigorous growth of the corals on the

outer rim. The outer rim thus becomes a barrier reef and the inner

belt of retarded growth is deepened by subsidence to a ship

channel, s's' representing sea level at this time. The final

stage, where the island has been carried completely beneath the

sea and overgrown by the contracting reef, whose outer ring now

forms an atoll, is represented by s"s".

In very many instances, however, atolls and barrier reefs may be

explained without subsidence. Thus a barrier reef may be formed by

the seaward growth of a fringing reef upon the talus of its sea

face. In Figure 162 f is a fringing reef whose outer wall rises

from about one hundred and fifty feet, the lower limit of the

reef-building species. At the foot of this submarine cliff a talus

of fallen blocks t accumulates, and as it reaches the zone of

coral growth becomes the foundation on which the reef is steadily

extended seaward. As the reef widens, the polyps of the

circumference flourish, while those of the inner belt are retarded

in their growth and at last perish. The coral rock of the inner

belt is now dissolved by sea water and scoured out by tidal

currents until it gives place to a gradually deepening ship

channel, while the outer margin is left as a barrier reef.

In much the same way atolls may be built on any shoal which lies

within the zone of coral growth. Such shoals may be produced when

volcanic islands are leveled by waves and ocean currents, and when

submarine plateaus, ridges, and peaks are built up by various

organic agencies, such as molluscous and foraminiferal shell

deposits. The reef-building corals, whose eggs are drifted widely

over the tropic seas by ocean currents, colonize such submarine

foundations wherever the conditions are favorable for their

growth. As the reef approaches the surface the corals of the inner

area are smothered by silt and starved, and their Submarine

Volcanic Peak hard parts are dissolved and scoured away; while

those of the circumference, with abundant food supply, nourish and

build the ring of the atoll. Atolls may be produced also by the

backward drift of sand from either end of a crescentic coral reef

or island, the spits uniting in the quiet water of the lee to

inclose a lagoon. In the Maldive Archipelago all gradations

between crescent-shaped islets and complete atoll rings have been

observed.

In a number of instances where coral reefs have been raised by

movements of the earth's crust, the reef formation is found to be

a thin veneer built upon a foundation of other deposits. Thus

Christmas Island, in the Indian Ocean, is a volcanic pile rising

eleven hundred feet above sea level and fifteen thousand five

hundred feet above the bottom of the sea. The summit is a plateau

surrounded by a rim of hills of reef formation, which represent

the ring of islets of an ancient atoll. Beneath the reef are thick

beds of limestone, composed largely of the remains of

foraminifers, which cover the lavas and fragraental materials of

the old submarine volcano.

Among the ancient sediments which now form the stratified rocks of

the land there occur many thin reef deposits, but none are known

of the immense thickness which modern reefs are supposed to reach

according to the theory of subsidence.

Barrier and fringing reefs are commonly interrupted off the mouths

of rivers. Why?

SUMMARY. We have seen that the ocean bed is the goal to which the

waste of the rocks of the land at last arrives. Their soluble

parts, dissolved by underground waters and carried to the sea by

rivers, are largely built up by living creatures into vast sheets

of limestone. The less soluble portions--the waste brought in by

streams and the waste of the shore--form the muds and sands of

continental deltas. All of these sea deposits consolidate and

harden, and the coherent rocks of the land are thus reconstructed

on the ocean floor. But the destination is not a final one. The

stratified rocks of the land are for the most part ancient

deposits of the sea, which have been lifted above sea level; and

we may believe that the sediments now being laid offshore are the

"dust of continents to be," and will some time emerge to form

additions to the land. We are now to study the movements of the

earth's crust which restore the sediments of the sea to the light

of day, and to whose beneficence we owe the habitable lands of the

present.

PART II

INTERNAL GEOLOGICAL AGENCIES

CHAPTER IX

MOVEMENTS OF THE EARTH'S CRUST

The geological agencies which we have so far studied--weathering,

streams, underground waters, glaciers, winds, and the ocean--all

work upon the earth from without, and all are set in motion by an

energy external to the earth, namely, the radiant energy of the

sun. All, too, have a common tendency to reduce the inequalities

of the earth's surface by leveling the lands and strewing their

waste beneath the sea.

But despite the unceasing efforts of these external agencies, they

have not destroyed the continents, which still rear their broad

plains and great plateaus and mountain ranges above the sea.

Either, then, the earth is very young and the agents of denudation

have not yet had time to do their work, or they have been opposed

successfully by other forces.

We enter now upon a department of our science which treats of

forces which work upon the earth from within, and increase the

inequalities of its surface. It is they which uplift and recreate

the lands which the agents of denudation are continually

destroying; it is they which deepen the ocean bed and thus

withdraw its waters from the shores. At times also these forces

have aided in the destruction of the lands by gradually lowering

them and bringing in the sea. Under the action of forces resident

within the earth the crust slowly rises or sinks; from time to

time it has been folded and broken; while vast quantities of

molten rock have been pressed up into it from beneath and

outpoured upon its surface. We shall take up these phenomena in

the following chapters, which treat of upheavals and depressions

of the crust, foldings and fractures of the crust, earthquakes,

volcanoes, the interior conditions of the earth, mineral veins,

and metamorphism.

OSCILLATIONS OF THE CRUST

Of the various movements of the crust due to internal agencies we

will consider first those called oscillations, which lift or

depress large areas so slowly that a long time is needed to

produce perceptible changes of level, and which leave the strata

in nearly their original horizontal attitude. These movements are

most conspicuous along coasts, where they can be referred to the

datum plane of sea level; we will therefore take our first

illustrations from rising and sinking shores.

NEW JERSEY. Along the coasts of New Jersey one may find awash at

high tide ancient shell heaps, the remains of tribal feasts of

aborigines. Meadows and old forest grounds, with the stumps still

standing, are now overflowed by the sea, and fragments of their

turf and wood are brought to shore by waves. Assuming that the sea

level remains constant, it is clear that the New Jersey coast is

now gradually sinking. The rate of submergence has been estimated

at about two feet per century.

On the other hand, the wide coastal plain of New Jersey is made of

stratified sands and clays, which, as their marine fossils show,

were outspread beneath the sea. Their present position above sea

level proves that the land now subsiding emerged in the recent

past.

The coast of New Jersey is an example of the slow and tranquil

oscillations of the earth's unstable crust now in progress along

many shores. Some are emerging from the sea, some are sinking

beneath it; and no part of the land seems to have been exempt from

these changes in the past.

EVIDENCES OF CHANGES OF LEVEL. Taking the surface of the sea as a

level of reference, we may accept as proofs of relative upheaval

whatever is now found in place above sea level and could have been

formed only at or beneath it, and as proofs of relative subsidence

whatever is now found beneath the sea and could only have been

formed above it.

Thus old strand lines with sea cliffs, wave-cut rock benches, and

beaches of wave-worn pebbles or sand, are striking proofs of

recent emergence to the amount of their present height above tide.

No less conclusive is the presence of sea-laid rocks which we may

find in the neighboring quarry or outcrop, although it may have

been long ages since they were lifted from the sea to form part of

the dry land.

Among common proofs of subsidence are roads and buildings and

other works of man, and vegetal growths and deposits, such as

forest grounds and peat beds, now submerged beneath the sea. In

the deltas of many large rivers, such as the Po, the Nile, the

Ganges, and the Mississippi, buried soils prove subsidences of

hundreds of feet; and in several cases, as in the Mississippi

delta, the depression seems to be now in progress.

Other proofs of the same movement are drowned land forms which are

modeled only in open air. Since rivers cannot cut their valleys

farther below the baselevel of the sea than the depths of their

channels, DROWNED VALLEYS are among the plainest proofs of

depression. To this class belong Narragansett, Delaware,

Chesapeake, Mobile, and San Francisco bays, and many other similar

drowned valleys along the coasts of the United States. Less

conspicuous are the SUBMARINE CHANNELS which, as soundings show,

extend from the mouths of a number of rivers some distance out to

sea. Such is the submerged channel which reaches from New York Bay

southeast to the edge of the continental shelf, and which is

supposed to have been cut by the Hudson River when this part of

the shelf was a coastal plain.

WARPING. In a region undergoing changes of level the rate of

movement commonly varies in different parts. Portions of an area

may be rising or sinking, while adjacent portions are stationary

or moving in the opposite direction. In this way a land surface

becomes WARPED. Thus, while Nova Scotia and New Brunswick are now

rising from the level of the sea, Prince Edward Island and Cape

Breton Island are sinking, and the sea now flows over the site of

the famous old town of Louisburg destroyed in 1758.

Since the close of the glacial epoch the coasts of Newfoundland

and Labrador have risen hundreds of feet, but the rate of

emergence has not been uniform. The old strand line, which stands

at five hundred and seventy-five feet above tide at St. John's,

Newfoundland, declines to two hundred and fifty feet near the

northern point of Labrador.

THE GREAT LAKES is now under-going perceptible warping. Rivers

enter the lakes from the south and west with sluggish currents and

deep channels resembling the estuaries of drowned rivers; while

those that enter from opposite directions are swift and shallow.

At the western end of Lake Erie are found submerged caves

containing stalactites, and old meadows and forest grounds are now

under water. It is thus seen that the water of the lakes is rising

along their southwestern shores, while from their north-eastern

shores it is being withdrawn. The region of the Great Lakes is

therefore warping; it is rising in the northeast as compared with

the southwest.

From old bench marks and records of lake levels it has been

estimated that the rate of warping amounts to five inches a

century for every one hundred miles. It is calculated that the

water of Lake Michigan is rising at Chicago at the rate of nine or

ten inches per century. The divide at this point between the

tributaries of the Mississippi and Lake Michigan is but eight feet

above the mean stage of the lake. If the canting of the region

continues at its present rate, in a thousand years the waters of

the lake will here overflow the divide. In three thousand five

hundred years all the lakes except Ontario will discharge by this

outlet, via the Illinois and Mississippi rivers, into the Gulf of

Mexico. The present outlet by the Niagara River will be left dry,

and the divide between the St. Lawrence and the Mississippi

systems will have shifted from Chicago to the vicinity of Buffalo.

PHYSIOGRAPHIC EFFECTS OF OSCILLATIONS. We have already mentioned

several of the most important effects of movements of elevation

and depression, such as their effects on rivers, the mantle of

waste, and the forms of coasts. Movements of elevation--including

uplifts by folding and fracture of the crust to be noticed later--

are the necessary conditions for erosion by whatever agent. They

determine the various agencies which are to be chiefly concerned m

the wear of any land,--whether streams or glaciers, weathering or

the wind,--and the degree of their efficiency. The lands must be

uplifted before they can be eroded, and since they must be eroded

before their waste can be deposited, movements of elevation are a

prerequisite condition for sedimentation also. Subsidence is a

necessary condition for deposits of great thickness, such as those

of the Great Valley of California and the Indo-Gangetic plain (p.

101), the Mississippi delta (p. 109), and the still more important

formations of the continental delta in gradually sinking troughs

(p. 183). It is not too much to say that the character and

thickness of each formation of the stratified rocks depend

primarily on these crustal movements.

Along the Baltic coast of Sweden, bench marks show that the sea is

withdrawing from the land at a rate which at the north amounts to

between three and four feet per century; Towards the south the

rate decreases. South of Stockholm, until recent years, the sea

has gained upon the land, and here in several seaboard towns

streets by the shore are still submerged. The rate of oscillation

increases also from the coast inland. On the other hand, along the

German coast of the Baltic the only historic fluctuations of sea

level are those which may be accounted for by variations due to

changes in rainfall. In 1730 Celsius explained the changes of

level of the Swedish coast as due to a lowering of the Baltic

instead of to an elevation of the land. Are the facts just stated

consistent with his theory?

At the little town of Tadousac--where the Saguenay River empties

into the St. Lawrence--there are terraces of old sea beaches, some

almost as fresh as recent railway fills, the highest standing two

hundred and thirty feet above the river. Here the Saguenay is

eight hundred and forty feet in depth, and the tide ebbs and flows

far up its stream. Was its channel cut to this depth by the river

when the land was at its present height? What oscillations are

here recorded, and to what amount?

A few miles north of Naples, Italy, the ruins of an ancient Roman

temple lie by the edge of the sea, on a narrow plain which is

overlooked in the rear by an old sea cliff (Fig. 166). Three

marble pillars are still standing. For eleven feet above their

bases these columns are uninjured, for to this height they were

protected by an accumulation of volcanic ashes; but from eleven to

nineteen feet they are closely pitted with the holes of boring

marine mollusks. From these facts trace the history of the

oscillations of the region.

FOLDINGS OF THE CRUST

The oscillations which we have just described leave the strata not

far from their original horizontal attitude. Figure 167 represents

a region in which movements of a very different nature have taken

place. Here, on either side of the valley V, we find outcrops of

layers tilted at high angles. Sections along the ridge r show that

it is composed of layers which slant inward from either side. In

places the outcropping strata stand nearly on edge, and on the

right of the valley they are quite overturned; a shale SH has come

to overlie a limestone LM although the shale is the older rock,

whose original position was beneath the limestone.

It is not reasonable to suppose that these rocks were deposited in

the attitude in which we find them now; we must believe that, like

other stratified rocks, they were outspread in nearly level sheets

upon the ocean floor. Since that time they must have been

deformed. Layers of solid rock several miles in thickness have

been crumpled and folded like soft wax in the hand, and a vast

denudation has worn away the upper portions of the folds, in part

represented in our section by dotted lines.

DIP AND STRIKE. In districts where the strata have been disturbed

it is desirable to record their attitude. This is most easily done

by taking the angle at which the strata are inclined and the

compass direction in which they slant. It is also convenient to

record the direction in which the outcrop of the strata trends

across the country.

The inclination of a bed of rocks to the horizon is its DIP. The

amount of the dip is the angle made with a horizontal plane. The

dip of a horizontal layer is zero, and that of a vertical layer is

90 degrees. The direction of the dip is taken with the compass.

Thus a geologist's notebook in describing the attitude of

outcropping strata contains many such entries as these: dip 32

degrees north, or dip 8 degrees south 20 degrees west,--meaning in

the latter case that the amount of the dip is 8 degrees and the

direction of the dip bears 20 degrees west of south.

The line of intersection of a layer with the horizontal plane is

the STRIKE. The strike always runs at right angles to the dip.

Dip and strike may be illustrated by a book set aslant on a shelf.

The dip is the acute angle made with the shelf by the side of the

book, while the strike is represented by a line running along the

book's upper edge. If the dip is north or south, the strike runs

east and west.

FOLDED STRUCTURES. An upfold, in which the strata dip away from a

line drawn along the crest and called the axis of the fold, is

known as an ANTICLINE. A downfold, where the strata dip from

either side toward the axis of the trough, is called a SYNCLINE.

There is sometimes seen a downward bend in horizontal or gently

inclined strata, by which they descend to a lower level. Such a

single flexure is a MONOCLINE.

DEGREES OF FOLDING. Folds vary in degree from broad, low swells,

which can hardly be detected, to the most highly contorted and

complicated structures. In SYMMETRIC folds the dips of the rocks

on each side the axis of the fold are equal. In UNSYMMETRICAL

folds one limb is steeper than the other, as in the anticline in

Figure 167. In OVERTURNED folds one limb is inclined beyond the

perpendicular. FAN FOLDS have been so pinched that the original

anticlines are left broader at the top than at the bottom.

In folds where the compression has been great the layers are often

found thickened at the crest and thinned along the limbs. Where

strong rocks such as heavy limestones are folded together with

weak rocks such as shales, the strong rocks are often bent into

great simple folds, while the weak rocks are minutely crumpled.

SYSTEMS OF FOLDS. As a rule, folds occur in systems. Over the

Appalachian mountain belt, for example, extending from

northeastern Pennsylvania to northern Alabama and Georgia, the

earth's crust has been thrown into a series of parallel folds

whose axes run from northeast to southwest (Fig. 175). In

Pennsylvania one may count a score or more of these earth waves,--

some but from ten to twenty miles in length, and some extending as

much as two hundred miles before they die away. On the eastern

part of this belt the folds are steeper and more numerous than on

the western side.

CAUSE AND CONDITIONS OF FOLDING. The sections which we have

studied suggest that rocks are folded by lateral pressure. While a

single, simple fold might be produced by a heave, a series of

folds, including overturns, fan folds, and folds thickened on

their crests at the expense of their limbs, could only be made in

one way,--by pressure from the side. Experiment has reproduced all

forms of folds by subjecting to lateral thrust layers of plastic

material such as wax.

Vast as the force must have been which could fold the solid rocks

of the crust as one may crumple the leaves of a magazine in the

fingers, it is only under certain conditions that it could have

produced the results which we see. Rocks are brittle, and it is

only when under a HEAVY LOAD and by GREAT PRESSURE SLOWLY APPLIED,

that they can thus be folded and bent instead of being crushed to

pieces. Under these conditions, experiments prove that not only

metals such as steel, but also brittle rocks such as marble, can

be deformed and molded and made to flow like plastic clay.

ZONE OF FLOW, ZONE OF FLOW AND FRACTURE, AND ZONE OF FRACTURE. We

may believe that at depths which must be reckoned in tens of

thousands of feet the load of overlying rocks is so great that

rocks of all kinds yield by folding to lateral pressure, and flow

instead of breaking. Indeed, at such profound depths and under

such inconceivable weight no cavity can form, and any fractures

would be healed at once by the welding of grain to grain. At less

depths there exists a zone where soft rocks fold and flow under

stress, and hard rocks are fractured; while at and near the

surface hard and soft rocks alike yield by fracture to strong

pressure.

STRUCTURES DEVELOPED IN COMPRESSED ROCKS

Deformed rocks show the effects of the stresses to which they have

yielded, not only in the immense folds into which they have been

thrown but in their smallest parts as well. A hand specimen of

slate, or even a particle under the microscope, may show

plications similar in form and origin to the foldings which have

produced ranges of mountains. A tiny flake of mica in the rocks of

the Alps may be puckered by the same resistless forces which have

folded miles of solid rock to form that lofty range.

SLATY CLEAVAGE. Rocks which have yielded to pressure often split

easily in a certain direction across the bedding planes. This

cleavage is known as slaty cleavage, since it is most perfectly

developed in fine-grained, homogeneous rocks, such as slates,

which cleave to the thin, smooth-surfaced plates with which we are

familiar in the slates used in roofing and for ciphering and

blackboards. In coarse-grained rocks, pressure develops more

distant partings which separate the rocks into blocks.

Slaty cleavage cannot be due to lamination, since it commonly

crosses bedding planes at an angle, while these planes have been

often well-nigh or quite obliterated. Examining slate with a

microscope, we find that its cleavage is due to the grain of the

rock. Its particles are flattened and lie with their broad faces

in parallel planes, along which the rock naturally splits more

easily than in any other direction. The irregular grains of the

mud which has been altered to slate have been squeezed flat by a

pressure exerted at right angles to the plane of cleavage.

Cleavage is found only in folded rocks, and, as we may see in

Figure 176, the strike of the cleavage runs parallel to the strike

of the strata and the axis of the folds. The dip of the cleavage

is generally steep, hence the pressure was nearly horizontal. The

pressure which has acted at right angles to the cleavage, and to

which it is due, is the same lateral pressure which has thrown the

strata into folds.

We find additional proof that slates have undergone compression at

right angles to their cleavage in the fact that any inclusions in

them, such as nodules and fossils, have been squeezed out of shape

and have their long diameters lying in the planes of cleavage.

That pressure is competent to cause cleavage is shown by

experiment. Homogeneous material of fine grain, such as beeswax,

when subjected to heavy pressure cleaves at right angles to the

direction of the compressing force.

RATE OF FOLDING. All the facts known with regard to rock

deformation agree that it is a secular process, taking place so

slowly that, like the deepening of valleys by erosion, it escapes

the notice of the inhabitants of the region. It is only under

stresses slowly applied that rocks bend without breaking. The

folds of some of the highest mountains have risen so gradually

that strong, well-intrenched rivers which had the right of way

across the region were able to hold to their courses, and as a

circular saw cuts its way through the log which is steadily driven

against it, so these rivers sawed their gorges through the fold as

fast as it rose beneath them. Streams which thus maintain the

course which they had antecedent to a deformation of the region

are known as ANTECEDENT streams. Examples of such are the Sutlej

and other rivers of India, whose valleys trench the outer ranges

of the Himalayas and whose earlier river deposits have been

upturned by the rising ridges. On the other hand, mountain crests

are usually divides, parting the head waters of different drainage

systems. In these cases the original streams of the region have

been broken or destroyed by the uplift of the mountain mass across

their paths.

On the whole, which have worked more rapidly, processes of

deformation or of denudation?

LAND FORMS DUE TO FOLDING

As folding goes on so slowly, it is never left to form surface

features unmodified by the action of other agencies. An anticlinal

fold is attacked by erosion as soon as it begins to rise above the

original level, and the higher it is uplifted, and the stronger

are its slopes, the faster is it worn away. Even while rising, a

young upfold is often thus unroofed, and instead of appearing as a

long, Smooth, boat-shaped ridge, it commonly has had opened along

the rocks of the axis, when these are weak, a valley which is

overlooked by the infacing escarpments of the hard layers of the

sides of the fold. Under long-continued erosion, anticlines may be

degraded to valleys, while the synclines of the same system may be

left in relief as ridges.

FOLDED MOUNTAINS. The vastness of the forces which wrinkle the

crust is best realized in the presence of some lofty mountain

range. All mountains, indeed, are not the result of folding. Some,

as we shall see, are due to upwarps or to fractures of the crust;

some are piles of volcanic material; some are swellings caused by

the intrusion of molten matter beneath the surface; some are the

relicts left after the long denudation of high plateaus.

But most of the mountain ranges of the earth, and some of the

greatest, such as the Alps and the Himalayas, were originally

mountains of folding. The earth's crust has wrinkled into a fold;

or into a series of folds, forming a series of parallel ridges and

intervening valleys; or a number of folds have been mashed

together into a vast upswelling of the crust, in which the layers

have been so crumpled and twisted, overturned and crushed, that it

is exceedingly difficult to make out the original structure.

The close and intricate folds seen in great mountain ranges were

formed, as we have seen, deep below the surface, within the zone

of folding. Hence they may never have found expression in any

individual surface features. As the result of these deformations

deep under ground the surface was broadly lifted to mountain

height, and the crumpled and twisted mountain structures are now

to be seen only because erosion has swept away the heavy cover of

surface rocks under whose load they were developed.

When the structure of mountains has been deciphered it is possible

to estimate roughly the amount of horizontal compression which the

region has suffered. If the strata of the folds of the Alps were

smoothed out, they would occupy a belt seventy-four miles wider

than that to which they have been compressed, or twice their

present width. A section across the Appalachian folds in

Pennyslvania shows a compression to about two thirds the original

width; the belt has been shortened thirty-five miles in every

hundred.

Considering the thickness of their strata, the compression which

mountains have undergone accounts fully for their height, with

enough to spare for all that has been lost by denudation.

The Appalachian folds involve strata thirty thousand feet in

thickness. Assuming that the folded strata rested on an unyielding

foundation, and that what was lost in width was gained in height,

what elevation would the range have reached had not denudation

worn it as it rose?

THE LIFE HISTORY OF MOUNTAINS. While the disturbance and uplift of

mountain masses are due to deformation, their sculpture into

ridges and peaks, valleys and deep ravines, and all the forms

which meet the eye in mountain scenery, excepting in the very

youngest ranges, is due solely to erosion. We may therefore

classify mountains according to the degree to which they have been

dissected. The Juras are an example of the stage of early youth,

in which the anticlines still persist as ridges and the synclines

coincide with the valleys; this they owe as much to the slight

height of their uplift as to the recency of its date.

The Alps were upheaved at various times, the last uplift being

later than the uplift of the Juras, but to so much greater height

that erosion has already advanced them well on towards maturity.

The mountain mass has been cut to the core, revealing strange

contortions of strata which could never have found expression at

the surface. Sharp peaks, knife-edged crests, deep valleys with

ungraded slopes subject to frequent landslides, are all features

of Alpine scenery typical of a mountain range at this stage in its

life history. They represent the survival of the hardest rocks and

the strongest structures, and the destruction of the weaker in

their long struggle for existence against the agents of erosion.

Although miles of rock have been removed from such ranges as the

Alps, we need not suppose that they ever stood much, if any,

higher than at present. All this vast denudation may easily have

been accomplished while their slow upheaval was going on; in

several mountain ranges we have evidence that elevation has not

yet ceased.

Under long denudation mountains are subdued to the forms

characteristic of old age. The lofty peaks and jagged crests of

their earlier life are smoothed down to low domes and rounded

crests. The southern Appalachians and portions of the Hartz

Mountains in Germany are examples of mountains which have reached

this stage.

There are numerous regions of upland and plains in which the rocks

are found to have the same structure that we have seen in folded

mountains; they are tilted, crumpled, and overturned, and have

clearly suffered intense compression. We may infer that their

folds were once lifted to the height of mountains and have since

been wasted to low-lying lands. Such a section as that of Figure

67 illustrates how ancient mountains may be leveled to their

roots, and represents the final stage to which even the Alps and

the Himalayas must sometime arrive. Mountains, perhaps of Alpine

height, once stood about Lake Superior; a lofty range once

extended from New England and New Jersey southwestward to Georgia

along the Piedmont belt. In our study of historic geology we shall

see more clearly how short is the life of mountains as the earth

counts time, and how great ranges have been lifted, worn away, and

again upheaved into a new cycle of erosion.

THE SEDIMENTARY HISTORY OF FOLDED MOUNTAINS. We may mention here

some of the conditions which have commonly been antecedent to

great foldings of the crust.

1. Mountain ranges are made of belts of enormously and

exceptionally thick sediments. The strata of the Appalachians are

thirty thousand feet thick, while the same formations thin out to

five thousand feet in the Mississippi valley. The folds of the

Wasatch Mountains involve strata thirty thousand feet thick, which

thin to two thousand feet in the region of the Plains.

2. The sedimentary strata of which mountains are made are for the

most part the shallow-water deposits of continental deltas.

Mountain ranges have been upfolded along the margins of

continents.

3. Shallow-water deposits of the immense thickness found in

mountain ranges can be laid only in a gradually sinking area. A

profound subsidence, often to be reckoned in tens of thousands of

feet, precedes the upfolding of a mountain range.

Thus the history of mountains of folding is as follows: For long

ages the sea bottom off the coast of a continent slowly subsides,

and the great trough, as fast as it forms, is filled with

sediments, which at last come to be many thousands of feet thick.

The downward movement finally ceases. A slow but resistless

pressure sets in, and gradually, and with a long series of many

intermittent movements, the vast mass of accumulated sediments is

crumpled and uplifted into a mountain range.

FRACTURES AND DISLOCATIONS OF THE CRUST

Considering the immense stresses to which the rocks of the crust

are subjected, it is not surprising to find that they often yield

by fracture, like brittle bodies, instead of by folding and

flowing, like plastic solids. Whether rocks bend or break depends

on the character and condition of the rocks, the load of overlying

rocks which they bear, and the amount of the force and the

slowness with which it is applied.

JOINTS. At the surface, where their load is least, we find rocks

universally broken into blocks of greater or less size by partings

known as joints. Under this name are included many division planes

caused by cooling and drying; but it is now generally believed

that the larger and more regular joints, especially those which

run parallel to the dip and strike of the strata, are fractures

due to up-and-down movements and foldings and twistings of the

rocks.

Joints are used to great advantage in quarrying, and we have seen

how they are utilized by the weather in breaking up rock masses,

by rivers in widening their valleys, by the sea in driving back

its cliffs, by glaciers in plucking their beds, and how they are

enlarged in soluble rocks to form natural passageways for

underground waters. The ends of the parted strata match along both

sides of joint planes; in. joints there has been little or no

displacement of the broken rocks.

FAULTS. In Figure 184 the rocks have been both broken and

dislocated along the plane ff'. One side must have been moved up

or down past the other. Such a dislocation is called a fault. The

amount of the displacement, as measured by the vertical distance

between the ends of a parted layer, is the throw. The angle which

the fault plane makes with the vertical is the HADE. In Figure 184

the right side has gone down relatively to the left; the right is

the side of the downthrow, while the left is the side of the

upthrow. Where the fault plane is not vertical the surfaces on the

two sides may be distinguished as the HANGING WALL and the FOOT

WALL. Faults differ in throw from a fraction of an inch to many

thousands of feet.

SLICKENSIDES. If we examine the walls of a fault, we may find

further evidence of movement in the fact that the surfaces are

polished and grooved by the enormous friction which they have

suffered as they have ground one upon the other. These

appearances, called sliekensides, have sometimes been mistaken for

the results of glacial action.

NORMAL FAULTS. Faults are of two kinds,--normal faults and thrust

faults. Normal faults, of which Figure 184 is an example, hade to

the downthrow; the hanging wall has gone down. The total length of

the strata has been increased by the displacement. It seems that

the strata have been stretched and broken, and that the blocks

have readjusted themselves under the action of gravity as they

settled.

THRUST FAULTS. Thrust faults hade to the upthrow; the hanging wall

has gone up. Clearly such faults, where the strata occupy less

space than before, are due to lateral thrust. Folds and thrust

faults are closely associated. Under lateral pressure strata may

fold to a certain point and then tear apart and fault along the

surface of least resistance. Under immense pressure strata also

break by shear without folding. Thus, in Figure 185, the rigid

earth block under lateral thrust has found it easier to break

along the fault plane than to fold. Where such faults are nearly

horizontal they are distinguished as THRUST PLANES.

In all thrust faults one mass has been pushed over another, so as

to bring the underlying and older strata upon younger beds; and

when the fault planes are nearly horizontal, and especially when

the rocks have been broken into many slices which have slidden far

one upon another, the true succession of strata is extremely hard

to decipher.

In the Selkirk Mountains of Canada the basement rocks of the

region have been driven east for seven miles on a thrust plane,

over rocks which originally lay thousands of feet above them.

Along the western Appalachians, from Virginia to Georgia, the

mountain folds are broken by more than fifteen parallel thrust

planes, running from northeast to southwest, along which the older

strata have been pushed westward over the younger. The longest

continuous fault has been traced three hundred and seventy-five

miles, and the greatest horizontal displacement has been estimated

at not less than eleven miles.

CRUSH BRECCIA. Rocks often do not fault with a clean and simple

fracture, but along a zone, sometimes several yards in width, in

which they are broken to fragments. It may occur also that strata

which as a whole yield to lateral thrust by folding include beds

of brittle rocks, such as thin-layered limestones, which are

crushed to pieces by the strain. In either case the fragments when

recemented by percolating waters form a rock known as a CRUSH

BRECCIA (pronounced BRETCHA).

Breccia is a term applied to any rock formed of cemented ANGULAR

fragments. This rock may be made by the consolidation of volcanic

cinders, of angular waste at the foot of cliffs, or of fragments

of coral torn by the waves from coral reefs, as well as of strata

crushed by crustal movements.

SURFACE FEATURES DUE TO DISLOCATIONS

FAULT SCARPS. A fault of recent date may be marked at surface by a

scarp, because the face of the upthrown block has not yet been

worn to the level of the downthrow side.

After the upthrown block has been worn down to this level,

differential erosion produces fault scarps wherever weak rocks and

resistant rocks are brought in contact along the fault plane; and

the harder rocks, whether on the upthrow or the downthrow side,

emerge in a line of cliffs. Where a fault is so old that no abrupt

scarps appear, its general course is sometimes marked by the line

of division between highland and lowland or hill and plain. Great

faults have sometimes brought ancient crystalline rocks in contact

with weaker and younger sedimentary rocks, and long after erosion

has destroyed all fault scarps the harder crystallines rise in an

upland of rugged or mountainous country which meets the lowland

along the line of faulting.

The vast majority of faults give rise to no surface features. The

faulted region may be old enough to have been baseleveled, or the

rocks on both sides of the line of dislocation may be alike in

their resistance to erosion and therefore have been worn down to a

common slope. The fault may be entirely concealed by the mantle of

waste, and in such cases it can be inferred from abrupt changes in

the character or the strike and dip of the strata where they may

outcrop near it.

The plateau trenched by the Grand Canyon of the Colorado River

exhibits a series of magnificent fault scarps whose general course

is from north to south, marking the edges of the great crust

blocks into which the country has been broken. The highest part of

the plateau is a crust block ninety miles long and thirty-five

miles in maximum width, which has been hoisted to nine thousand

three hundred feet above, sea level. On the east it descends four

thousand feet by a monoclinal fold, which passes into a fault

towards the north. On the west it breaks down by a succession of

terraces faced by fault scarps. The throw of these faults varies

from seven hundred feet to more than a mile. The escarpments,

however, are due in a large degree to the erosion of weaker rock

on the downthrow side.

The Highlands of Scotland meet the Lowlands on the south with a

bold front of rugged hills along a line of dislocation which runs

across the country from sea to sea. On the one side are hills of

ancient crystalline rocks whose crumpled structures prove that

they are but the roots of once lofty mountains; on the other lies

a lowland of sandstone and other stratified rocks formed from the

waste of those long-vanished mountain ranges. Remnants of

sandstone occur in places on the north of the great fault, and are

here seen to rest on the worn and fairly even surface of the

crystallines. We may infer that these ancient mountains were

reduced along their margins to low plains, which were slowly

lowered beneath the sea to receive a cover of sedimentary rocks.

Still later came an uplift and dislocation. On the one side

erosion has since stripped off the sandstones for the most part,

but the hard crystalline rocks yet stand in bold relief. On the

other side the weak sedimentary rocks have been worn down to

lowlands.

RIFT VALLEYS. In a broken region undergoing uplift or the unequal

settling which may follow, a slice inclosed between two fissures

may sink below the level of the crust blocks on either side, thus

forming a linear depression known as a rift valley, or valley of

fracture.

One of the most striking examples of this rare type of valley is

the long trough which runs straight from the Lebanon Mountains of

Syria on the north to the Red Sea on the south, and whose central

portion is occupied by the Jordan valley and the Dead Sea. The

plateau which it gashes has been lifted more than three thousand

feet above sea level, and the bottom of the trough reaches a depth

of two thousand six hundred feet below that level in parts of the

Dead Sea. South of the Dead Sea the floor of the trough rises

somewhat above sea level, and in the Gulf of Akabah again sinks

below it. This uneven floor could be accounted for either by the

profound warping of a valley of erosion or by the unequal

depression of the floor of a rift valley. But that the trough is a

true valley of fracture is proved by the fact that on either side

it is bounded by fault scarps and monoclinal folds. The keystone

of the arch has subsided. Many geologists believe that the Jordan-

Akabah trough, the long narrow basin of the Red Sea, and the chain

of down-faulted valleys which in Africa extends from the strait of

Bab-el-Mandeb as far south as Lake Nyassa--valleys which contain

more than thirty lakes--belong to a single system of dislocation.

Should you expect the lateral valleys of a rift valley at the time

of its formation to enter it as hanging valleys or at a common

level?

BLOCK MOUNTAINS. Dislocations take place on so grand a scale that

by the upheaval of blocks of the earth's crust or the down-

faulting of the blocks about one which is relatively stationary,

mountains known as block mountains are produced. A tilted crust

block may present a steep slope on the side upheaved and a more

gentle descent on the side depressed.

THE BASIN RANGES. The plateaus of the United States bounded by the

Rocky Mouirtains on the east, and on the west by the ranges which

front the Pacific, have been profoundly fractured and faulted. The

system of great fissures by which they are broken extends north

and south, and the long, narrow, tilted crust blocks intercepted

between the fissures give rise to the numerous north-south ranges

of the region. Some of the tilted blocks, as those of southern

Oregon, are as yet but moderately carved by erosion, and shallow

lakes lie on the waste that has been washed into the depressions

between them. We may therefore conclude that their displacement is

somewhat recent. Others, as those of Nevada, are so old that they

have been deeply dissected; their original form has been destroyed

by erosion, and the intermontane depressions are occupied by wide

plains of waste.

DISLOCATIONS AND RIVER VALLEYS. Before geologists had proved that

rivers can by their own unaided efforts cut deep canyons, it was

common to consider any narrow gorge as a gaping fissure of the

crust. This crude view has long since been set aside. A map of the

plateaus of northern Arizona shows how independent of the immense

faults of the region is the course of the Colorado River. In the

Alps the tunnels on the Saint Gotthard railway pass six times

beneath the gorge of the Reuss, but at no point do the rocks show

the slightest trace of a fault.

RATE OF DISLOCATION. So far as human experience goes, the earth

movements which we have just studied, some of which have produced

deep-sunk valleys and lofty mountain ranges, and faults whose

throw is to be measured in thousands of feet, are slow and

gradual. They are not accomplished by a single paroxysmal effort,

but by slow creep and a series of slight slips continued for vast

lengths of time.

In the Aspen mining district in Colorado faulting is now going on

at a comparatively rapid rate. Although no sudden slips take

place, the creep of the rock along certain planes of faulting

gradually bends out of shape the square-set timbers in horizontal

drifts and has closed some vertical shafts by shifting the upper

portion across the lower. Along one of the faults of this region

it is estimated that there has been a movement of at least four

hundred feet since the Glacial epoch. More conspicuous are the

instances of active faulting by means of sudden slips. In 1891

there occurred along an old fault plane in Japan a slip which

produced an earth rent traced for fifty miles (Fig. 192). The

country on one side was depressed in places twenty feet below that

on the other, and also shifted as much as thirteen feet

horizontally in the direction of the fault line.

In 1872 a slip occurred for forty miles on the great line of

dislocation which runs along the eastern base of the Sierra Nevada

Mountains. In the Owens valley, California, the throw amounted to

twenty-five feet in places, with a horizontal movement along the

fault line of as much as eighteen feet. Both this slip and that in

Japan just mentioned caused severe earthquakes.

For the sake of clearness we have described oscillations,

foldings, and fractures of the crust as separate processes, each

giving rise to its own peculiar surface features, but in nature

earth movements are by no means so simple,--they are often

implicated with one another: folds pass into faults; in a deformed

region certain rocks have bent, while others under the same

strain, but under different conditions of plasticity and load,

have broken; folded mountains have been worn to their roots, and

the peneplains to which they have been denuded have been upwarped

to mountain height and afterwards dissected,--as in the case of

the Alleghany ridges, the southern Carpathians, and other ranges,

--or, as in the case of the Sierra Nevada Mountains, have been

broken and uplifted as mountains of fracture.

Draw the following diagrams, being careful to show the direction

in which the faulted blocks have moved, by the position of the two

parts of some well-defined layer of limestone, sandstone, or

shale, which occurs on each side of the fault plane, as in Figure

184.

1. A normal fault with a hade of 15 degrees, the original fault

scarp remaining.

2. A normal fault with a hade of 50 degrees, the original fault

scarp worn away, showing cliffs caused by harder strata on the

downthrow side.

3. A thrust fault with a hade of 30 degrees, showing cliffs due to

harder strata outcropping on the downthrow.

4. A thrust fault with a hade of 80 degrees, with surface

baseleveled.

5. In a region of normal faults a coal mine is being worked along

the seam of coal AB (Fig. 193). At B it is found broken by a fault

f which hades toward A. To find the seam again, should you advise

tunneling up or down from B?

6. In a vertical shaft of a coal mine the same bed of coal is

pierced twice at different levels because of a fault. Draw a

diagram to show whether the fault is normal or a thrust.

7. Copy the diagram in Figure 194, showing how the two ridges may

be accounted for by a single resistant stratum dislocated by a

fault. Is the fault a STRIKE FAULT, i.e. one running parallel with

the strike of the strata, or a DIP FAULT, one running parallel

with the direction of the dip?

8. Draw a diagram of the block in Figure 195 as it would appear if

dislocated along the plane efg by a normal fault whose throw

equals one fourth the height of the block. Is the fault a strike

or a dip fault? Draw a second diagram showing the same block after

denudation has worn it down below the center of the upthrown side.

Note that the outcrop of the coal seam is now deceptively

repeated. This exercise may be done in blocks of wood instead of

drawings.

9. Draw diagrams showing by dotted lines the conditions both of A

and of B, Figure 196, after deformation had given the strata their

present attitude.

10. What is the attitude of the strata of this earth block, Figure

197? What has taken place along the plane bef? When did the

dislocation occur compared with the folding of the strata? With

the erosion of the valleys on the right-hand side of the mountain?

With the deposition of the sediments? Do you find any remnants of

the original surface baf produced by the dislocation? From the

left-hand side of the mountain infer what was the relief of the

region before the dislocation. Give the complete history recorded

in the diagram from the deposition of the strata to the present.

11. Which is the older fault, in Figure 198, or When did the lava

flow occur? How long a time elapsed between the formation of the

two faults as measured in the work done in the interval? How long

a time since the formation of the later fault?

12. Measure by the scale the thickness lie of the coal-bearing

strata outcropping from a to b in Figure 199. On any convenient

scale draw a similar section of strata with a dip of 30 degrees

outcropping along a horizontal line normal to the strike one

thousand feet in length, and measure the thickness of the strata

by the scale employed. The thickness may also be calculated by

trigonometry.

UNCONFORMITY

Strata deposited one upon, another in an unbroken succession are

said to be conformable. But the continuous deposition of strata is

often interrupted by movements of the earth's crust, Old sea

floors are lifted to form land and are again depressed beneath the

sea to receive a cover of sediments only after an interval during

which they were carved by subaerial erosion. An erosion surface

which thus parts older from younger strata is known as an

UNCONFORMITY, and the strata above it are said to be UNCONFORMABLE

with the rocks below, or to rest unconformably upon them. An

unconformity thus records movements of the crust and a consequent

break in the deposition of the strata. It denotes a period of land

erosion of greater or less length, which may sometimes be roughly

measured by the stage in the erosion cycle which the land surface

had attained before its burial. Unconformable strata may be

parallel, as in Figure 200, where the record includes the

deposition of strata, their emergence, the erosion of the land

surface, a submergence and the deposit of the strata, and lastly,

emergence and the erosion of the present surface.

Often the earth movements to which the uplift or depression was

due involved tilting or folding of the earlier strata, so that the

strata are now nonparallel as well as unconformable. In Figure

201, for example, the record includes deposition, uplift, and

tilting of a; erosion, depression, the deposit of b; and finally

the uplift which has brought the rocks to open air and permitted

the dissection by which the unconformity is revealed. From this

section infer that during early Silurian times the area was sea,

and thick sea muds were laid upon it. These were later altered to

hard slates by pressure and upfolded into mountains. During the

later Silurian and the Devonian the area was land and suffered

vast denudation. In the Carboniferous period it was lowered

beneath the sea and received a cover of limestone.

THE AGE OF MOUNTAINS. It is largely by means of unconformities

that we read the history of mountain making and other deformations

and movements of the crust. In Figure 203, for example, the

deformation which upfolded the range of mountains took place after

the deposit of the series of strata a of which the mountains are

composed, and before the deposit of the stratified rocks, which

rest unconformably on a and have not shared their uplift.

Most great mountain ranges, like the Sierra Nevada and the Alps,

mark lines of weakness along which the earth's crust has yielded

again and again during the long ages of geological time. The

strata deposited at various times about their flanks have been

infolded by later crumplings with the original mountain mass, and

have been repeatedly crushed, inverted, faulted, intruded with

igneous rocks, and denuded. The structure of great mountain ranges

thus becomes exceedingly complex and difficult to read. A

comparatively simple case of repeated uplift is shown in Figure

204. In the section of a portion of the Alps shown in Figure 179 a

far more complicated history may be deciphered.

UNCONFORMITIES IN THE COLORADO CANYON, ARIZONA. How geological

history may be read in unconformities is further illustrated in

Figures 207 and 208. The dark crystalline rocks a at the bottom of

the canyon are among the most ancient known, and are overlain

unconformably by a mass of tilted coarse marine sandstones b,

whose total thickness is not seen in the diagram and measures

twelve thousand feet perpendicularly to the dip. Both a and b rise

to a common level nn and upon them rest the horizontal sea-laid

strata c, in which the upper portion of the canyon has been cut.

Note that the crystalline rocks a have been crumpled and crushed.

Comparing their structure with that of folded mountains, what do

you infer as to their relief after their deformation? To which

surface were they first worn down, mm' or nm? Describe and account

for the surface mm'. How does it differ from the surface of the

crystalline rocks seen in the Torridonian Mountains, and why? This

surface mm' is one of the oldest land surfaces of which any

vestige remains.

It is a bit of fossil geography buried from view since the

earliest geological ages and recently brought to light by the

erosion of the canyon.

How did the surface mm' come to receive its cover of sandstones b?

From the thickness and coarseness of these sediments draw

inferences as to the land mass from which they were derived. Was

it rising or subsiding? high or low? Were its streams slow or

swift? Was the amount of erosion small or great?

Note the strong dip of these sandstones b. Was the surface mm'

tilted as now when the sandstones were deposited upon it? When was

it tilted? Draw a diagram showing the attitude of the rocks after

this tilting occurred, and their height relative to sea level.

The surface nn' is remarkably even, although diversified by some

low hills which rise into the bedded rocks of c, and it may be

traced for long distances up and down the canyon. Were the layers

of b and the surface mm' always thus cut short by nn' as now? What

has made the surface nn' so even? How does it come to cross the

hard crystalline rocks a and the weaker sandstones b at the same

impartial level? How did the sediments of c come to be laid upon

it? Give now the entire history recorded in the section, and in

addition that involved in the production of the platform P, shown

in Figure 130, and that of the cutting of the canyon. How does the

time involved in the cutting of the canyon compare with that

required for the production of the surfaces mm', nn', and P?

CHAPTER X

EARTHQUAKES

Any sudden movement of the rocks of the crust, as when they tear

apart when a fissure is formed or extended, or slip from time to

time along a growing fault, produces a jar called an earthquake,

which spreads in all directions from the place of disturbance.

THE CHARLESTON EARTHQUAKE. On the evening of August 31, 1886, the

city of Charleston, S.C., was shaken by one of the greatest

earthquakes which has occurred in the United States. A slight

tremor which rattled the windows was followed a few seconds later

by a roar, as of subterranean thunder, as the main shock passed

beneath the city. Houses swayed to and fro, and their heaving

floors overturned furniture and threw persons off their feet as,

dizzy and nauseated, they rushed to the doors for safety. In sixty

seconds a number of houses were completely wrecked, fourteen

thousand chimneys were toppled over, and in all the city scarcely

a building was left without serious injury. In the vicinity of

Charleston railways were twisted and trains derailed. Fissures

opened in the loose superficial deposits, and in places spouted

water mingled with sand from shallow underlying aquifers.

The point of origin, or FOCUS, of the earthquake was inferred from

subsequent investigations to be a rent in the rocks about twelve

miles beneath the surface. From the center of greatest

disturbance, which lay above the focus, a few miles northwest of

the city, the surface shock traveled outward in every direction,

with decreasing effects, at the rate of nearly two hundred miles

per minute. It was felt from Boston to Cuba, and from eastern Iowa

to the Bermudas, over a circular area whose diameter was a

thousand miles.

An earthquake is transmitted from the focus through the elastic

rocks of the crust, as a wave, or series of waves, of compression

and rarefaction, much as a sound wave is transmitted through the

elastic medium of the air. Each earth particle vibrates with

exceeding swiftness, but over a very short path. The swing of a

particle in firm rock seldom exceeds one tenth of an inch in

ordinary earthquakes, and when it reaches one half an inch and an

inch, the movement becomes dangerous and destructive.

The velocity of earthquake waves, like that of all elastic waves,

varies with the temperature and elasticity of the medium. In the

deep, hot, elastic rocks they speed faster than in the cold and

broken rocks near the surface. The deeper the point of origin and

the more violent the initial shock, the faster and farther do the

vibrations run.

Great earthquakes, caused by some sudden displacement or some

violent rending of the rocks, shake the entire planet. Their waves

run through the body of the earth at the rate of about three

hundred and fifty miles a minute, and more slowly round its

circumference, registering their arrival at opposite sides of the

globe on the exceedingly delicate instruments of modern earthquake

observatories.

GEOLOGICAL EFFECTS. Even great earthquakes seldom produce

geological effects of much importance. Landslides may be shaken

down from the sides of mountains and hills, and cracks may be

opened in the surface deposits of plains; but the transient

shiver, which may overturn cities and destroy thousands of human

lives, runs through the crust and leaves it much the same as

before.

EARTHQUAKES ATTENDING GREAT DISPLACEMENTS. Great earthquakes

frequently attend the displacement of large masses of the rocks of

the crust. In 1822 the coast of Chile was suddenly raised three or

four feet, and the rise was five or six feet a mile inland. In

1835 the same region was again upheaved from two to ten feet. In

each instance a destructive earthquake was felt for one thousand

miles along the coast.

THE GREAT CALIFORNIA EARTHQUAKE OF 1906. A sudden dislocation

occurred in 1906 along an ancient fault plane which extends for

300 miles through western California. The vertical displacement

did not exceed four feet, while the horizontal shifting reached a

maximum of twenty feet. Fences, rows of trees, and roads which

crossed the fault were broken and offset. The latitude and

longitude of all points over thousands of square miles were

changed. On each side of the fault the earth blocks moved in

opposite directions, the block on the east moving southward and

that on the west moving northward and to twice the distance. East

and west of the fault the movements lessened with increasing

distance from it.

This sudden slip set up an earthquake lasting sixty-five seconds,

followed by minor shocks recurring for many days. In places the

jar shook down the waste on steep hillsides, snapped off or

uprooted trees, and rocked houses from their foundations or threw

down their walls or chimneys. The water mains of San Francisco

were broken, and the city was thus left defenseless against a

conflagration which destroyed $500,000,000 worth of property. The

destructive effects varied with the nature of the ground.

Buildings on firm rock suffered least, while those on deep

alluvium were severely shaken by the undulations, like water

waves, into which the loose material was thrown. Well-braced steel

structures, even of the largest size, were earthquake proof, and

buildings of other materials, when honestly built and

intelligently designed to withstand earthquake shocks, usually

suffered little injury. The length of the intervals between severe

earthquakes in western California shows that a great dislocation

so relieves the stresses of the adjacent earth blocks that scores

of years may elapse before the stresses again accumulate and cause

another dislocation.

Perhaps the most violent earthquake which ever visited the United

States attended the depression, in 1812, of a region seventy-five

miles long and thirty miles wide, near New Madrid, Mo. Much of the

area was converted into swamps and some into shallow lakes, while

a region twenty miles in diameter was bulged up athwart the

channel of the Mississippi. Slight quakes are still felt in this

region from time to time, showing that the strains to which the

dislocation was due have not yet been fully relieved.

EARTHQUAKES ORIGINATING BENEATH THE SEA. Many earthquakes

originate beneath the sea, and in a number of examples they seem

to have been accompanied, as soundings indicate, by local

subsidences of the ocean bottom. There have been instances where

the displacement has been sufficient to set the entire Pacific

Ocean pulsating for many hours. In mid ocean the wave thus

produced has a height of only a few feet, while it may be two

hundred miles in width. On shores near the point of origin

destructive waves two or three score feet in height roll in, and

on coasts thousands of miles distant the expiring undulations may

be still able to record themselves on tidal gauges.

DISTRIBUTION OF EARTHQUAKES. Every half hour some considerable

area of the earth's surface is sensibly shaken by an earthquake,

but earthquakes are by no means uniformly distributed over the

globe. As we might infer from what we know as to their causes,

earthquakes are most frequent in regions now undergoing

deformation. Such are young rising mountain ranges, fault lines

where readjustments recur from time to time, and the slopes of

suboceanic depressions whose steepness suggests that subsidence

may there be in progress.

Earthquakes, often of extreme severity, frequently visit the lofty

and young ranges of the Andes, while they are little known in the

subdued old mountains of Brazil. The Highlands of Scotland are

crossed by a deep and singularly straight depression called the

Great Glen, which has been excavated along a very ancient line of

dislocation. The earthquakes which occur from time to time in this

region, such as the Inverness earthquake in 1891, are referred to

slight slips along this fault plane.

In Japan, earthquakes are very frequent. More than a thousand are

recorded every year, and twenty-nine world-shaking earthquakes

occurred in the three years ending with 1901. They originate, for

the most part, well down on the eastern flank of the earth fold

whose summit is the mountainous crest of the islands, and which

plunges steeply beneath the sea to the abyss of the Tuscarora

Deep.

MINOR CAUSES OF EARTHQUAKES. Since any concussion within the crust

sets up an earth jar, there are several minor causes of

earthquakes, such as volcanic explosions and even the collapse of

the roofs of caves. The earthquakes which attend the eruption of

volcanoes are local, even in the case of the most violent volcanic

paroxysms known. When the top of a volcano has been blown to

fragments, the accompanying earth shock has sometimes not been

felt more than twenty-five miles away.

DEPTH OF FOCUS. The focus of the Charleston earthquake, estimated

at about twelve miles below the surface, was exceptionally deep.

Volcanic earthquakes are particularly shallow, and probably no

earthquakes known have started at a greater depth than fifteen or

twenty miles. This distance is so slight compared with the earth's

radius that we may say that earthquakes are but skin-deep.

Should you expect the velocity of an earthquake to be greater in a

peneplain or in a river delta?

After an earthquake, piles on which buildings rested were found

driven into the ground, and chimneys crushed at base. From what

direction did the shock come?

Chimneys standing on the south walls of houses toppled over on the

roof. Should you infer that the shock in this case came from the

north or south?

How should you expect a shock from the east to affect pictures

hanging on the east and the west walls of a room? how the pictures

hanging on the north and the south walls?

In parts of the country, as in southwestern Wisconsin, slender

erosion pillars, or "monuments," are common. What inference could

you draw as to the occurrence in such regions of severe

earthquakes in the recent past?

CHAPTER XI

VOLCANOES

Connected with movements of the earth's crust which take place so

slowly that they can be inferred only from their effects is one of

the most rapid and impressive of all geological processes,--the

extrusion of molten rock from beneath the surface of the earth,

giving rise to all the various phenomena of volcanoes.

In a volcano, molten rock from a region deep below, which we may

call its reservoir, ascends through a pipe or fissure to the

surface. The materials erupted may be spread over vast areas, or,

as is commonly the case, may accumulate about the opening, forming

a conical pile known as the volcanic cone. It is to this cone that

popular usage refers the word VOLCANO; but the cone is simply a

conspicuous part of the volcanic mechanism whose still more

important parts, the reservoir and the pipe, are hidden from view.

Volcanic eruptions are of two types,--EFFUSIVE eruptions, in which

molten rock wells up from below and flows forth in streams of LAVA

(a comprehensive term applied to all kinds of rock emitted from

volcanoes in a molten state), and EXPLOSIVE eruptions, in which

the rock is blown out in fragments great and small by the

expansive force of steam.

ERUPTIONS OF THE EFFUSIVE TYPE

THE HAWAIIAN VOLCANOES. The Hawaiian Islands are all volcanic in

origin, and have a linear arrangement characteristic of many

volcanic groups in all parts of the world. They are strung along a

northwest-southeast line, their volcanoes standing in two parallel

rows as if reared along two adjacent lines of fracture or folding.

In the northwestern islands the volcanoes have long been extinct

and are worn low by erosion. In the southeastern island. Hawaii,

three volcanoes are still active and in process of building. Of

these Mauna Loa, the monarch of volcanoes, with a girth of two

hundred miles and a height of nearly fourteen thousand feet above

sea level, is a lava dome the slope of whose sides does not

average more than five degrees. On the summit is an elliptical

basin ten miles in circumference and several hundred feet deep.

Concentric cracks surround the rim, and from time to time the

basin is enlarged as great slices are detached from the vertical

walls and engulfed.

Such a volcanic basin, formed by the insinking of the top of the

cone, is called a CALDERA.

On the flanks of Mauna Loa, four thousand feet above sea level,

lies the caldera of Kilauea, an independent volcano whose dome has

been joined to the larger mountain by the gradual growth of the

two. In each caldera the floor, which to the eye is a plain of

black lava, is the congealed surface of a column of molten rock.

At times of an eruption lakes of boiling lava appear which may be

compared to air holes in a frozen river. Great waves surge up,

lifting tons of the fiery liquid a score of feet in air, to fall

back with a mighty plunge and roar, and occasionally the lava

rises several hundred feet in fountains of dazzling brightness.

The lava lakes may flood the floor of the basin, but in historic

times have never been known to fill it and overflow the rim.

Instead, the heavy column of lava breaks way through the sides of

the mountain and discharges in streams which flow down the

mountain slopes for a distance sometimes of as much as thirty-five

miles. With the drawing off of the lava the column in the duct of

the volcano lowers, and the floor of the caldera wholly or in part

subsides. A black and steaming abyss marks the place of the lava

lakes. After a time the lava rises in the duct, the floor is

floated higher, and the boiling lakes reappear.

The eruptions of the Hawaiian volcanoes are thus of the effusive

type. The column of lava rises, breaks through the side of the

mountain, and discharges in lava streams. There are no explosions,

and usually no earthquakes, or very slight ones, accompany the

eruptions. The lava in the calderas boils because of escaping

steam, but the vapor emitted is comparatively little, and seldom

hangs above the summits in heavy clouds. We see here in its

simplest form the most impressive and important fact in all

volcanic action, molten rock has been driven upward to the surface

from some deep-lying source.

LAVA FLOWS. As lava issues from the side of a volcano or overflows

from the summit, it flows away in a glowing stream resembling

molten iron drawn white-hot from an iron furnace. The surface of

the stream soon cools and blackens, and the hard crust of

nonconducting rock may grow thick and firm enough to form a

tunnel, within which the fluid lava may flow far before it loses

its heat to any marked degree. Such tunnels may at last be left as

caves by the draining away of the lava, and are sometimes several

miles in length.

PAHOEHOE AND AA. When the crust of highly fluid lava remains

unbroken after its first freezing, it presents a smooth, hummocky,

and ropy surface known by the Hawaiian term PAHOEHOE. On the other

hand, the crust of a viscid flow may be broken and splintered as

it is dragged along by the slowly moving mass beneath. The stream

then appears as a field of stones clanking and grinding on, with

here and there from some chink a dull red glow or a wisp of steam.

It sets to a surface called AA, of broken, sharp-edged blocks,

which is often both difficult and dangerous to traverse.

FISSURE ERUPTIONS. Some of the largest and most important outflows

of lava have not been connected with volcanic cones, but have been

discharged from fissures, flooding the country far and wide with

molten rock. Sheet after sheet of molten rock has been

successively outpoured, and there have been built up, layer upon

layer, plateaus of lava thousands of feet in thickness and many

thousands of square miles in area.

ICELAND. This island plateau has been rent from time to time by

fissures from which floods of lava have outpoured. In some

instances the lava discharges along the whole length of the

fissure, but more often only at certain points upon it. The Laki

fissure, twenty miles long, was in eruption in 1783 for seven

months. The inundation of fluid rock which poured from it is the

largest of historic record, reaching a distance of forty-seven

miles and covering two hundred and twenty square miles to an

average depth of a hundred feet. At the present time the fissure

is traced by a line of several hundred insignificant mounds of

fragmental materials which mark where the lava issued.

The distance to which the fissure eruptions of Iceland flow on

slopes extremely gentle is noteworthy. One such stream is ninety

miles in length, and another seventy miles long has a slope of

little more than one half a degree.

Where lava is emitted at one point and flows to a less distance

there is gradually built up a dome of the shape of an inverted

saucer with an immense base but comparatively low. Many LAVA DOMES

have been discovered in Iceland, although from their exceedingly

gentle slopes, often but two or three degrees, they long escaped

the notice of explorers.

The entire plateau of Iceland, a region as large as Ohio, is

composed of volcanic products,--for the most part of successive

sheets of lava whose total thickness falls little short of two

miles. The lava sheets exposed to view were outpoured in open air

and not beneath the sea; for peat bogs and old forest grounds are

interbedded with them, and the fossil plants of these vegetable

deposits prove that the plateau has long been building and is very

ancient. On the steep sea cliffs of the island, where its

structure is exhibited, the sheets of lava are seen to be cut with

many DIKES,--fissures which have been filled by molten rock,--and

there is little doubt that it was through these fissures that the

lava outwelled in successive flows which spread far and wide over

the country and gradually reared the enormous pile of the plateau.

ERUPTIONS OF THE EXPLOSIVE TYPE

In the majority of volcanoes the lava which rises in the pipe is

at least in part blown into fragments with violent explosions and

shot into the air together with vast quantities of water vapor and

various gases. The finer particles into--which the lava is

exploded are called VOLCANIC DUST or VOLCANIC ASHES, and are often

carried long distances by the wind before they settle to the

earth. The coarser fragments fall about the vent and there

accumulate in a steep, conical, volcanic mountain. As successive

explosions keep open the throat of the pipe, there remains on the

summit a cup-shaped depression called the CRATER.

STROMBOLI. To study the nature of these explosions we may visit

Stromboli, a low volcano built chiefly of fragmental materials,

which rises from the sea off the north coast of Sicily and is in

constant though moderate action.

Over the summit hangs a cloud of vapor which strikingly resembles

the column of smoke puffed from the smokestack of a locomotive, in

that it consists of globular masses, each the product of a

distinct explosion. At night the cloud of vapor is lighted with a

red glow at intervals of a few minutes, like the glow on the trail

of smoke behind the locomotive when from time to time the fire bos

is opened. Because of this intermittent light flashing thousands

of feet above the sea, Stromboli has been given the name of the

Lighthouse of the Mediterranean.

Looking down into the crater of the volcano, one sees a viscid

lava slowly seething. The agitation gradually increases. A great

bubble forms. It bursts with an explosion which causes the walls

of the crater to quiver with a miniature earthquake, and an

outrush of steam carries the fragments of the bubble aloft for a

thousand feet to fall into the crater or on the mountain side

about it. With the explosion the cooled and darkened crust of the

lava is removed, and the light of the incandescent liquid beneath

is reflected from the cloud of vapor which overhangs the cone.

At Stromboli we learn the lesson that the explosive force in

volcanoes is that of steam. The lava in the pipe is permeated with

it much as is a thick boiling porridge. The steam in boiling

porridge is unable to escape freely and gathers into bubbles which

in breaking spurt out drops of the pasty substance; in the same

way the explosion of great bubbles of steam in the viscid lava

shoots clots and fragments of it into the air.

KRAKATOA. The most violent eruption of history, that of Krakatoa,

a small volcanic island in the strait between Sumatra and Java,

occurred in the last week of August, 1883. Continuous explosions

shot a column of steam and ashes. seventeen miles in air. A black

cloud, beneath which was midnight darkness and from which fell a

rain of ashes and stones, overspread the surrounding region to a

distance of one hundred and fifty miles. Launched on the currents

of the upper air, the dust was swiftly carried westward to long

distances. Three days after the eruption it fell on the deck of a

ship sixteen hundred miles away, and in thirteen days the finest

impalpable powder from the volcano had floated round the globe.

For many months the dust hung over Europe and America as a faint

lofty haze illuminated at sunrise and sunset with brilliant

crimson. In countries nearer the eruption, as in India and Africa,

the haze for some time was so thick that it colored sun and moon

with blue, green, and copper-red tints and encircled them with

coronas.

At a distance of even a thousand miles the detonations of the

eruption sounded like the booming of heavy guns a few miles away.

In one direction they were audible for a distance as great as that

from San Francisco to Cleveland. The entire atmosphere was thrown

into undulations under which all barometers rose and fell as the

air waves thrice encircled the earth. The shock of the explosions

raised sea waves which swept round the adjacent shores at a height

of more than fifty feet, and which were perceptible halfway around

the globe.

At the close of the eruption it was found that half the mountain

had been blown away, and that where the central part of the island

had been the sea was a thousand feet deep.

MARTINIQUE AND ST. VINCENT. In 1902 two dormant volcanoes of the

West Indies, Mt. Pelee in Martinique and Soufriere in St. Vincent,

broke into eruption simultaneously. No lava was emitted, but there

were blown into the air great quantities of ashes, which mantled

the adjacent parts of the islands with a pall as of gray snow. In

early stages of the eruption lakes which occupied old craters were

discharged and swept down the ash-covered mountain valleys in

torrents of boiling mud.

On several occasions there was shot from the crater of each

volcano a thick and heavy cloud of incandescent ashes and steam,

which rushed down the mountain side like an avalanche, red with

glowing stones and scintillating with lightning flashes. Forests

and buildings in its path were leveled as by a tornado, wood was

charred and set on fire by the incandescent fragments, all

vegetation was destroyed, and to breathe the steam and hot,

suffocating dust of the cloud was death to every living creature.

On the morning of the 8th of May, 1902, the first of these

peculiar avalanches from Mt. Pelee fell on the city of St. Pierre

and instantly destroyed the lives of its thirty thousand

inhabitants.

The eruptions of many volcanoes partake of both the effusive and

the explosive types: the molten rock in the pipe is in part blown

into the air with explosions of steam, and in part is discharged

in streams of lava over the lip of the crater and from fissures in

the sides of the cone. Such are the eruptions of Vesuvius, one of

which is illustrated in Figure 219.

SUBMARINE ERUPTIONS. The many volcanic islands of the ocean and

the coral islands resting on submerged volcanic peaks prove that

eruptions have often taken place upon the ocean floor and have

there built up enormous piles of volcanic fragments and lava. The

Hawaiian volcanoes rise from a depth of eighteen thousand feet of

water and lift their heads to about thirty thousand feet above the

ocean bed. Christmas Island (see p. 194), built wholly beneath the

ocean, is a coral-capped volcanic peak, whose total height, as

measured from the bottom of the sea, is more than fifteen thousand

feet. Deep-sea soundings have revealed the presence of numerous

peaks which fail to reach sea level and which no doubt are

submarine volcanoes. A number of volcanoes on the land were

submarine in their early stages, as, for example, the vast pile of

Etna, the celebrated Sicilian volcano, which rests on stratified

volcanic fragments containing marine shells now uplifted from the

sea.

Submarine outflows of lava and deposits of volcanic fragments

become covered with sediments during the long intervals between

eruptions. Such volcanic deposits are said to be CONTEMPORANEOUS,

because they are formed during the same period as the strata among

which they are imbedded. Contemporaneous lava sheets may be

expected to bake the surface of the stratum on which they rest,

while the sediments deposited upon them are unaltered by their

heat. They are among the most permanent records of volcanic

action, far outlasting the greatest volcanic mountains built in

open air.

From upraised submarine volcanoes, such as Christmas Island, it is

learned that lava flows which are poured out upon the bottom of

the sea do not differ materially either in composition or texture

from those of the land.

VOLCANIC PRODUCTS

Vast amounts of steam are, as we have seen, emitted from

volcanoes, and comparatively small quantities of other vapors,

such as various acid and sulphurous gases. The rocks erupted from

volcanoes differ widely in chemical composition and in texture.

ACIDIC AND BASIC LAVAS. Two classes of volcanic rocks may be

distinguished,--those containing a large proportion of silica

(silicic acid, SiO2) and therefore called ACIDIC, and those

containing less silica and a larger proportion of the bases (lime,

magnesia, soda, etc.) and therefore called BASIC. The acidic

lavas, of which RHYOLITE and THRACHYTE are examples, are

comparatively light in color and weight, and are difficult to

melt. The basic lavas, of which BASALT is a type, are dark and

heavy and melt at a lower temperature.

SCORIA AND PUMICE. The texture of volcanic rocks depends in part

on the degree to which they were distended by the steam which

permeated them when in a molten state. They harden into compact

rock where the steam cannot expand. Where the steam is released

from pressure, as on the surface of a lava stream, it forms

bubbles (steam blebs) of various sizes, which give the hardened

rock a cellular structure (Fig. 220), In this way are formed the

rough slags and clinkers called SCORIA, which are found on the

surface of flows and which are also thrown out as clots of lava in

explosive eruptions.

On the surface of the seething lava in the throat of the volcano

there gathers a rock foam, which, when hurled into the air, is

cooled and falls as PUMICE,--a spongy gray rock so light that it

floats on water.

AMYGDULES. The steam blebs of lava flows are often drawn out from

a spherical to an elliptical form resembling that of an almond,

and after the rock has cooled these cavities are gradually filled

with minerals deposited from solution by underground water. From

their shape such casts are called amygdules (Greek, amygdalon, an

almond). Amygdules are commonly composed of silica. Lavas contain

both silica and the alkalies, potash and soda, and after

dissolving the alkalies, percolating water is able to take silica

also into solution. Most AGATES are banded amygdules in which the

silica has been laid in varicolored, concentric layers.

GLASSY AND STONY LAVAS. Volcanic rocks differ in texture according

also to the rate at which they have solidified. When rapidly

cooled, as on the surface of a lava flow, molten rock chills to a

glass, because the minerals of which it is composed have not had

time to separate themselves from the fused mixture and form

crystals. Under slow cooling, as in the interior of the flow, it

becomes a stony mass composed of crystals set in a glassy paste.

In thin slices of volcanic glass one may see under the microscope

the beginnings of crystal growth in filaments and needles and

feathery forms, which are the rudiments of the crystals of various

minerals.

Spherulites, which also mark the first changes of glassy lavas

toward a stony condition, are little balls within the rock,

varying from microscopic size to several inches in diameter, and

made up of radiating fibers.

Perlitic structure, common among glassy lavas, consists of

microscopic curving and interlacing cracks, due to contraction.

FLOW LINES are exhibited by volcanic rocks both to the naked eye

and under the microscope. Steam blebs, together with crystals and

their embryonic forms, are left arranged in lines and streaks by

the currents of the flowing lava as it stiffened into rock.

PORPHYRITIC STRUCTURE. Rocks whose ground mass has scattered

through it large conspicuous crystals are said to be PORPHYRITIC,

and it is especially among volcanic rocks that this structure

occurs. The ground mass of porphyries either may be glassy or may

consist in part of a felt of minute crystals; in either case it

represents the consolidation of the rock after its outpouring upon

the surface. On the other hand, the large crystals of porphyry

have slowly formed deep below the ground at an earlier date.

COLUMNAR STRUCTURE. Just as wet starch contracts on drying to

prismatic forms, so lava often contracts on cooling to a mass of

close-set, prismatic, and commonly six-sided columns, which stand

at right angles to the cooling surface. The upper portion of a

flow, on rapid cooling from the surface exposed to the air, may

contract to a confused mass of small and irregular prisms; while

the remainder forms large and beautifully regular columns, which

have grown upward by slow cooling from beneath.

FRAGMENTAL MATERIALS

Rocks weighing many tons are often thrown from a volcano at the

beginning of an outburst by the breaking up of the solidofied

floor of the crater; and during the progress of an eruption large

blocks may be torn from the throat of the volcano by the outrush

of steam. But the most important fragmental materials are those

derived from the lava itself. As lava rises in the pipe, the steam

which permeates it is released from pressure and explodes, hurling

the lava into the air in fragments of all sizes,--large pieces of

scoria, LAPILLI (fragments the size of a pea or walnut), volcanic

"sand" and volcanic "ashes." The latter resemble in appearance the

ashes of wood or coal, but they are not in any sense, like them, a

residue after combustion.

Volcanic ashes are produced in several ways: lava rising in the

volcanic duct is exploded into fine dust by the steam which

permeates it; glassy lava, hurled into the air and cooled

suddenly, is brought into a state of high strain and tension, and,

like Prince Rupert's drops, flies to pieces at the least

provocation. The clash of rising and falling projectiles also

produces some dust, a fair sample of which may be made by grating

together two pieces of pumice.

Beds of volcanic ash occur widely among recent deposits in the

western United States. In Nebraska ash beds are found in twenty

counties, and are often as white as powdered pumice. The beds grow

thicker and coarser toward the southwestern part of the state,

where their thickness sometimes reaches fifty feet. In what

direction would you look for the now extinct volcano whose

explosive eruptions are thus recorded?

TUFF. This is a convenient term designating any rock composed of

volcanic fragments. Coarse tuffs of angular fragments are called

VOLCANIC BRECIA, and when the fragments have been rounded and

sorted by water the rock is termed a VOLCANIC CONGLOMERATE. Even

when deposited in the open air, as on the slopes of a volcano,

tuffs may be rudely bedded and their fragments more or less

rounded, and unless marine shells or the remains of land plants

and animals are found as fossils in them, there is often

considerable difficulty in telling whether they were laid in water

or in air. In either case they soon become consolidated. Chemical

deposits from percolating waters fill the interstices, and the bed

of loose fragments is cemented to hard rock.

The materials of which tuffs are composed are easily recognized as

volcanic in their origin. The fragments are more or less cellular,

according to the degree to which they were distended with steam

when in a molten state, and even in the finest dust one may see

the glass or the crystals of lava from which it was derived. Tuffs

often contain VOCLANIC BOMBS,--balls of lava which took shape

while whirling in the air, and solidified before falling to the

ground.

ANCIENT VOLCANIC ROCKS. It is in these materials and structures

which we have described that volcanoes leave some of their most

enduring records. Even the volcanic rocks of the earliest geological

ages, uplifted after long burial beneath the sea and exposed to view

by deep erosion, are recognized and their history read despite the

many changes which they may have undergone. A sheet of ancient lava

may be distinguished by its composition from the sediments among

which it is imbedded. The direction of its flow lines may be noted.

The cellular and slaggy surface where the pasty lava was distended

by escaping steam is recognized by the amygdules which now fill the

ancient steam blebs. In a pile of successive sheets of lava each

flow may be distinguished and its thickness measured; for the

surface of each sheet is glassy and scoriaceous, while beneath its

upper portions the lava of each flow is more dense and stony. The

length of time which elapsed before a sheet was buried beneath the

materials of succeeding eruptions may be told by the amount of

weathering which it had undergone, the depth of ancient soil--now

baked to solid rock--upon it, and the erosion which it had suffered

in the interval.

If the flow occurred from some submarine volcano, we may recognize

the fact by the sea-laid sediments which cover it, filling the

cracks and crevices of its upper surface and containing pieces of

lava washed from it in their basal layers.

Long-buried glassy lavas devitrify, or pass to a stony condition,

under the unceasing action of underground waters; but their flow

lines and perlitic and spherulitic structures remain to tell of

their original state.

Ancient tuffs are known by the fragmental character of their

volcanic material, even though they have been altered to firm

rock. Some remains of land animals and plants may be found

imbedded to tell that the beds were laid in open air; while the

remains of marine organisms would prove as surely that the tuffs

were deposited in the sea.

In these ways ancient volcanoes have been recognized near Boston,

in southeastern Pennsylvania, about Lake Superior, and in other

regions of the United States.

THE LIFE HISTORY OF A VOLCANO

The invasion of a region by volcanic forces is attended by

movements of the crust heralded by earthquakes. A fissure or a

pipe is opened and the building of the cone or the spreading of

wide lava sheets is begun.

VOLCANIC CONES. The shape of a volcanic cone depends chiefly on

the materials erupted. Cones made of fragments may have sides as

steep as the angle of repose, which in the case of coarse scoria

is sometimes as high as thirty or forty degrees. About the base of

the mountain the finer materials erupted are spread in more gentle

slopes, and are also washed forward by rains and streams. The

normal profile is thus a symmetric cone with a flaring base.

Cones built of lava vary in form according to the liquidity of the

lava. Domes of gentle slope, as those of Hawaii, for example, are

formed of basalt, which flows to long distances before it

congeals. When superheated and emitted from many vents, this

easily melted lava builds great plateaus, such as that of Iceland.

On the other hand, lavas less fusible, or poured out at a lower

temperature, stiffen when they have flowed but a short distance,

and accumulate in a steep cone. Trachyte has been extruded in a

state so viscid that it has formed steepsided domes like that of

Sarcoui.

Most volcanoes are built, like Vesuvius, both of lava flows and of

tuffs, and sections show that the structure of the cone consists

of outward-dipping, alternating layers of lava, scoria, and ashes.

From time to time the cone is rent by the violence of explosions

and by the weight of the column of lava in the pipe. The fissures

are filled with lava and some discharge on the sides of the

mountain, building parasitic cones, while all form dikes, which

strengthen the pile with ribs of hard rock and make it more

difficult to rend.

Great catastrophes are recorded in the shape of some volcanoes

which consist of a circular rim perhaps miles in diameter,

inclosing a vast crater or a caldera within which small cones may

rise. We may infer that at some time the top of the mountain has

been blown off, or has collapsed and been engulfed because some

reservoir beneath had been emptied by long-continued eruptions.

The cone-building stage may be said to continue until eruptions of

lava and fragmental materials cease altogether. Sooner or later

the volcanic forces shift or die away, and no further eruptions

add to the pile or replace its losses by erosion during periods of

repose. Gases however are still emitted, and, as sulphur vapors

are conspicuous among them, such vents are called SOLFATARAS.

Mount Hood, in Oregon, is an example of a volcano sunk to this

stage. From a steaming rift on its side there rise sulphurous

fumes which, half a mile down the wind, will tarnish a silver

coin.

GEYSERS AND HOT SPRINGS. The hot springs of volcanic regions are

among the last vestiges of volcanic heat. Periodically eruptive

boiling springs are termed geysers. In each of the geyser regions

of the earth--the Yellowstone National Park, Iceland, and New

Zealand--the ground water of the locality is supposed to be heated

by ancient lavas that, because of the poor conductivity of the

rock, still remain hot beneath the surface.

OLD FAITHFUL, one of the many geysers of the Yellowstone National

Park, plays a fountain of boiling water a hundred feet in air;

while clouds of vapor from the escaping steam ascend to several

times that height. The eruptions take place at intervals of from

seventy to ninety minutes. In repose the geyser is a quiet pool,

occupying a craterlike depression in a conical mound some twelve

feet high. The conduit of the spring is too irregular to be

sounded. The mound is composed of porous silica deposited by the

waters of the geyser.

Geysers erupt at intervals instead of continuously boiling,

because their long, narrow, and often tortuous conduits do not

permit a free circulation of the water. After an eruption the tube

is refilled and the water again gradually becomes heated. Deep in

the tube where it is in contact with hot lavas the water sooner or

later reaches the boiling point, and bursting into steam shoots

the water above it high in air.

CARBONATED SPRINGS. After all the other signs of life have gone,

the ancient volcano may emit carbon dioxide as its dying breath.

The springs of the region may long be charged with carbon dioxide,

or carbonated, and where they rise through limestone may be

expected to deposit large quantities of travertine. We should

remember, however, that many carbonated springs, and many hot

springs, are wholly independent of volcanoes.

THE DESTRUCTION OF THE CONE. As soon as the volcanic cone ceases

to grow by eruptions the agents of erosion begin to wear it down,

and the length of time that has elapsed since the period of active

growth may be roughly measured by the degree to which the cone has

been dissected. We infer that Mount Shasta, whose conical shape is

still preserved despite the gullies one thousand feet deep which

trench its sides, is younger than Mount Hood, which erosive

agencies have carved to a pyramidal form. The pile of materials

accumulated about a volcanic vent, no matter how vast in bulk, is

at last swept entirely away. The cone of the volcano, active or

extinct, is not old as the earth counts time; volcanoes are short-

lived geological phenomena.

CRANDALL VOLCANO. This name is given to a dissected ancient

volcano in the Yellowstone National Park, which once, it is

estimated, reared its head thousands of feet above the surrounding

country and greatly exceeded in bulk either Mount Shasta or Mount

Etna. Not a line of the original mountain remains; all has been

swept away by erosion except some four thousand feet of the base

of the pile. This basal wreck now appears as a rugged region about

thirty miles in diameter, trenched by deep valleys and cut into

sharp peaks and precipitous ridges. In the center of the area is

found the nucleus (N, Fig. 237),--a mass of coarsely crystalline

rock that congealed deep in the old volcanic pipe. From it there

radiate in all directions, like the spokes of a wheel, long dikes

whose rock grows rapidly finer of grain as it leaves the vicinity

of the once heated core. The remainder of the base of the ancient

mountain is made of rudely bedded tuffs and volcanic breccia, with

occasional flows of lava, some of the fragments of the breccia

measuring as much as twenty feet in diameter. On the sides of

canyons the breccia is carved by rain erosion to fantastic

pinnacles. At different levels in the midst of these beds of tuff

and lava are many old forest grounds. The stumps and trunks of the

trees, now turned to stone, still in many cases stand upright

where once they grew on the slopes of the mountain as it was

building (Fig. 238). The great size and age of some of these trees

indicate, the lapse of time between the eruption whose lavas or

tuffs weathered to the soil on which they grew and the subsequent

eruption which buried them beneath showers of stones and ashes.

Near the edge of the area lies Death Gulch, in which carbon

dioxide is given off in such quantities that in quiet weather it

accumulates in a heavy layer along the ground and suffocates the

animals which may enter it.

CHAPTER XII

UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN

It is because long-continued erosion lays bare the innermost

anatomy of an extinct volcano, and even sweeps away the entire

pile with much of the underlying strata, thus leaving the very

roots of the volcano open to view, that we are able to study

underground volcanic structures. With these we include, for

convenience, intrusions of molten rock which have been driven

upward into the crust, but which may not have succeeded in

breaking way to the surface and establishing a volcano. All these

structures are built of rock forced when in a fluid or pasty state

into some cavity which it has found or made, and we may classify

them therefore, according to the shape of the molds in which the

molten rock has congealed, as (1) dikes, (2) volcanic necks, (3)

intrusive sheets, and (4) intrusive masses.

DIKES. The sheet of once molten rock with which a fissure has been

filled is known as a dike. Dikes are formed when volcanic cones

are rent by explosions or by the weight of the lava column in the

duct, and on the dissection of the pile they appear as radiating

vertical ribs cutting across the layers of lava and tuff of which

the cone is built. In regions undergoing deformation rocks lying

deep below the ground are often broken and the fissures are filled

with molten rock from beneath, which finds no outlet to the

surface. Such dikes are common in areas of the most ancient rocks,

which have been brought to light by long erosion.

In exceptional cases dikes may reach the length of fifty or one

hundred miles. They vary in width from a fraction of a foot to

even as much as three hundred feet.

Dikes are commonly more fine of grain on the sides than in the

center, and may have a glassy and crackled surface where they meet

the inclosing rock. Can you account for this on any principle

which you have learned?

VOLCANIC NECKS. The pipe of a volcano rises from far below the

base of the cone,--from the deep reservoir from which its

eruptions are supplied. When the volcano has become extinct this

great tube remains filled with hardened lava. It forms a

cylindrical core of solid rock, except for some distance below the

ancient crater, where it may contain a mass of fragments which had

fallen back into the chimney after being hurled into the air.

As the mountain is worn down, this central column known as the

VOLCANIC NECK is left standing as a conical hill (Fig. 240). Even

when every other trace of the volcano has been swept away, erosion

will not have passed below this great stalk on which the volcano

was borne as a fiery flower whose site it remains to mark. In

volcanic regions of deep denudation volcanic necks rise solitary

and abrupt from the surrounding country as dome-shaped hills. They

are marked features in the landscape in parts of Scotland and in

the St. Lawrence valley about Montreal (Fig. 241).

INTRUSIVE SHEETS. Sheets of igneous rocks are sometimes found

interleaved with sedimentary strata, especially in regions where

the rocks have been deformed and have suffered from volcanic

action. In some instances such a sheet is seen to be

CONTEMPORANEOUS (p. 248). In other instances the sheet must be

INTRUSIVE. The overlying stratum, as well as that beneath, has

been affected by the heat of the once molten rock. We infer that

the igneous rock when in a molten state was forced between the

strata, much as a card may be pushed between the leaves of a

closed book. The liquid wedged its way between the layers, lifting

those above to make room for itself. The source of the intrusive

sheet may often be traced to some dike (known therefore as the

FEEDING DIKE), or to some mass of igneous rock.

Intrusive sheets may extend a score and more of miles, and, like

the longest surface flows, the most extensive sheets consist of

the more fusible and fluid lavas,--those of the basic class of

which basalt is an example. Intrusive sheets are usually harder

than the strata in which they lie and are therefore often left in

relief after long denudation of the region (Fig. 315).

On the west bank of the Hudson there extends from New York Bay

north for thirty miles a bold cliff several hundred feet high,--

the PALISADES OF THE HUDSON. It is the outcropping edge of a sheet

of ancient igneous rock, which rests on stratified sandstones and

is overlain by strata of the same series. Sandstones and lava

sheet together dip gently to the west arid the latter disappears

from view two miles back from the river.

It is an interesting question whether the Palisades sheet is

CONTEMPORANEOUS or INTRUSIVE. Was it outpoured on the sandstones

beneath it when they formed the floor of the sea, and covered

forthwith by the sediments of the strata above, or was it intruded

among these beds at a later date?

The latter is the case: for the overlying stratum is intensely

baked along the zone of contact. At the west edge of the sheet is

found the dike in which the lava rose to force its way far and

wide between the strata.

ELECTRIC PEAK, one of the prominent mountains of the Yellowstone

National Park, is carved out of a mass of strata into which many

sheets of molten rock have been intruded. The western summit

consists of such a sheet several hundred feet thick. Studying the

section of Figure 244, what inference do you draw as to the source

of these intrusive sheets?

INTRUSIVE MASSES

BOSSES. This name is generally applied to huge irregular masses of

coarsely crystalline igneous rock lying in the midst of other

formations. Bosses vary greatly in size and may reach scores of

miles in extent. Seldom are there any evidences found that bosses

ever had connection with the surface. On the other hand, it is

often proved that they have been driven, or have melted their way,

upward into the formations in which they lie; for they give off

dikes and intrusive sheets, and have profoundly altered the rocks

about them by their heat.

The texture of the rock of bosses proves that consolidation

proceeded slowly and at great depths, and it is only because of

vast denudation that they are now exposed to view. Bosses are

commonly harder than the rocks about them, and stand up,

therefore, as rounded hills and mountainous ridges long after the

surrounding country has worn to a low plain.

The base of bosses is indefinite or undetermined, and in this

respect they differ from laccoliths. Some bosses have broken and

faulted the overlying beds; some have forced the rocks aside and

melted them away.

The SPANISH PEAKS of southeastern Colorado were formed by the

upthrust of immense masses of igneous rock, bulging and breaking

the overlying strata. On one side of the mountains the throw of

the fault is nearly a mile, and fragments of deep-lying beds were

dragged upward by the rising masses. The adjacent rocks were

altered by heat to a distance of several thousand feet. No

evidence appears that the molten rock ever reached the surface,

and if volcanic eruptions ever took place either in lava flows or

fragmental materials, all traces of them have been effaced. The

rock of the intrusive masses is coarsely crystalline, and no doubt

solidified slowly under the pressure of vast thicknesses of

overlying rock, now mostly removed by erosion.

A magnificent system of dikes radiates from the Peaks to a

distance of fifteen miles, some now being left by long erosion as

walls a hundred feet in height (Fig. 239). Intrusive sheets fed by

the dikes penetrate the surrounding strata, and their edges are

cut by canyons as much as twenty-five miles from the mountain. In

these strata are valuable beds of lignite, an imperfect coal,

which the heat of dikes and sheets has changed to coke.

LACCOLITHS. The laccolith (Greek laccos, cistern; lithos, stone)

is a variety of intrusive masses in which molten rock has spread

between the strata, and, lifting the strata above it to a dome-

shaped form, has collected beneath them in a lens-shaped body with

a flat base.

The HENRY MOUNTAINS, a small group of detached peaks in southern

Utah, rise from a plateau of horizontal rocks. Some of the peaks

are carved wholly in separate domelike uplifts of the strata of

the plateau. In others, as Mount Hillers, the largest of the

group, there is exposed on the summit a core of igneous rock from

which the sedimentary rocks of the flanks dip steeply outward in

all directions. In still others erosion has stripped off the

covering strata and has laid bare the core to its base; and its

shape is here seen to be that of a plano-convex lens or a baker's

bun, its flat base resting on the undisturbed bedded rocks

beneath. The structure of Mount Hillers is shown in Figure 248.

The nucleus of igneous rock is four miles in diameter and more

than a mile in depth.

REGIONAL INTRUSIONS. These vast bodies of igneous rock, which may

reach hundreds of miles in diameter, differ little from bosses

except in their immense bulk. Like bosses, regional intrusions

give off dikes and sheets and greatly change the rocks about them

by their heat. They are now exposed to view only because of the

profound denudation which has removed the upheaved dome of rocks

beneath which they slowly cooled. Such intrusions are accompanied

--whether as cause or as effect is still hardly known--by

deformations, and their masses of igneous rock are thus found as

the core of many great mountain ranges. The granitic masses of

which the Bitter Root Mountains and the Sierra Nevadas have been

largely carved are each more than three hundred miles in length.

Immense regional intrusions, the cores of once lofty mountain

ranges, are found upon the Laurentian peneplain.

PHYSIOGRAPHIC EFFECTS OF INTRUSIVE MASSES. We have already seen

examples of the topographic effects of intrusive masses in Mount

Hillers, the Spanish Peaks, and in the great mountain ranges

mentioned in the paragraph on regional intrusions, although in the

latter instances these effects are entangled with the effects of

other processes. Masses of igneous rock cannot be intruded within

the crust without an accompanying deformation on a scale

corresponding to the bulk of the intruded mass. The overlying

strata are arched into hills or mountains, or, if the molten

material is of great extent, the strata may conceivably be floated

upward to the height of a plateau. We may suppose that the

transference of molten matter from one region to another may be

among the causes of slow subsidences and elevations. Intrusions

give rise to fissures, dikes, and intrusive sheets, and these

dislocations cannot fail to produce earthquakes. Where intrusive

masses open communication with the surface, volcanoes are

established or fissure eruptions occur such as those of Iceland.

THE INTRUSIVE ROCKS

The igneous rocks are divided into two general classes,--the

VOLCANIC or ERUPTIVE rocks, which have been outpoured in open air

or on the floor of the sea, and the INTRUSIVE rocks, which have

been intruded within the rocks of the crust and have solidified

below the surface. The two classes are alike in chemical

composition and may be divided into acidic and basic groups. In

texture the intrusive rocks differ from the volcanic rocks because

of the different conditions under which they have solidified. They

cooled far more slowly beneath the cover of the rocks into which

they were pressed than is permitted to lava flows in open air.

Their constituent minerals had ample opportunity to sort

themselves and crystallize from the fluid mixture, and none of

that mixture was left to congeal as a glassy paste.

They consolidated also under pressure. They are never scoriaceous,

for the steam with which they were charged was not allowed to

expand and distend them with steam blebs. In the rocks of the

larger intrusive masses one may see with a powerful microscope

exceedingly minute cavities, to be counted by many millions to the

cubic inch, in which the gaseous water which the mass contained

was held imprisoned under the immense pressure of the overlying

rocks.

Naturally these characteristics are best developed in the

intrusives which cooled most slowly, i.e. in the deepest-seated

and largest masses; while in those which cooled more rapidly, as

in dikes and sheets, we find gradations approaching the texture of

surface flows.

VARIETIES OF THE INTRUSIVE ROCKS. We will now describe a few of

the varieties of rocks of deep-seated intrusions. All are even

grained, consisting of a mass of crystalline grains formed during

one continuous stage of solidification, and no porphyritic

crystals appear as in lavas.

GRANITE, as we have learned already, is composed of three

minerals,--quartz, feldspar, and mica. According to the color of

the feldspar the rock may be red, or pink, or gray. Hornblende--a

black or dark green mineral, an iron-magnesian silicate, about as

hard as feldspar--is sometimes found as a fourth constituent, and

the rock is then known as HORNBLENDIC GRANITE. Granite is an

acidic rock corresponding to rhyolite in chemical composition. We

may believe that the same molten mass which supplies this acidic

lava in surface flows solidifies as granite deep below ground in

the volcanic reservoir.

SYENITE, composed of feldspar and mica, has consolidated from a

less siliceous mixture than has granite.

DIORITE, still less siliceous, is composed of hornblende and

feldspar,--the latter mineral being of different variety from the

feldspar of granite and syenite.

GABBRO, a typical basic rock, corresponds to basalt in chemical

composition. It is a dark, heavy, coarsely crystalline aggregate

of feldspar and AUGITE (a dark mineral allied to hornblende). It

often contains MAGNETITE (the magnetic black oxide of iron) and

OLIVINE (a greenish magnesian silicate).

In the northern states all these types, and many others also of

the vast number of varieties of intrusive rocks, can be found

among the rocks of the drift brought from the areas of igneous

rock in Canada and the states of our northern border.

SUMMARY. The records of geology prove that since the earliest of

their annals tremendous forces have been active in the earth. In

all the past, under pressures inconceivably great, molten rock has

been driven upward into the rocks of the crust. It has squeezed

into fissures forming dikes; it has burrowed among the strata as

intrusive sheets; it has melted the rocks away or lifted the

overlying strata, filling the chambers which it has made with

intrusive masses. During all geological ages molten rock has found

way to the surface, and volcanoes have darkened the sky with

clouds of ashes and poured streams of glowing lava down their

sides. The older strata,--the strata which have been most deeply

buried,--and especially those which have suffered most from

folding and from fracture, show the largest amount of igneous

intrusions. The molten rock which has been driven from the earth's

interior to within the crust or to the surface during geologic

time must be reckoned in millions of cubic miles.

THE INTERIOR CONDITION OF THE EARTH AND CAUSES OF VULCANISM AND

DEFORMATION

The problems of volcanoes and of deformation are so closely

connected with that of the earth's interior that we may consider

them together. Few of these problems are solved, and we may only

state some known facts and the probable conclusions which may be

drawn as inferences from them.

THE INTERIOR OF THE EARTH IS HOT. Volcanoes prove that in many

parts of the earth there exist within reach of the surface regions

of such intense heat that the rock is in a molten condition. Deep

wells and mines show everywhere an increase in temperature below

the surface shell affected by the heat of summer and the cold of

winter,--a shell in temperate latitudes sixty or seventy feet

thick. Thus in a boring more than a mile deep at Schladebach,

Germany, the earth grows warmer at the rate of 1 degrees F. for

every sixty-seven feet as we descend. Taking the average rate of

increase at one degree for every sixty feet of descent, and

assuming that this rate, observed at the moderate distances open

to observation, continues to at least thirty-five miles, the

temperature at that depth must be more than three thousand

degrees,--a temperature at which all ordinary rocks would melt at

the earth's surface. The rate of increase in temperature probably

lessens as we go downward, and it may not be appreciable below a

few hundred miles. But there is no reason to doubt that THE

INTERIOR OF THE EARTH IS INTENSELY HOT. Below a depth of one or

two score miles we may imagine the rocks everywhere glowing with

heat.

Although the heat of the interior is great enough to melt all

rocks at atmospheric pressure, it does not follow that the

interior is fluid. Pressure raises the fusing point of rocks, and

the weight of the crust may keep the interior in what may be

called a solid state, although so hot as to be a liquid or a gas

were the pressure to be removed.

THE INTERIOR OF THE EARTH IS RIGID AND HEAVY. The earth behaves as

a globe more rigid than glass under the attractions of the sun and

moon. It is not deformed by these stresses as is the ocean in the

tides, proving that it is not a fluid ball covered with a yielding

crust a few miles thick. Earthquakes pass through the earth faster

than they would were it of solid steel. Hence the rocks of the

interior are highly elastic, being brought by pressure to a

compact, continuous condition unbroken by the cracks and vesicles

of surface rocks. THE INTERIOR OF THE EARTH IS RIGID

The common rocks of the crust are about two and a half times

heavier than water, while the earth as a whole weighs five and

six-tenths times as much as a globe of water of the same size. THE

INTERIOR IS THEREFORE MUCH MORE HEAVY THAN THE CRUST. This may be

caused in part by compression of the interior under the enormous

weight of the crust, and in part also by an assortment of

material, the heavier substances, such as the heavy metals, having

gravitated towards the center.

Between the crust, which is solid because it is cool, and the

interior, which is hot enough to melt were it not for the pressure

which keeps it dense and rigid, there may be an intermediate zone

in which heat and pressure are so evenly balanced that here rock

liquefies whenever and wherever the pressure upon it may be

relieved by movements of the crust. It is perhaps from such a

subcrustal layer that the lava of volcanoes is supplied.

THE CAUSES OF VOLCANIC ACTION. It is now generally believed that

the HEAT of volcanoes is that of the earth's interior. Other

causes, such as friction and crushing in the making of mountains

and the chemical reactions between oxidizing agents of the crust

and the unoxidized interior, have been suggested, but to most

geologists they seem inadequate.

There is much difference of opinion as to the FORCE which causes

molten rock to rise to the surface in the ducts of volcanoes.

Steam is so evidently concerned in explosive eruptions that many

believe that lava is driven upward by the expansive force of the

steam with which it is charged, much as a viscid liquid rises and

boils over in a test tube or kettle.

But in quiet eruptions, and still more in the irruption of

intrusive sheets and masses, there is little if any evidence that

steam is the driving force. It is therefore believed by many

geologists that it is PRESSURE DUE TO CRUSTAL MOVEMENTS AND

INTERNAL STRESSES which squeezes molten rock from below into

fissures and ducts in the crust. It is held by some that where

considerable water is supplied to the rising column of lava, as

from the ground water of the surrounding region, and where the

lava is viscid so that steam does not readily escape, the eruption

is of the explosive type; when these conditions do not obtain, the

lava outwells quietly, as in the Hawaiian volcanoes. It is held by

others not only that volcanoes are due to the outflow of the

earth's deep-seated heat, but also that the steam and other

emitted gases are for the most part native to the earth's interior

and never have had place in the circulation of atmospheric and

ground waters.

VOLCANIC ACTION AND DEFORMATION. Volcanoes do not occur on wide

plains or among ancient mountains. On the other hand, where

movements of the earth's crust are in progress in the uplift of

high plateaus, and still more in mountain making, molten rock may

reach the surface, or may be driven upward toward it forming great

intrusive masses. Thus extensive lava flows accompanied the

upheaval of the block mountains of western North America and the

uplift of the Colorado plateau. A line of recent volcanoes may be

traced along the system of rift valleys which extends from the

Jordan and Dead Sea through eastern Africa to Lake Nyassa. The

volcanoes of the Andes show how conspicuous volcanic action may be

in young rising ranges. Folded mountains often show a core of

igneous rock, which by long erosion has come to form the axis and

the highest peaks of the range, as if the molten rock had been

squeezed up under the rising upfolds. As we decipher the records

of the rocks in historical geology we shall see more fully how, in

all the past, volcanic action has characterized the periods of

great crustal movements, and how it has been absent when and where

the earth's crust has remained comparatively at rest.

THE CAUSES OF DEFORMATION. As the earth's interior, or nucleus, is

highly heated it must be constantly though slowly losing its heat

by conduction through the crust and into space; and since the

nucleus is cooling it must also be contracting. The nucleus has

contracted also because of the extrusion of molten matter, the

loss of constituent gases given off in volcanic eruptions, and

(still more important) the compression and consolidation of its

material under gravity. As the nucleus contracts, it tends to draw

away from the cooled and solid crust, and the latter settles,

adapting itself to the shrinking nucleus much as the skin of a

withering apple wrinkles down upon the shrunken fruit. The

unsupported weight of the spherical crust develops enormous

tangential pressures, similar to the stresses of an arch or dome,

and when these lateral thrusts accumulate beyond the power of

resistance the solid rock is warped and folded and broken.

Since the planet attained its present mass it has thus been

lessening in volume. Notwithstanding local and relative upheavals

the earth's surface on the whole has drawn nearer and nearer to

the center. The portions of the lithosphere which have been

carried down the farthest have received the waters of the oceans,

while those portions which have been carried down the least have

emerged as continents.

Although it serves our convenience to refer the movements of the

crust to the sea level as datum plane, it is understood that this

level is by no means fixed. Changes in the ocean basins increase

or reduce their capacity and thus lower or raise the level of the

sea. But since these basins are connected, the effect of any

change upon the water level is so distributed that it is far less

noticeable than a corresponding change would be upon the land.

CHAPTER XIII

METAMORPHISM AND MINERAL VEINS

Under the action of internal agencies rocks of all kinds may be

rendered harder, more firmly cemented, and more crystalline. These

processes are known as METAMORPHISM, and the rocks affected,

whether originally sedimentary or igneous, are called METAMORPHIC

ROCKS. We may contrast with metamorphism the action of external

agencies in weathering, which render rocks less coherent by

dissolving their soluble parts and breaking down their crystalline

grains.

CONTACT METAMORPHISM. Rocks beneath a lava flow or in contact with

igneous intrusions are found to be metamorphosed to various

degrees by the heat of the cooling mass. The adjacent strata may

be changed only in color, hardness, and texture. Thus, next to a

dike, bituminous coal may be baked to coke or anthracite, and

chalk and limestone to crystalline marble. Sandstone may be

converted into quartzite, and shale into ARGILLITE, a compact,

massive clay rock. New minerals may also be developed. In

sedimentary rocks there may be produced crystals of mica and of

GARNET (a mineral as hard as quartz, commonly occurring in red,

twelve-sided crystals). Where the changes are most profound, rocks

may be wholly made over in structure and mineral composition.

In contact metamorphism, thin sheets of molten rock produce less

effect than thicker ones. The strongest heat effects are naturally

caused by bosses and regional intrusions, and the zone of change

about them may be several miles in width. In these changes heated

waters and vapors from the masses of igneous rocks undoubtedly

play a very important part.

Which will be more strongly altered, the rocks about a closed dike

in which lava began to cool as soon as it filled the fissure, or

the rocks about a dike which opened on the surface and through

which the molten rock flowed for some time?

Taking into consideration the part played by heated waters, which

will produce the most far-reaching metamorphism, dikes which cut

across the bedding planes or intrusive sheets which are thrust

between the strata?

REGIONAL METAMORPHISM. Metamorphic rocks occur wide-spread in many

regions, often hundreds of square miles in area, where such

extensive changes cannot be accounted for by igneous intrusions.

Such are the dissected cores of lofty mountains, as the Alps, and

the worn-down bases of ancient ranges, as in New England, large

areas in the Piedmont Belt, and the Laurentian peneplain.

In these regions the rocks have yielded to immense pressure. They

have been folded, crumpled, and mashed, and even their minute

grains, as one may see with a microscope, have often been

puckered, broken, and crushed to powder. It is to these mechanical

movements and strains which the rocks have suffered in every part

that we may attribute their metamorphism, and the degree to which

they have been changed is in direct proportion to the degree to

which they have been deformed and mashed.

Other factors, however, have played important parts. Rock crushing

develops heat, and allows a freer circulation of heated waters and

vapors. Thus chemical reactions are greatly quickened; minerals

are dissolved and redeposited in new positions, or their chemical

constituents may recombine in new minerals, entirely changing the

nature of the rock, as when, for example, feldspar recrystallizes

as quartz and mica.

Early stages of metamorphism are seen in SLATE. Pressure has

hardened the marine muds, the arkose, or the volcanic ash from

which slates are derived, and has caused them to cleave by the

rearrangement of their particles.

Under somewhat greater pressure, slate becomes PHYLLITE, a clay

slate whose cleavage surfaces are lustrous with flat-lying mica

flakes. The same pressure which has caused the rock to cleave has

set free some of its mineral constituents along the cleavage

planes to crystallize there as mica.

FOLIATION. Under still stronger pressure the whole structure of

the rock is altered. The minerals of which it is composed, and the

new minerals which develop by heat and pressure, arrange

themselves along planes of cleavage or of shear in rudely parallel

leaves, or FOLIA. Of this structure, called FOLIATION, we may

distinguish two types,--a coarser feldspathic type, and a fine

type in which other minerals than feldspar predominate.

GNEISS is the general name under which are comprised coarsely

foliated rocks banded with irregular layers of feldspar and other

minerals. The gneisses appear to be due in many cases to the

crushing and shearing of deep-seated igneous rocks, such as

granite and gabbro.

THE CRYSTALLINE SCHISTS, representing the finer types of

foliation, consist of thin, parallel, crystalline leaves, which

are often remarkably crumpled. These folia can be distinguished

from the laminae of sedimentary rocks by their lenticular form and

lack of continuity, and especially by the fact that they consist

of platy, crystalline grains, and not of particles rounded by

wear.

MICA SCHIST, the most common of schists, and in fact of all

metamorphic rocks, is composed of mica and quartz in alternating

wavy folia. All gradations between it and phyllite may be traced,

and in many cases we may prove it due to the metamorphism of

slates and shales. It is widespread in New England and along the

eastern side of the Appalachians. TALC SCHIST consists of quartz

and TALC, a light-colored magnesian mineral of greasy feel, and so

soft that it can be scratched with the thumb nail.

HORNBLENDE SCHIST, resulting in many cases from the foliation of

basic igneous rocks, is made of folia of hornblende alternating

with bands of quartz and feldspar. Hornblende schist is common

over large areas in the Lake Superior region.

QUARTZ SCHIST is produced from quartzite by the development of

fine folia of mica along planes of shear. All gradations may be

found between it and unfoliated quartzite on the one hand and mica

schist on the other.

Under the resistless pressure of crustal movements almost any

rocks, sandstones, shales, lavas of all kinds, granites, diorites,

and gabbros may be metamorphosed into schists by crushing and

shearing. Limestones, however, are metamorphosed by pressure into

marble, the grains of carbonate of lime recrystallizing freely to

interlocking crystals of calcite.

These few examples must suffice of the great class of metamorphic

rocks. As we have seen, they owe their origin to the alteration of

both of the other classes of rocks--the sedimentary and the

igneous--by heat and pressure, assisted usually by the presence of

water. The fact of change is seen in their hardness arid

cementation, their more or less complete recrystallization, and

their foliation; but the change is often so complete that no trace

of their original structure and mineral composition remains to

tell whether the rocks from which they were derived were

sedimentary or igneous, or to what variety of either of these

classes they belonged.

In many cases, however, the early history of a metamorphic rock

can be deciphered. Fossils not wholly obliterated may prove it

originally water-laid. Schists may contain rolled-out pebbles,

showing their derivation from a conglomerate. Dikes of igneous

rocks may be followed into a region where they have been foliated

by pressure. The most thoroughly metamorphosed rocks may sometimes

be traced out into unaltered sedimentary or igneous rocks, or

among them may be found patches of little change where their

history maybe read.

Metamorphism is most common among rocks of the earlier geological

ages, and most rare among rocks of recent formation. No doubt it

is now in progress where deep-buried sediments are invaded

by heat either from intrusive igneous masses or from the earth's

interior, or are suffering slow deformation under the thrust of

mountain-making forces.

Suggest how rocks now in process of metamorphism may sometimes be

exposed to view. Why do metamorphic rocks appear on the surface

to-day?

MINERAL VEINS

In regions of folded and broken rocks fissures are frequently

found to be filled with sheets of crystalline minerals deposited

from solution by underground water, and fissures thus filled are

known as mineral veins. Much of the importance of mineral veins is

due to the fact that they are often metalliferous, carrying

valuable native metals and metallic ores disseminated in fine

particles, in strings, and sometimes in large masses in the midst

of the valueless nonmetallic minerals which make up what is known

as the VEIN STONE.

The most common vein stones are QUARTZ and CALCITE. FLUORITE

(calcium fluoride), a mineral harder than calcite and

crystallizing in cubes of various colors, and BARITE (barium

sulphate), a heavy white mineral, are abundant in many veins.

The gold-bearing quartz veins of California traverse the

metamorphic slates of the Sierra Nevada Mountains. Below the zone

of solution (p. 45) these veins consist of a vein stone of quartz

mingled with pyrite (p. 13), the latter containing threads and

grains of native gold. But to the depth of about fifty feet from

the surface the pyrite of the vein has been dissolved, leaving a

rusty, cellular quartz with grains of the insoluble gold scattered

through it.

The PLACER DEPOSITS of California and other regions are gold-

bearing deposits of gravel and sand in river beds. The heavy gold

is apt to be found mostly near or upon the solid rock, and its

grains, like those of the sand, are always rounded. How the gold

came in the placers we may leave the pupil to suggest.

Copper is found in a number of ores, and also in the native metal.

Below the zone of surface changes the ore of a copper vein is

often a double sulphide of iron and copper called CHALCOPYRITE, a

mineral softer than pyrite--it can easily be scratched with a

knife--and deeper yellow in color. For several score of feet below

the ground the vein may consist of rusty quartz from which the

metallic ores have been dissolved; but at the base of the zone of

solution we may find exceedingly rich deposits of copper ores,--

copper sulphides, red and black copper oxides, and green and blue

copper carbonates, which have clearly been brought down in

solution from the leached upper portion of the vein.

ORIGIN OF MINERAL VEINS. Both vein stones and ores have been

deposited slowly from solution in water, much as crystals of salt

are deposited on the sides of a jar of saturated brine. In our

study of underground water we learned that it is everywhere

circulating through the permeable rocks of the crust, descending

to profound depths under the action of gravity and again driven to

the surface by hydrostatic pressure. Now fissures, wherever they

occur, form the trunk channels of the underground circulation.

Water descends from the surface along these rifts; it moves

laterally from either side to the fissure plane, just as ground

water seeps through the surrounding rocks from every direction to

a well; and it ascends through these natural water ways as in an

artesian well, whenever they intersect an aquifer in which water

is under hydrostatic pressure.

The waters which deposit vein stones and ores are commonly hot,

and in many cases they have derived their heat from intrusions of

igneous rock still uncooled within the crust. The solvent power of

the water is thus greatly increased, and it takes up into solution

various substances from the igneous and sedimentary rocks which it

traverses. For various reasons these substances stances are

deposited in the vein as ores and vein stones. On rising through

the fissure the water cools and loses pressure, and its capacity

to hold minerals in solution is therefore lessened. Besides, as

different currents meet in the fissure, some ascending, some

descending, and some coming in from the sides, the chemical

reaction of these various weak solutions upon one another and upon

the walls of the vein precipitates the minerals of vein stuffs and

ores.

As an illustration of the method of vein deposits we may cite the

case of a wooden box pipe used in the Comstock mines, Nevada, to

carry the hot water of the mine from one level to another, which

in ten years was lined with calcium carbonate more than half an

inch thick.

The Steamboat Springs, Nevada, furnish examples of mineral veins

in process of formation. The steaming water rises through fissures

in volcanic rocks and is now depositing in the rifts a vein stone

of quartz, with metallic ores of iron, mercury, lead, and other

metals.

RECONCENTRATION. Near the base of the zone of solution veins are

often stored with exceptionally large and valuable ore deposits.

This local enrichment of the vein is due to the reconcentration of

its metalliferous ores. As the surface of the land is slowly

lowered by weathering and running water, the zone of solution is

lowered at an equal rate and encroaches constantly on the zone of

cementation. The minerals of veins are therefore constantly being

dissolved along their upper portions and carried down the fissures

by ground water to lower levels, where they are redeposited.

Many of the richest ore deposits are thus due to successive

concentrations: the ores were leached originally from the rocks to

a large extent by laterally seeping waters; they were concentrated

in the ore deposits of the vein chiefly by ascending currents;

they have been reconcentrated by descending waters in the way just

mentioned.

THE ORIGINAL SOURCE OF THE METALS. It is to the igneous rocks that

we may look for the original source of the metals of veins. Lavas

contain minute percentages of various metallic compounds, and no

doubt this was the case also with the igneous rocks which formed

the original earth crust. By the erosion of the igneous rocks the

metals have been distributed among sedimentary strata, and even

the sea has taken into solution an appreciable amount of gold and

other metals, but in this widely diffused condition they are

wholly useless to man. The concentration which has made them

available is due to the interaction of many agencies. Earth

movements fracturing deeply the rocks of the crust, the intrusion

of heated masses, the circulation of underground waters, have all

cooperated in the concentration of the metals of mineral veins.

While fissure veins are the most important of mineral veins, the

latter term is applied also to any water way which has been filled

by similar deposits from solution. Thus in soluble rocks, such as

limestones, joints enlarged by percolating water are sometimes

filled with metalliferous deposits, as, for example, the lead and

zinc deposits of the upper Mississippi valley. Even a porous

aquifer may be made the seat of mineral deposits, as in the case

of some copper-bearing and silver-bearing sandstones of New

Mexico.

PART III

HISTORICAL GEOLOGY