CHAPTER IX.

THE ROCKS AND THEIR ORDER.

In the preceding chapters of this book the attention of the student

has been directed mainly to the operations of those natural forces

which act upon the surface of the earth. Incidentally the consequences

arising from the applications of energy to the outer part of the

planet have been attended to, but the main aim has been to set forth

the work which solar energy, operating in the form of heat,

accomplishes upon the lands. We have now to consider one of the great

results of these actions, which is exhibited in the successive strata

that make up the earth's crust.

The most noteworthy effect arising from the action of the solar forces

on the earth and their co-operation with those which originate in our

sphere is found in the destruction of beds or other deposits of rock,

and the removal of the materials to the floors of water basins, where

they are again aggregated in strata, and gradually brought once more

into a stable condition within the earth. This work is accomplished by

water in its various states, the action being directly affected by

gravitation. In the form of steam, water which has been built into

rocks and volcanically expelled by tensions, due to the heat which it

has acquired at great depths below the surface, blows forth great

quantities of lava, which is contributed to the formation of strata,

either directly in the solid form or indirectly, after having been

dissolved in the sea. Acting as waves, water impelled by solar energy

transmitted to it by the winds beats against the shores, wearing away

great quantities of rock, which is dragged off to the neighbouring sea

bottoms, there to resume the bedded form. Moving ice in glaciers,

water again applying solar energy given to it by its elevation above

the sea, most effectively grinds away the elevated parts of the crust,

the _débris_ being delivered to the ocean. In the rain the same work

is done, and even in the wind the power of the sun serves to abrade

the high-lying rocks, making new strata of their fragments.

As gravity enters as an element in all the movements of divided rock,

the tendency of the waste worn from the land is to gather on to the

bottoms of basins which contain water. Rarely, and only in a small

way, this process results in the accumulation of lake deposits; the

greater part of the work is done upon the sea floor. When the beds are

formed in lake basins, they may be accumulated in either of two very

diverse conditions. They may be formed in what are called dead seas,

in which case the detrital materials are commonly small in amount, for

the reason that the inflowing streams are inconsiderable; in such

basins there is normally a large share of saline materials, which are

laid down by the evaporation of the water. In ordinary lakes the

deposits which are formed are mostly due to the sediment that the

rivers import. These materials are usually fine-grained, and the sand

or pebbles which they contain are plentifully mingled with clay. Hence

lake deposits are usually of an argillaceous nature. As organic life,

such as secretes limestone, is rarely developed to any extent in lake

basins, limy beds are very rarely formed beneath those areas of water.

Where they occur, they are generally due to the fact that rivers

charged with limy matter import such quantities of the substance that

it is precipitated on the bottom.

As lake deposits are normally formed in basins above the level of the

sea, and as the drainage channels of the basins are always cutting

down, the effect is to leave such strata at a considerable height

above the sea level, where the erosive agents may readily attack them.

In consequence of this condition, lacustrine beds are rarely found of

great antiquity; they generally disappear soon after they are formed.

Where preserved, their endurance is generally to be attributed to the

fact that the region they occupy has been lowered beneath the sea and

covered by marine strata.

The great laboratory in which the sedimentary deposits are

accumulated, the realm in which at least ninety-nine of the hundred

parts of these materials are laid down, is the oceanic part of the

earth. On the floors of the seas and oceans we have not only the

region where the greater part of the sedimentation is effected, but

that in which the work assumes the greatest variety. The sea bottoms,

as regards the deposits formed upon them, are naturally divided into

two regions--the one in which the _débris_ from the land forms an

important part of the sediment, and the other, where the remoteness

of the shores deprives the sediment of land waste, or at least of

enough of that material in any such share as can affect the character

of the deposits.

What we may term the littoral or shore zone of the sea occupies a belt

of prevailingly shallow water, varying in width from a few score to a

few hundred miles. Where the bottom descends steeply from the coast,

where there are no strong off-shore setting currents, and where the

region is not near the mouth of a large river which bears a great tide

of sediment to the sea, the land waste may not affect the bottom for

more than a mile or two from the shore. Where these conditions are

reversed, the _débris_ from the air-covered region may be found three

or four hundred miles from the coast line. It should also be noted

that the incessant up-and-down goings of the land result in a constant

change in the position of the coast line, and consequently in the

extension of the land sediment, in the course of a few geological

periods over a far wider field of sea bottom than that to which they

would attain if the shores remained steadfast.

It is characteristic of the sediments deposited within the influence

of the continental detritus that they vary very much in their action,

and that this variation takes place not only horizontally along the

shores in the same stratum, but vertically, in the succession of the

beds. It also may be traced down the slope from the coast line to deep

water. Thus where all the _débris_ comes from the action of the waves,

the deposits formed from the shore outwardly will consist of coarse

materials, such as pebbles near the coast, of sand in the deeper and

remoter section, and of finer silt in the part of the deposit which is

farthest out. With each change in the level of the coast line the

position of these belts will necessarily be altered. Where a great

river enters the sea, the changes in the volume of sediment which it

from time to time sends forth, together with the alternations in the

position of its point of discharge, led to great local complexities in

the strata. Moreover, the turbid water sent forth by the stream may,

as in the case of the tide from the Amazon, be drifted for hundreds of

miles along the coast line or into the open sea.

The most important variations which occur in the deposits of the

littoral zone are brought about by the formations of rocks more or

less composed of limestone. Everywhere the sea is, as compared with

lake waters, remarkably rich in organic life. Next the shore, partly

because the water is there shallow, but also because of its relative

warmth and the extent to which it is in motion, organic life, both

that of animals and plants, commonly develops in a very luxuriant way.

Only where the bottom is composed of drifting sands, which do not

afford a foothold for those species which need to rest upon the shore,

do we fail to find that surface thickly tenanted with varied forms.

These are arranged according to the depth of the bottom. The species

of marine plants which are attached to fixed objects are limited to

the depth within which the sunlight effectively penetrates the water;

in general, it may be said that they do not extend below a depth of

one hundred feet. The animal forms are distributed, according to their

kinds, over the floor, but few species having the capacity to endure

any great range in the pressure of the sea water. Only a few forms,

indeed, extend from low tide to the depth of a thousand feet.

The greatest development of organic life, the realm in which the

largest number of species occur, and where their growth is most rapid,

lies within about a hundred feet of the low-tide level. Here sunlight,

warmth, and motion in the water combine to favour organic development.

It is in this region that coral reefs and other great accumulations of

limestone, formed from the skeletons of polyps and mollusks, most

abundantly occur. These deposits of a limy nature depend upon a very

delicate adjustment of the conditions which favour the growth of

certain creatures; very slight geographic changes, by inducing

movements of sand or mud, are apt to interrupt their formation,

bringing about a great and immediate alteration in the character of

the deposits. Thus it is that where geologists find considerable

fields of rock, where limestones are intercalated with sandstones and

deposits of clay, they are justified in assuming that the strata were

laid down near some ancient shore. In general, these coast deposits

become more and more limy as we go toward the tropical realms, and

this for the reason that the species which secrete large amounts of

lime are in those regions most abundant and attain the most rapid

growth. The stony polyps, the most vigorous of the limestone makers,

grow in large quantities only in the tropical realm, or near to it,

where ocean streams of great warmth may provide the creatures with the

conditions of temperature and food which they need.

As we pass from the shore to the deeper sea, the share of land

detritus rapidly diminishes until, as before remarked, at the distance

of five hundred miles from the coast line, very little of that waste,

except that from volcanoes, attains the bottom of the sea. By far the

larger part of the contributions which go to the formation of these

deep-sea strata come from organic remains, which are continually

falling upon the sea floor. In part, this waste is derived from

creatures which dwell upon the bottom; in considerable measure,

however, it is from the dead bodies of those forms which live near the

surface of the sea, and which when dying sink slowly through the

intermediate realm to the bottom.

Owing to the absence of sunlight, the prevailingly cold water of the

deeper seas, and the lack of vegetation in those realms, the growth of

organic forms on the deep-sea floor is relatively slow. Thus it

happens that each shell or other contribution to the sediment lies for

some time on the bottom before it is buried. While in this condition

it is apt to be devoured by some of the many species which dwell on

the bottom and subsist from the remains of animals and plants which

they find there. In all cases the fossilization of any form depends

upon the accumulation of sediment before the processes of destruction

have overtaken them, and among these processes we must give the first

place to the creatures which subsist on shells, bones, or other

substances of like nature which find their way to the ocean floor. In

the absolute darkness, the still water, and the exceeding cold of the

deeper seas, animals find difficult conditions for development.

Moreover, in this deep realm there is no native vegetation, and, in

general, but little material of this nature descends to the bottom

from the surface of the sea. The result is, the animals have to

subsist on the remains of other animals which at some step in the

succession have obtained their provender from the plants which belong

on the surface or in the shallow waters of the sea. This limitation

of the food supply causes the depths of the sea to be a realm of

continual hunger, a region where every particle of organic matter is

apt to be seized upon by some needy creature.

In consequence of the fact that little organic matter on the deeper

sea floors escapes being devoured, the most of the material of this

nature which goes into strata enters that state in a finely divided

condition. In the group of worms alone--forms which in a great

diversity of species inhabit the sea floor--we find creatures which

are specially adapted to digesting the _débris_ which gathers on the

sea bottom. Wandering over this surface, much in the manner of our

ordinary earthworms, these creatures devour the mud, voiding the

matter from their bodies in a yet more perfectly divided form. Hence

it comes about that the limestone beds, so commonly formed beneath the

open seas, are generally composed of materials which show but few and

very imperfect fossils. Studying any series of limestone beds, we

commonly find that each layer, in greater or less degree, is made up

of rather massive materials, which evidently came to their place in

the form of a limy mud. Very often this lime has crystallized, and

thus has lost all trace of its original organic structure.

One of the conspicuous features which may be observed in any

succession of limestone beds is the partings or divisions into layers

which occur with varied frequency. Sometimes at vertical intervals of

not more than one or two inches, again with spacings of a score of

feet, we find divisional planes, which indicate a sudden change in the

process of rock formation. The lime disappears, and in place of it we

have a thin layer of very fine detritus, which takes on the form of a

clay. Examining these partings with care, we observe that on the upper

surface on the limestone the remains of the animal which dwelt on the

ancient sea floor are remarkably well preserved, they having evidently

escaped the effect of the process which reduced their ancestors,

whose remains constitute the layer, to mud. Furthermore, we note that

the shaly layer is not only lacking in lime, but commonly contains no

trace of animals such as might have dwelt on the bottom. The fossils

it bears are usually of species which swam in the overlying water and

came to the bottom after death. Following up through the layer of

shale, we note that the ordinary bottom life gradually reappears, and

shortly becomes so plentiful that the deposit resumes the character

which it had before the interruption began. Often, however, we note

that the assemblage of species which dwelt on the given area of sea

floor has undergone a considerable change. Forms in existence in the

lower layer may be lacking in the upper, their place being taken by

new varieties.

So far the origin of these divisional planes in marine deposits has

received little attention from geologists; they have, indeed, assumed

that each of these alterations indicates some sudden disturbance of

the life of the sea floors. They have, however, generally assumed that

the change was due to alterations in the depth of the sea or in the

run of ocean currents. It seems to the writer, however, that while

these divisions may in certain cases be due to the above-mentioned

and, indeed, to a great variety of causes, they are in general best to

be explained by the action of earthquakes. Water being an exceedingly

elastic substance, an earthquake passes through it with much greater

speed than it traverses the rocks which support the ocean floor. The

result is that, when the fluid and solid oscillate in the repeated

swingings which a shock causes, they do not move together, but rub

over each other, the independent movements having the swing of from a

few inches to a foot or two in shocks of considerable energy.

When the sea bottom and the overlying water, vibrating under the

impulse of an earthquake shock, move past each other, the inevitable

result is the formation of muddy water; the very fine silt of the

bottom is shaken up into the fluid, which afterward descends as a

sheet to its original position. It is a well-known fact that such

muddying of water, in which species accustomed to other conditions

dwell, inevitably leads to their death by covering their breathing

organs and otherwise disturbing the delicately balanced conditions

which enable them to exist. We find, in fact, that most of the tenants

of the water, particularly the forms which dwell upon the bottom, are

provided with an array of contrivances which enable them to clear away

from their bodies such small quantities of silt as may inconvenience

them. Thus, in the case of our common clam, the breathing organs are

covered with vibratory cilia, which, acting like brooms, sweep off any

foreign matter which may come upon their surfaces. Moreover, the

creature has a long, double, spoutlike organ, which it can elevate

some distance above the bottom, through which it draws and discharges

the water from which it obtains food and air. Other forms, such as the

crinoids, or sea lilies, elevate the breathing parts on top of tall

stems of marvellous construction, which brings those vital organs at

the level, it may be, of three or four feet above the zone of mud. In

consequence of the peculiar method of growth, the crinoids often

escape the damage done by the disturbance of the bottom, and thus form

limestone beds of remarkable thickness; sometimes, indeed, we find

these layers composed mainly of crinoidal remains, which exhibit only

slight traces of partings such as we have described, being essentially

united for the depth of ten or twenty feet. Where the layers have been

mainly accumulated by shellfish, their average thickness is less than

half a foot.

When we examine the partitions between the layers of limestone, we

commonly find that, however thin, they generally extend for an

indefinite distance in every direction. The writer has traced some of

these for miles; never, indeed, has he been able to find where they

disappeared. This fact makes it clear that the destruction which took

place at the stage where these partings were formed was widespread; so

far as it was due to earthquake shocks, we may fairly believe that in

many cases it occurred over areas which were to be measured by tens of

thousands of square miles. Indeed, from what we know of earthquake

shocks, it seems likely that the devastation may at times have

affected millions of square miles.

Another class of accidents connected with earthquakes may also

suddenly disturb the mud on the sea bottom. When, as elsewhere noted,

a shock originates beneath the sea, the effect is suddenly to elevate

the water over the seat of the jarring and the regions thereabouts to

the height of some feet. This elevation quickly takes the shape of a

ringlike wave, which rolls off in every direction from its point of

origin. Where the sea is deep, the effect of this wave on the bottom

may be but slight; but as the undulation attains shallower water, and

in proportion to the shoaling, the front of the surge is retarded in

its advance by the friction of the bottom, while the rear part, being

in deeper water, crowds upon the advancing line. The action is

precisely that which has been described as occurring in wind-made

waves as they approach the beach; but in this last-named group of

undulations, because of the great width of the swell, the effect of

the shallowing is evident in much deeper water. It is likely that at

the depth of a thousand feet the passing of one of these vast surges

born of earthquakes may so stir the mud of the sea floor as to bring

about a widespread destruction of life, and thus give rise to many of

the partitions between strata.

If we examine with the microscope the fine-grained silts which make up

the shaly layers between limestones, we find the materials to be

mostly of inorganic origin. It is hard to trace the origin of the

mineral matter which it contains; some of the fragments are likely to

prove of Volcanic origin; others, bits of dust from meteorites; yet

others, dust blown from the land, which may, as we know, be conveyed

for any distance across the seas. Mingled with this sediment of an

inorganic origin we almost invariably find a share of organic waste,

derived not from creatures which dwelt upon the bottom, but from those

which inhabited the higher-lying waters. If, now, we take a portion of

the limestone layer which lies above or below the shale parting, and

carefully dissolve out with acids the limy matter which it contains,

we obtain a residuum which in general character, except so far as the

particles may have been affected by the acid, is exactly like the

material which forms the claylike partition. We are thus readily led

to the conclusion that on the floors of the deeper seas there is

constantly descending, in the form of a very slow shower, a mass of

mineral detritus. Where organic life belonging to the species which

secrete hard shells or skeletons is absent, this accumulation,

proceeding with exceeding slowness, gradually accumulates layers,

which take on a shaly character. Where limestone-making animals

abound, they so increase the rate of deposition that the proportion of

the mineral material in the growing strata is very much reduced; it

may, indeed, become as small as one per cent of the mass. In this case

we may say that the deposit of limestone grew a hundred times as fast

as the intervening beds of shale.

The foregoing considerations make it tolerably clear that the sea

floor is in receipt of two diverse classes of sediment--those of a

mineral and those of an organic origin. The mineral, or inorganic,

materials predominate along the shores. They gradually diminish in

quantity toward the open sea, where the supply is mainly dependent on

the substances thrown forth from volcanoes, on pumice in its massive

or its comminuted form--i.e., volcanic dust, states of lava in which

the material, because of the vesicles which it contains, can float for

ages before it comes to rest on the sea bottom. Variations in the

volcanic waste contributed to the sea floor may somewhat affect the

quantity of the inorganic sediments, but, as a whole, the downfalling

of these fragments is probably at a singularly uniform rate. It is

otherwise with the contributions of sediment arising from organic

forms. This varies in a surprising measure. On the coral reefs, such

as form in the mid oceans, the proportion of matter which has not come

into the accumulation through the bodies of animals and plants may be

as small as one tenth of one per cent, or less. In the deeper seas, it

is doubtful whether the rate of animal growth is such as to permit the

formation of any beds which have less than one half of their mass made

up of materials which fell through the water.

In certain areas of the open seas the upper part of the water is dwelt

in by a host of creatures, mostly foraminifera, which extract

limestone from the water, and, on dying, send their shells to the

bottom. Thus in the North Atlantic, even where the sea floor is of

great depth beneath the surface, there is constantly accumulating a

mass of limy matter, which is forming very massive limestone strata,

somewhat resembling chalk deposits, such as abundantly occur in Great

Britain, in the neighbouring parts of Europe, in Texas, and elsewhere.

Accumulations such as this, where the supply is derived from the

surface of the water, are not affected by the accidents which divide

beds made on the bottom in the manner before described. They may,

therefore, have the singularly continuous character which we note in

the English chalk, where, for the thickness of hundreds of feet, we

may have no evident partitions, except certain divisions, which have

evidently originated long after the beds were formed.

We have already noted the fact that, while the floors of the deeper

seas appear to lack mountainous elevations, those arising from the

folding of strata, they are plentifully scattered over with volcanic

cones. We may therefore suppose that, in general, the deposits formed

on the sea floor are to a great extent affected by the materials which

these vents cast forth. Lava streams and showers represent only a

part of the contributions from volcanoes, which finally find their way

to the bottom. In larger part, the materials thrown forth are probably

first dissolved in the water and then taken up by the organic species;

only after the death of these creatures does the waste go to the

bottom. As hosts of these creatures have no solid skeleton to

contribute to the sea floor, such mineral matter as they may obtain is

after their death at once restored to the sea.

Not only does the contribution of organic sediment diminish in

quantity with the depth which is attained, but the deeper parts of the

ocean bed appear to be in a condition where no accumulations of this

nature are made, and this for the reason that the water dissolves the

organic matter more rapidly than it is laid down. Thus in place of

limestone, which would otherwise form, we have only a claylike

residuum, such as is obtained when we dissolve lime rocks in acids.

This process of solution, by which the limy matter deposited on the

bottom is taken back into the water, goes on everywhere, but at a rate

which increases with the depth. This increase is due in part to the

augmentation of pressure, and in part to the larger share of carbonic

dioxide which the water at great depths holds. The result is, that

explorations with the dredge seem to indicate that on certain parts of

the deeper sea floors the rocks are undergoing a process of

dissolution comparable to that which takes place in limestone caverns.

So considerable is the solvent work that a large part of the inorganic

waste appears to be taken up by the waters, so as to leave the bottom

essentially without sedimentary accumulations. The sea, in a word,

appears to be eating into rocks which it laid down before the

depression attained its present great depth.

We should here note something of the conditions which determine the

supply of food which the marine animals obtain. First of all, we may

recur to the point that the ocean waters appear to contain something

of all the earth materials which do not readily decompose when they

are taken into the state of solution. These mineral substances,

including the metals, are obtained in part from the lands, through the

action of the rain water and the waves, but perhaps in larger share

from the volcanic matter which, in the form of floating lava, pumice,

or dust, is plentifully delivered to the sea. Except doubtfully, and

at most in a very small way, this chemical store of the sea water can

not be directly taken into the structures of animals; it can only be

immediately appropriated by the marine plants. These forms can only

develop in that superficial realm of the seas which is penetrated by

the sunlight, or say within the depth of five hundred feet, mostly

within one hundred feet of the surface, about one thirtieth of the

average, and about one fiftieth of the maximum ocean depth. On this

marine plant life, and in a small measure on the vegetable matter

derived from the land, the marine animals primarily depend for their

provender. Through the conditions which bring about the formation of

_Sargassum_ seas, those areas of the ocean where seaweeds grow afloat,

as well as by the water-logging and weighting down of other vegetable

matter, some part of the plant remains is carried to the sea floor,

even to great depths; but the main dependence of the deep-sea forms of

animals is upon other animal forms, which themselves may have obtained

their store from yet others. In fact, in any deep-sea form we might

find it necessary to trace back the food by thousands of steps before

we found the creature which had access to the vegetable matter. It is

easy to see how such conditions profoundly limit the development of

organic being in the abysm of the ocean.

The sedentary animals, or those which are fixed to the sea bottom--a

group which includes the larger part of the marine species--have to

depend for their sustenance on the movement of the water which passes

their station. If the seas were perfectly still, none of these

creatures except the most minute could be fed; therefore the currents

of the ocean go far by their speed to determine the rate at which life

may flourish. At great depths, as we have seen, these movements are

practically limited to that which is caused by the slow movement which

the tide brings about. The amount of this motion is proportional to

the depth of the sea; in the deeper parts, it carries the water to and

fro twice each day for the distance of about two hundred and fifty

feet. In the shallower water this motion increases in proportion to

the shoaling, and in the regions near the shores the currents of the

sea which, except the massive drift from the poles, do not usually

touch the bottom, begin to have their influence. Where the water is

less than a hundred feet in depth, each wave contributes to the

movement, which attains its maximum near the shore, where every surge

sweeps the water rapidly to and fro. It is in this surge belt, where

the waves are broken, that marine animals are best provided with food,

and it is here that their growth is most rapid. If the student will

obtain a pint of water from the surf, he will find that it is clouded

by fragments of organic matter, the quantity in a pound of the fluid

often amounting to the fiftieth part of its weight. He will thus

perceive that along the shore line, though the provision of victuals

is most abundant, the store is made from the animals and plants which

are ground up in the mill. In a word, while the coast is a place of

rapid growth, it is also a region of rapid destruction; only in the

case of the coral animals, which associate their bodies with a number

of myriads in large and elaborately organized communities, do we find

animals which can make such head against the action of the waves that

they can build great deposits in their realm.

It should be noted that a part of the advantage which is afforded to

organic life by the shore belt is due to the fact that the waters are

there subjected to a constant process of aëration by the whipping into

foam and spray which occurs where the waves overturn.

It will be interesting to the student to note the great number of

mechanical contrivances which have been devised to give security to

animals and plants which face these difficult conditions arising from

successive violent blows of falling water. Among these may be briefly

noted those of the limpets--mollusks which dwell in a conical shell,

which faces the water with a domelike outside, and which at the moment

of the stroke is drawn down upon the rock by the strong muscle which

fastens the creature to its foundation. The barnacles, which with

their wedge-shaped prows cut the water at the moment of the stroke,

but open in the pauses between the waves, so that the creature may

with its branching arms grasp at the food which floats about it; the

nullipores, forms of seaweed which are framed of limestone and cling

firmly to the rock--afford yet other instances of protective

adaptations contrived to insure the safety of creatures which dwell in

the field of abundant food supply.

* * * * *

The facts above presented will show the reader that the marine

sediments are formed under conditions which permit a great variety in

the nature of the materials of which they are composed. As soon as the

deposits are built into rocks and covered by later accumulations,

their materials enter the laboratory of the under earth, where they

are subjected to progressive changes. Even before they have attained a

great depth, through the laying down of later deposits upon them,

changes begin which serve to alter their structure. The fragments of a

soluble kind begin to be dissolved, and are redeposited, so that the

mass commonly becomes much more solid, passing from the state of

detritus to that of more or less solid rock. When yet more deeply

buried, and thereby brought into a realm of greater warmth, or perhaps

when penetrated by dikes and thereby heated, these changes go yet

further. More of the material is commonly rearranged by solution and

redeposition, so that limestone may be converted into crystalline

marble, granular sandstones into firm masses, known as quartzites, and

clays into the harder form of slate. Where the changes go to the

extreme point, rocks originally distinctly bedded probably may be so

taken to pieces and made over that all traces of their stratification

may be destroyed, all fossils obliterated, and the stone transformed

into mica schist, or granite or other crystalline rock. It may be

injected into the overlying strata in the form of dikes, or it may be

blown forth into the air through volcanoes. Involved in

mountain-folding, after being more or less changed in the manner

described, the beds may become tangled together like the rumpled

leaves of a book, or even with the complexity of snarled thread. All

these changes of condition makes it difficult for the geologist to

unravel the succession of strata so that he may know the true order of

the rocks, and read from them the story of the successive geological

periods. This task, though incomplete, has by the labours of many

thousand men been so far advanced that we are now able to divide the

record into chapters, the divisions of the geologic ages, and to give

some account of the succession of events, organic and geographic,

which have occurred since life began to write its records.

EARTHQUAKES.

In ordinary experience we seem to behold the greater part of the earth

which meets our eyes as fixed in its position. A better understanding

shows us that nothing in this world is immovable. In the realm of the

inorganic world the atoms and molecules even in solid bodies have to

be conceived as endowed with ceaseless though ordered motions. Even

when matter is built into the solid rock, it is doubtful whether any

grain of it ever comes really to rest. Under the strains which arise

from the contraction of the earth's interior and the chemical changes

which the rocks undergo, each bit is subject to ever-changing

thrusts, which somewhat affect its position. If we in any way could

bring a grain of sand from any stratum under a microscope, so that we

could perceive its changes of place, we should probably find that it

was endlessly swaying this way and that, with reference to an ideally

fixed point, such as the centre of the earth. But even that centre,

whether of gravity or of figure, is probably never at rest.

Earth movements may be divided into two groups--those which arise from

the bodily shifting of matter, which conveys the particles this way or

that, or, as we say, change their place, and those which merely

produce vibration, in which the particles, after their vibratory

movement, return to their original place. For purposes of illustration

the first, or translatory motion, may be compared to that which takes

place when a bell is carried along upon a locomotive or a ship; and

the second, or vibratory movement, to what takes place when the bell

is by a blow made to ring. It is with these ringing movements, as we

may term them, that we find ourselves concerned when we undertake the

study of earthquakes.

It is desirable that the reader should preface his study of

earthquakes by noting the great and, at the same time, variable

elasticity of rocks. In the extreme form this elasticity is very well

shown when a toy marble, which is made of a close-textured rock, such

as that from which it derives its name, is thrown upon a pavement

composed of like dense material. Experiment will show that the little

sphere can often be made to bounce to the height of twenty feet

without breaking. If, then, with the same energy the marble is thrown

upon a brick floor, the rebound will be very much diminished. It is

well to consider what happens to produce the rebound. When the sphere

strikes the floor it changes its shape, becoming shorter in the axis

at right angles to the point which was struck, and at the same instant

expanded along the equator of that axis. The flattening remains for

only a small fraction of a second; the sphere vibrates so that it

stretches along the line on which it previously shortened, and, as

this movement takes place with great swiftness, it may be said to

propel itself away from the floor. At the same time a similar movement

goes on in the rock of the floor, and, where the rate of vibration is

the same, the two kicks are coincident, and so the sphere is impelled

violently away from the point of contact. Where the marble comes in

contact with brick, in part because of the lesser elasticity of that

material, due to its rather porous structure, and partly because it

does not vibrate at the same rate as the marble, the expelling blow is

much less strong.

All rocks whatever, even those which appear as incoherent sands, are

more or less set into vibratory motion whenever they are struck by a

blow. In the crust of the earth various accidents occur which may

produce that sudden motion which we term a blow. When we have examined

into the origin of these impulses, and the way in which they are

transmitted through the rocks, we obtain a basis for understanding

earthquake shocks. The commonest cause of the jarrings in the earth is

found in the formation of fractures, known as faults. If the reader

has ever been upon a frozen lake at a time when the weather was

growing colder, and the ice, therefore, was shrinking, he may have

noted the rending sound and the slight vibration which comes with the

formation of a crack traversing the sheet of ice. At such a time he

feels a movement which is an earthquake, and which represents the

simpler form of those tremors arising from the sudden rupture of fault

planes. If he has a mind to make the experiment, he may hang a bullet

by a thread from a small frame which rests upon the ice, and note that

as the vibration occurs the little pendulum sways to and fro, thus

indicating the oscillations of the ice. The same instrument will move

in an identical manner when affected by a quaking in the rocks.

Where the rocks are set in vibration by a rent which is formed in

them, the phenomena are more complicated, and often on a vastly larger

scale than in the simple conditions afforded by a sheet of ice. The

rocks on either side of the rupture generally slide over each other,

and the opposing masses are rent in their friction upon one another;

the result is, not only the first jar formed by the initial fracture,

but a great many successive movements from the other breakages which

occur. Again, in the deeper parts of the crust, the fault fissures are

often at the moment of their formation filled by a violent inrush of

liquid rock. This, as it swiftly moves along, tears away masses from

the walls, and when it strikes the end of the opening delivers a blow

which may be of great violence. The nature of this stroke may be

judged by the familiar instance where the relatively slow-flowing

stream from a hydrant pipe is suddenly choked by closing the stopcock.

Unless the plumber provides a cushion of air to diminish the energy of

the blow, it is often strong enough to shake the house. Again, when

steam or other gases are by a sudden diminution of pressure enabled to

expand, they may deliver a blow which is exactly like that caused by

the explosion of gunpowder, which, even when it rushes against the

soft cushion of the air, may cause a jarring that may be felt as well

as heard to a great distance. Such movements very frequently occur in

the eruptions of volcanoes; they cause a quivering of the earth, which

may be felt for a great distance from the immediate seat of the

disturbance.

When by any of the sudden movements which have been above described a

jar is applied to the rocks, the wave flies through the more or less

elastic mass until the energy involved in it is exhausted. This may

not be brought about until the motion has travelled for the distance

of hundreds of miles. In the great earthquake of 1755, known as the

Lisbon shock, the records make it seem probable that the movement was

felt over one eighth part of the earth's surface. Such great

disturbances probably bring about a motion of the rocks near the point

of origin, which may be expressed in oscillations having an amplitude

of one to two feet; but in the greater number of earthquakes the

maximum swing probably does not exceed the tenth of that amount. Very

sensible shaking, even such as may produce considerable damage to

buildings, are caused by shocks in which the earth vibrates with less

than an inch of swing.

When a shock originates, the wave in the rocks due to the compression

which the blow inflicts runs at a speed varying with the elasticity of

the substance, but at the rate of about fifteen hundred feet a second.

The movements of this wave are at right angles to the seat of the

originating disturbance, so that the shock may come to the surface in

a line forming any angle between the vertical and the nearly

horizontal. Where, as in a volcanic eruption, the shock originates

with an explosion, these waves go off in circles. Where, however, as

is generally the case, the shock originates in a fault plane, which

may have a length and depth of many miles, the movement has an

elliptical form.

If the earthquake wave ran through a uniform and highly elastic

substance, such as glass, it would move everywhere with equal speed,

and, in the case of the greater disturbances, the motion might be felt

over the whole surface of the earth. But as the motion takes place

through rocks of varying elasticity, the rate at which it journeys is

very irregular. Moving through materials of one density, and with a

rate of vibration determined by those conditions, the impulse is with

difficulty communicated to strata which naturally vibrate at another

speed. In many cases, as where a shock passing through dense

crystalline strata encounters a mass of soft sandstone, the wave, in

place of going on, is reflected back toward its point of origin. These

earthquake echoes sometimes give rise to very destructive movements.

It often happens that before the original tremors of a shock have

passed away from a point on the surface the reflex movements rush in,

making a very irregular motion, which may be compared to that of the

waves in a cross-sea.

The foregoing account of earthquake action will serve to prepare the

reader for an understanding of those very curious and important

effects which these accidents produce in and on the earth. Below the

surface the sensible action of earthquake shocks is limited. It has

often been observed that people in mines hardly note a swaying which

may be very conspicuous to those on the surface, the reason for this

being that underground, where the rocks are firmly bound together, all

those swingings which are due to the unsupported position of such

objects as buildings, columnar rocks, trees, and the waters of the

earth, are absent. The effect of the movements which earthquakes

impress on the under earth is mainly due to the fact that in almost

every part of the crust tensions or strains of other kinds are

continually forming. These may for ages prove without effect until the

earth is jarred, when motions will suddenly take place which in a

moment may alter the conditions of the rocks throughout a wide field.

In a word, a great earthquake caused by the formation of an extensive

fault is likely to produce any number of slight dislocations, each of

which is in turn shock-making, sending its little wave to complicate

the great oscillation. Nor does the perturbing effect of these jarring

movements cease with the fractures which they set up and the new

strains which are in turn developed by the motions which they induce.

The alterations of the rocks which are involved in chemical changes

are favoured by such motions. It is a familiar experience that a

vessel of water, if kept in the state of repose, may have its

temperature lowered three or four degrees below the freezing point

without becoming frozen. If the side of the vessel is then tapped with

the finger, so as to send a slight quake through the mass, it will

instantly congeal. Molecular rearrangements are thus favoured by

shocks, and the consequences of those which run through the earth are,

from a chemical point of view, probably important.

The reader may help himself to understand something of the complicated

problem of earth tensions, and the corresponding movements of the

rocks, by considering certain homely illustrations. He may observe how

the soil cracks as it shrinks in times of drought, the openings

closing when it rains. In a similar way the frozen earth breaks open,

sometimes with a shock which is often counted as an earthquake. Again,

the ashes in a sifter or the gravel on a sieve show how each shaking

may relieve certain tensions established by gravity, while they create

others which are in turn to be released by the next shock. An ordinary

dwelling house sways and strains with the alternations of temperature

and moisture to which it is subjected in the round of climatal

alterations. Now and then we note the movements in a cracking sound,

but by far the greater part of them escape observation.

With this sketch of the mechanism of earthquake shocks we now turn to

consider their effects upon the surface of the earth. From a

geological point of view, the most important effect of earthquake

shocks is found in the movement of rock masses down steep slopes,

which is induced by the shaking. Everywhere on the land the agents of

decay and erosion tend to bring heavy masses into position where

gravitation naturally leads to their downfall, but where they may

remain long suspended, provided they are not disturbed. Thus, wherever

there are high and steep cliffs, great falls of rock are likely to

occur when the earthquake movements traverse the under earth. In more

than one instance observers, so placed that they commanded a view of

distant mountains, have noticed the downfall of precipices in the path

of the shock before the trembling affected the ground on which they

stood. In the famous earthquake of 1783, which devastated southern

Italy, the Prince of Scylla persuaded his people to take refuge in

their boats, hoping that they might thereby escape the destruction

which threatened them on the land. No sooner were the unhappy folk on

the water than the fall of neighbouring cliffs near the sea produced a

great wave, which overwhelmed the vessels.

Where the soil lies upon steep slopes, in positions in which it has

accumulated during ages of tranquillity, a great shock is likely to

send it down into the valleys in vast landslides. Thus, in the

earthquake of 1692, the Blue Mountains of Jamaica were so violently

shaken that the soil and the forests which stood on it were

precipitated into the river beds, so that many tree-clad summits

became fields of bare rock. The effect of this action is immensely to

increase the amount of detritus which the streams convey to the sea.

After the great Jamaica shock, above noted, the rivers for a while

ceased to flow, their waters being stored in the masses of loose

material. Then for weeks they poured forth torrents of mud and the

_débris_ of vegetation--materials which had to be swept away as the

streams formed new channels.

In all regions where earthquake movements are frequent, and the shock

of considerable violence, the trained observer notes that the surfaces

of bare rock are singularly extensive, the fact being that many of

these areas, where the slope lies at angles of from ten to thirty

degrees, which in an unshaken region would be thickly soil-covered,

are deprived of the coating by the downward movement of the waste

which the disturbances bring about. A familiar example of this action

may be had by watching the workmen engaged in sifting sand, by casting

the material on a sloping grating. The work could not be done but for

an occasional blow applied to the sifter. An arrangement for such a

jarring motion is commonly found in various ore-dressing machines,

where the object is to move fragments of matter over a sloping

surface.

Even where the earth is so level that an earthquake shock does not

cause a sliding motion of the materials, such as above described,

other consequences of the shaking may readily be noted. As the motion

runs through the mass, provided the movement be one of considerable

violence, crevices several feet in width, and sometimes having the

length of miles, are often formed. In most cases these fissures,

opened by one pulsation of the shock, are likely to be closed by the

return movement, which occurs the instant thereafter. The consequences

of this action are often singular, and in cases constitute the most

frightful elements of a shock which the sufferer beholds. In the great

earthquake of 1811, which ravaged the section of the Mississippi

Valley between the mouth of the Ohio and Vicksburg, these crevices

were so numerously formed that the pioneers protected themselves from

the danger of being caught in their jaws by felling trees so that they

lay at right angles to the direction in which the rents extended,

building on these timbers platforms to support their temporary

dwelling places. The records of earthquakes supply many instances in

which people have been caught in these earth fissures, and in a single

case it is recorded that a man who disappeared into the cavity was in

a moment cast forth in the rush of waters which in this, as in many

other cases, spouts forth as the walls of the opening come together.

Sometimes these rents are attended by a dislocation, which brings the

earth on one side much higher than on the other. The step thus

produced may be many miles in length, and may have a height of twenty

feet or more. It needs no argument to show that we have here the top

of a fault such as produced the shock, or it may be one of a secondary

nature, such as any earthquake is likely to bring about in the strata

which it traverses. In certain cases two faults conjoin their action,

so that a portion of the surface disappears beneath the earth,

entombing whatever may have stood on the vanished site. Thus in the

great shock known as that of Lisbon, which occurred in 1755, the stone

quay along the harbour, where many thousand people had sought refuge

from the falling buildings of the city, suddenly sank down with the

multitude, and the waters closed over it; no trace of the people or of

the structure was to be found after the shock was over. There is a

story to the effect that during the same earthquake an Arab village in

northern Africa sank down, the earth on either side closing over it,

so that no trace of the habitations remained. In both these instances

the catastrophes are best explained by the diagram.

[Illustration: Fig. 21.--Diagram showing how a portion of the earth's

surface may be sunk by faulting. Fig. A shows the original position;

B, the position after faulting; b b' and c c' the planes of the

faults; the arrows the direction of the movement.]

In the earthquake of 1811 the alluvial plains on either side of the

Mississippi at many points sank down so that arable land was converted

into lakes; the area of these depressions probably amounted to some

hundred square miles. The writer, on examining these sunken lands,

found that the subsidences had occurred where the old moats or

abandoned channels of the great river had been filled in with a

mixture of decaying timber and river silt. When violently shaken, this

loose-textured _débris_ naturally settled down, so that it formed a

basin occupied by a crescent-shaped lake. The same process of settling

plentifully goes on wherever the rocks are still in an uncemented

state. The result is often the production of changes which lead to the

expulsion of gases. Thus, in the Charleston earthquake of 1883, the

surface over an area of many hundred square miles was pitted with

small craters, formed by the uprush of water impelled by its contained

gases. These little water volcanoes--for such we may call

them--sometimes occur to the number of a dozen or more on each acre of

ground in the violently shaken district. They indicate one result of

the physical and chemical alterations which earthquake shocks bring

about. As earthquakes increase in violence their effect upon the soil

becomes continually greater, until in the most violent shocks all the

loose materials on the surface of the earth may be so shaken about as

to destroy even the boundaries of fields. After the famous earthquake

of Riobamba, which occurred on the west coast of South America in

1797, the people of the district in which the town of that name was

situated were forced to redivide their land, the original boundaries

having disappeared. Fortunately, shocks of this description are

exceedingly rare. They occur in only a few parts of the world.

Certain effects of earthquakes where the shock emerges beneath the sea

have been stated in the account of volcanic eruptions (see page 299).

We may therefore note here only certain of the more general facts.

While passing through the deep seas, this wave may have a height of

not more than two or three feet and a width of some score miles. As it

rolls in upon the shore the front of the undulation is retarded by the

friction of the bottom in such a measure that its speed is diminished,

while the following part of the waves, being less checked, crowds up

toward this forward part. The result is, that the surge mounts ever

higher and higher as it draws near the shore, upon which it may roll

as a vast wave having the height of fifty feet or more and a width

quite unparalleled by any wave produced from wind action. Waves of

this description are most common in the Pacific Ocean. Although but

occasional, the damage which they may inflict is very great. As the

movement approaches the shore, vessels, however well anchored, are

dragged away to seaward by the great back lash of the wave, a

phenomenon which may be perceived even in the case of the ordinary

surf. Thus forced to seaward, the crews of the ships may find their

vessels drawn out for the distance of some miles, until they come near

the face of the advancing billow. This, as it approaches the shore,

straightens up to the wall-fronted form, and then topples upon the

land. Those vessels which are not at once crushed down by the blow are

generally hurled far inland by the rush of waters. In the great

Jamaica earthquake of 1692 a British man-of-war was borne over the

tops of certain warehouses and deposited at a distance from the shore.

Owing to the fact that water is a highly elastic material, the shocks

transmitted to it from the bottom are sent onward with their energy

but little diminished. While the impulse is very violent, these

oscillations may prove damaging to shipping. The log-books of mariners

abound in stories of how vessels were dismasted or otherwise badly

shaken by a sudden blow received in the midst of a quiet sea. The

impression commonly conveyed to the sailors is that the craft has

struck upon a rock. The explanation is that an earthquake jar, in

traversing the water, has delivered its blow to the ship. As the speed

of this jarring movement is very much greater than that of any

ordinary wave, the blow which it may strike may be most destructive.

There seems, indeed, little reason to doubt that a portion of the

vessels which are ever disappearing in the wilderness of the ocean are

lost by the crushing effect of these quakings which pass through the

waters of the deep.

We have already spoken of the earthquake shock as an oscillation. It

is a quality of all bodies which oscillate under the influence of a

blow, such as originates in earthquake shocks, to swing to and fro,

after the manner of the metal in a bell or a tuning fork, in a

succession of movements, each less than the preceding, until the

impulse is worn out, or rather, we should in strict sense say,

changed to other forms of energy. The result is, that even in the

slightest earthquake shock the earth moves not once to and fro, but

very many times. In a considerable shock the successive diminishing

swingings amount to dozens before they become so slight as to elude

perception. Although the first swaying is the strongest, and generally

the most destructive, the quick to-and-fro motions are apt to continue

and to complete the devastation which the first brings about. The

vibrations due to any one shock take place with great rapidity. They

may, indeed, be compared to those movements which we perceive in the

margin of a large bell when it has received a heavy blow from the

clapper. The reader has perhaps seen that for a moment the rim of the

bell vibrates with such rapidity that it has a misty look--that is,

the motions elude the sight. It is easy to see that a shaking of this

kind is particularly calculated to disrupt any bodies which stand free

in the air and are supported only at their base.

In what we may call the natural architecture of the earth, the

pinnacles and obelisks, such as are formed in many high countries, the

effect of these shakings is destructive, and, as we have seen, even

the firmer-placed objects, such as the strong-walled cliffs and steep

slopes of earth, break down under the assaults. It is therefore no

matter of surprise that the buildings which man erects, where they are

composed of masonry, suffer greatly from these tremblings. In almost

all cases human edifices are constructed without regard to other

problems of strength than those which may be measured by their weight

and the resistance to fracture from gravitation alone. They are not

built with expectation of a quaking, but of a firm-set earth.

The damage which earthquakes do to buildings is in most cases due to

the fact that they sway their walls out of plumb, so that they are no

longer in position to support the weight which they have to bear. The

amount of this swaying is naturally very much greater than that which

the earth itself experiences in the movement. A building of any height

with its walls unsupported by neighbouring structures may find its

roof rocked to and fro through an arc which has a length of feet,

while its base moves only through a length of inches. The reader may

see an example of this nature if he will poise a thin book or a bit of

plank a foot long on top of a small table; then jarring the table so

that it swings through a distance of say a quarter of an inch, he will

see that the columnar object swings at its top through a much greater

distance, and is pretty sure to be overturned.

Where a building carries a load in its upper parts, such as may be

afforded by its heavy roof or the stores which it contains, the effect

of an earthquake shock such as carries the earth to and fro becomes

much more destructive than it might otherwise be. This weight lags

behind when the earth slips forward in the first movement of the

oscillation, with the effect that the walls of the building are pretty

sure to be thrust so far beyond the perpendicular that they give way

and are carried down by the weight which they bore. It has often been

remarked in earthquake shocks that tall columns, even where composed

of many blocks, survive a shock which overturns lower buildings where

thin walls support several floors, on each of which is accumulated a

considerable amount of weight. In the case of the column, the strains

are even, and the whole structure may rock to and fro without toppling

over. As the energy of the undulations diminish, it gradually regains

the quiet state without damage. In the ordinary edifice the irregular

disposition of the weight does not permit the uniform movement which

may insure safety. Thus, if the city of Washington should ever be

violently shaken, the great obelisk, notwithstanding that it is five

hundred feet high, may survive a disturbance which would wreck the

lower and more massive edifices which lie about it.

Where, as is fortunately rarely the case, the great shock comes to

the earth in a vertical direction, the effect upon all movable objects

is in the highest measure disastrous. In such a case buildings are

crushed as if by the stroke of a giant's hand. The roofs and floors

are at one stroke thrown to the foundations, and all the parts of the

walls which are not supported by strong masonry continuous from top to

bottom are broken to pieces. In such cases it has been remarked that

the bodies of men are often thrown considerable distances. It is

asserted, indeed, that in the Riobamba shock they were cast upward to

the height of more than ninety feet. It is related that the solo

survivor of a congregation which had hastened at the outset of the

disturbance into a church was thrown by the greatest and most

destructive shock upward and through a window the base of which was at

the height of more than twenty feet from the ground.

It is readily understood that an earthquake shock may enter a building

in any direction between the vertical and the horizontal. As the

movement exhausts itself in passing from the place of its origin, the

horizontal shocks are usually of least energy. Those which are

accurately vertical are only experienced where the edifices are placed

immediately over the point where the motion originates. It follows,

therefore, that the destructive work of earthquakes is mainly

performed in that part of the field where the motion is, as regards

its direction, between the vertical and the horizontal--a position in

which the edifice is likely to receive at once the destructive effect

arising from the sharp upward thrust of the vertical movement and the

oscillating action of that which is in a horizontal direction. Against

strains of this description, where the movements have an amplitude of

more than a few inches, no ordinary masonry edifice can be made

perfectly safe; the only tolerable security is attained where the

building is of well-framed timber, which by its elasticity permits a

good deal of motion without destructive consequences. Even such

buildings, however, those of the strongest type, may be ruined by the

greater earthquakes. Thus, in the Mississippi Valley earthquake of

1811, the log huts of the frontiersmen, which are about as strong as

any buildings can be made, were shaken to pieces by the sharp and

reiterated shocks.

It is by no means surprising to find that the style of architecture

adopted in earthquake countries differs from that which is developed

in regions where the earth is firm-set. The people generally learn

that where their buildings must meet the trials of earthquakes they

have to be low and strong, framed in the manner of fortifications, to

withstand the assault of this enemy. We observe that Gothic

architecture, where a great weight of masonry is carried upon slender

columns and walls divided by tall windows, though it became the

dominant style in the relatively stable lands of northern Europe,

never gained a firm foothold in those regions about the Mediterranean

which are frequently visited by severe convulsions of the earth. There

the Grecian or the Romanesque styles, which are of a much more massive

type, retain their places and are the fashions to the present day.

Even this manner of building, though affording a certain security

against slight tremblings, is not safe in the greater shocks. Again

and again large areas in southern Italy have been almost swept of

their buildings by the destructive movements which occur in that

realm. The only people who have systematically adapted their

architectural methods to earthquake strains are the Japanese, who in

certain districts where such risks are to be encountered construct

their dwellings of wood, and place them upon rollers, so that they may

readily move to and fro as the shock passes beneath them. In a measure

the people of San Francisco have also provided against this danger by

avoiding dangerous weights in the upper parts of their buildings, as

well as the excessive height to which these structures are lifted in

some of our American towns.

Earthquakes of sensible energy appear to be limited to particular

parts of the earth's crust. The regions, indeed, where within the

period of human history shocks of devastating energy have occurred do

not include more than one fifteenth part of the earth's surface. There

is a common notion that these movements are most apt to happen in

volcanic regions. It is, indeed, true that sensible shocks commonly

attend the explosions from great craters, but the records clearly show

that these movements are very rarely of destructive energy. Thus in

the regions about the base of Vesuvius and of Ætna, the two volcanoes

of which most is known, the shocks have never been productive of

extensive disaster. In fact, the reiterated slight jarrings which

attend volcanic action appear to prevent the formation of those great

and slowly accumulated strains which in their discharge produce the

most violent tremblings of the earth. The greatest and most continuous

earthquake disturbances of history--that before noted in the early

days of this century, in the Mississippi Valley, where shocks of

considerable violence continued for two years--came about in a field

very far removed from active volcanoes. So, too, the disturbances

beneath the Atlantic floor which originated the shocks that led to the

destruction of Lisbon, and many other similar though less violent

movements, are developed in a field apparently remote from living

volcanoes. Eastern New England, which has been the seat of several

considerable earthquakes, is about as far away from active vents as

any place on the habitable globe. We may therefore conclude that,

while volcanoes necessarily produce shocks resulting from the

discharge of their gases and the intrusion of lava into the dikes

which are formed about them, the greater part of the important shocks

are in no wise connected with volcanic explosions.

With the exception of the earthquake in the Mississippi Valley, all

the great shocks of which we have a record have occurred in or near

regions where the rocks have been extensively disturbed by

mountain-building forces, and where the indications lead us to

believe that dislocations of strata, such as are competent to rive the

beds asunder, may still be in progress. This, taken in connection with

the fact that many of these shocks are attended by the formation of

fault planes, which appear on the surface, lead us to the conclusion

that earthquakes of the stronger kind are generally formed by the

riving of fissures, which may or may not be developed upward to the

surface. This view is supported by many careful observations on the

effect which certain great earthquakes have exercised on the buildings

which they have ravaged. The distinguished observer, Mr. Charles

Mallet, who visited the seat of the earthquake which, in 1854,

occurred in the province of Calabria in Italy, with great labour and

skill determined the direction in which the shock moved through some

hundreds of edifices on which it left the marks of its passage.

Platting these lines of motion, he found that they were all referred

to a vertical plane lying at the depth of some miles beneath the

surface, and extending for a great distance in a north and south

direction. This method of inquiry has been applied to other fields,

with the result that in the case of all the instances which have been

subjected to this inquiry the seat of the shock has been traced to

such a plane, which can best be accounted for by the supposition of a

fault.

The method pursued by Mr. Mallet in his studies of the origin of

earthquakes, and by those who have continued his inquiry, may be

briefly indicated as follows: Examining disrupted buildings, it is

easy to determine those which have been wrecked by a shock that

emerged from the earth in a vertical direction. In these cases, though

tall walls may remain standing, the roofs and floors are thrown into

the cellars. With a dozen such instances the plane of what is called

the seismic vertical is established (_seismos_ is the Greek for

earthquake). Then on either side of this plane, which indicates the

line but not the depth of the disturbance, other observations may be

made which give the clew to the depth. Thus a building may be found

where the northwest corner at its upper part has been thrown off. Such

a rupture was clearly caused by an upward but oblique movement, which

in the first half of the oscillation heaved the structure upwardly

into the northwest, and then in the second half, or rebound, drew the

mass of the building away from the unsupported corner, allowing that

part of the masonry to fly off and fall to the ground. Constructing a

line at right angles to the plane of the fracture, it will be found to

intersect the plane, the position of which has been in part determined

by finding the line where it intersects the earth, or the seismic

vertical before noted. Multiplying such observations on either side of

the last-mentioned line, the attitude of the underground parts of the

plane, as well as the depth to which it attained, can be approximately

determined.

It is worth while to consider the extent to which earthquake shocks

may affect the general quality of the people who dwell in countries

where these disturbances occur with such frequency and violence as to

influence their lives. There can be no question that wherever

earthquakes occur in such a measure as to produce widespread terror,

where, recurring from time to time, they develop in men a sense of

abiding insecurity, they become potent agents of degradation. All the

best which men do in creating a civilization rests upon a sense of

confidence that their efforts may be accumulated from year to year,

and that even after death the work of each man may remain as a

heritage to his kind. It is likely, indeed, that in certain realms, as

in southern Italy, a part of the failure of the people to advance in

culture is due to their long experience of such calamities, and the

natural expectation that they will from time to time recur. In a

similar way the Spanish settlements in Central and South America,

which lie mostly in lands that are subject to disastrous shocks, may

have been retarded by the despair, as well as the loss of property

and life, which these accidents have so frequently inflicted upon

them. It will not do, however, to attribute too much to such

terrestrial influences. By far the most important element in

determining the destiny of a people is to be found in their native

quality, that which they owe to their ancestors of distant

generations. In this connection it is well to consider the history of

the Icelandic people, where a small folk has for a thousand years been

exposed to a range and severity of trials, such as earthquakes,

volcanic explosions, and dearth of harvests may produce, and all these

in a measure that few if any other countries experience.

Notwithstanding these misfortunes, the Icelanders have developed and

maintained a civilization which in all else, except its material

results, on the average transcends that which has been won by any

other folk in modern times. If a people have the determining spirit

which leads to high living, they can successfully face calamities far

greater than those which earthquakes inflict.

It was long supposed that the regions where earthquakes are not

noticeable by the unaided senses were exempt from all such

disturbances. The observations which seismologists have made in recent

years point to the conclusion that no part of the earth's surface is

quite exempt from movements which, though not readily perceived, can

be made visible by the use of appropriate instruments. With an

apparatus known as the horizontal pendulum it is possible to observe

vibrations which do not exceed in amplitude the hundredth part of an

inch. This mechanism consists essentially of a slender bar supported

near one end by two wires, one from above, the other from below. It

may readily be conceived that any measurable movement will cause the

longer end of the rod to sway through a considerable arc. Wherever

such a pendulum has been carefully observed in any district, it has

been found that it indicates the occurrence of slight tremors. Even

certain changes of the barometer, which alter the weight of the

atmosphere that rests upon the earth to the amount indicated by an

inch in the height of the mercury column, appears in all cases to

create such tremors. Many of these slight shocks may be due to the

effect of more violent quakings, which have run perhaps for thousands

of miles from their point of origin, and have thus been reduced in the

amplitude of their movement. Others are probably due to the slight

motion brought about through the chemical changes of the rocks, which

are continuously going on. The ease with which even small motions are

carried to a great distance may be judged by the fact that when the

ground is frozen the horizontal pendulum will indicate the jarring due

to a railway train at the distance of a mile or more from the track.

In connection with the earth jarring, it would be well to note the

occurrence of another, though physically different, kind of movement,

which we may term earth swayings, or massive movements, which slowly

dislocate the vertical, and doubtless also the horizontal, position of

points upon its surface. It has more than once been remarked that in

mountain countries, where accurate sights have been taken, the heights

of points between the extremities of a long line appear somewhat to

vary in the course of a term of years. Thus at a place in the

Apennines, where two buildings separated by some miles of distance are

commonly intervisible over the crest of a neighbouring peak, it has

happened that a change of level of some one of the points has made it

impossible to see the one edifice from the other. Knowing as we do

that the line of the seacoast is ever-changing, uprising taking place

at some points and down-sinking at others, it seems not unlikely that

these irregular swayings are of very common occurrence. Moreover,

astronomers are beginning to remark the fact that their observatories

appear not to remain permanently in the same position--that is, they

do not have exactly the same latitude and longitude. Certain of these

changes have recently been explained by the discovery of a new and

hitherto unnoted movement of the polar axis. It is not improbable,

however, that the irregular swaying of the earth's crust, due to the

folding of strata and to the alterations in the volume of rocks which

are continually going on, may have some share in bringing about these

dislocations.

Measured by the destruction which was wrought to the interests of man,

earthquakes deserve to be reckoned among the direst calamities of

Nature. Since the dawn of history the records show us that the

destruction of life which is to be attributed to them is to be counted

by the millions. A catalogue of the loss of life in the accidents of

this description which have occurred during the Christian era has led

the writer to suppose that probably over two million persons have

perished from these shocks in the last nineteen centuries.

Nevertheless, as compared with other agents of destruction, such as

preventable disease, war, or famine, the loss which has been inflicted

by earth movements is really trifling, and almost all of it is due to

an obstinate carelessness in the construction of buildings without

reference to the risks which are known to exist in earthquake-ridden

countries.

Although all our exact knowledge concerning the distribution of

earthquakes is limited to the imperfect records of two or three

thousand years, it is commonly possible to measure in a general way

the liability to such accidents which may exist in any country by a

careful study of the details of its topography. In almost every large

area the process of erosion naturally leaves quantities of rock,

either in the form of detached columns or as detrital accumulations

deposited on steep slopes. These features are of relatively slow

formation, and it is often possible to determine that they have been

in their positions for a time which is to be measured by thousands of

years. Thus, on inspecting a country such as North America, where the

historic records cover but a brief time, we may on inquiry determine

which portions of its area have long been exempt from powerful shocks.

Where natural obelisks and steep taluses abound--features which would

have disappeared if the region had been moved by great shocks--we may

be sure that the field under inspection has for a great period been

exempt from powerful shaking. Judged by this standard, we may safely

say that the region occupied by the Appalachian Mountains has been

exempt from serious trouble. So, too, the section of the Cordilleras

lying to the east of what is commonly called the Great Basin, between

the Rocky Mountains and the Sierra Nevada, has also enjoyed a long

reign of peace. In glaciated countries the record is naturally less

clear than in those parts of the world which have been subjected to

long-continued, slow decay of the rocks. Nevertheless, in those fields

boulders are often found poised in position which they could not have

maintained if subjected to violent shaking. Judged by this evidence,

we may say that a large part of the northern section of this

continent, particularly the area about the Great Lakes, has been

exempt from considerable shocks since the glacier passed away.

The shores which are subject to the visitations of the great marine

waves, caused by earthquake shocks occurring beneath the bottom of the

neighbouring ocean, are so swept by those violent inundations that

they lose many features which are often found along coasts that have

been exempted from such visitations. Thus wherever we find extensive

and delicately moulded dunes, poised stones, or slender pinnacled

rocks along a coast, we may be sure that since these features were

formed the district has not been swept by these great waves.

[Illustration: Fig. 22.--Poised rocks indicating a long exemption from

strong earthquakes in the places where such features occur.]

Around the northern Atlantic we almost everywhere find the glacial

waste here and there accumulated near the margin of the sea in the

complicated sculptured outlines which are assumed by kame sands and

gravels. From a study of these features just above the level of high

tide, the writer has become convinced that the North Atlantic district

has long been exempt from the assaults of other waves than those which

are produced during heavy storms. At the present time the waves

formed by earthquakes appear to be of destructive violence only on the

west coast of South America, where they roll in from a region of the

Pacific lying to the south of the equator and a few hundred miles from

the shore of the continent, which appears to be the seat of

exceedingly violent shocks. A similar field occurs in the Atlantic

between the Lesser Antilles and the Spanish peninsula, but no great

waves have come thence since the time of the Lisbon earthquake. The

basin of the Caribbean and the region about Java appear to be also

fields where these disturbances may be expected, though in each but

one wave of this nature has been recorded. Therefore we may regard

these secondary results of a submarine earthquake as seldom phenomena.

DURATION OF GEOLOGICAL TIME.

Although it is beyond the power of man to conceive any such lapses of

time as have taken place in the history of this earth, it is

interesting, and in certain ways profitable, to determine as near as

possible in the measure of years the duration of the events which are

recorded in the rocks. Some astronomers, basing their conclusions on

the heat-containing power of matter, and on the rate at which energy

in this form flows from the sun, have come to the conclusion that our

planet could not have been in independent existence for more than

about twenty million years. The geologist, however, resting his

conclusions on the records which are the subject of his inquiry, comes

on many different lines to an opinion which traverses that entertained

by some distinguished astronomers. The ways in which the student of

the earth arrives at this opinion will now be set forth.

By noting the amount of sediment carried forth to the sea by the

rivers, the geologist finds that the lands of the earth--those, at

least, which are protected by their natural envelopes of

vegetation--are wearing down at a rate which pretty certainly does

not exceed one foot in about five thousand years, or two hundred feet

in a million years. Discovering at many places on the earth's surface

deposits which originally had a thickness of five thousand feet or

more, which have been worn down to the depths of thousands of feet in

a single rather brief section of geological time, the student readily

finds himself prepared to claim that a period of from five to ten

million years has often been required for the accomplishment of but a

very small part of the changes which he knows to have occurred on this

earth.

As the geologist follows down through the sections of the stratified

rocks, and from the remains of strata determines the erosion which has

borne away the greater part of the thick deposits which have been

exposed to erosion, he comes upon one of those breaks in the

succession, or encounters what is called an unconformity, as when

horizontal strata lie against those which are tilted. In many cases he

may observe that at this time there was a great interval unrepresented

by deposits at the place where his observations are made, yet a great

lapse of time is indicated by the fact that a large amount of erosion

took place in the interval between the two sets of beds.

Putting together the bits of record, and assuming that the rate of

erosion accomplished by the agents which operate on the land has

always been about the same, the geologist comes to the conclusion that

the section of the rocks from the present day to the lowest strata of

the Laurentian represents in the time required for their formation not

less than a hundred million years; more likely twice that duration. To

this argument objection is made by some naturalists that the agents of

erosion may have been more active in the past than they are at

present. They suggest that the rainfall may have been much greater or

the tides higher than they now are. Granting all that can be claimed

on this score, we note the fact that the rate of erosion evidently

does not increase in anything like a proportionate way with the

amount of rainfall. Where a country is protected by its natural

coating of vegetation, the rain is delivered to the streams without

making any considerable assault upon the surface of the earth, however

large the fall may be. Moreover, the tides have little direct cutting

power; they can only remove detritus which other agents have brought

into a condition to be borne away. The direct cutting power of the

tidal movement does not seem to be much greater in the Bay of Fundy,

where the maximum height of the waves amounts to fifty feet, than on

the southern coast of Massachusetts, where the range is not more than

five. So far as the observer can judge, the climatal conditions and

the other influences which affect the wear of rocks have not greatly

varied in the past from what they are at the present day. Now and then

there have been periods of excessive erosion; again, ages in which

large fields were in the conditions of exceeding drought. It is,

however, a fair presumption that these periods in a way balance each

other, and that the average state was much like that which we find at

present.

If after studying the erosive phenomena exhibited in the structure of

the earth the student takes up the study of the accumulations of

strata, and endeavours to determine the time required for the laying

down of the sediments, he finds similar evidence of the earth's great

antiquity. Although the process of deposition, which has given us the

rocks visible in the land masses, has been very much interrupted, the

section which is made by grouping the observations made in various

fields shows that something like a maximum thickness of a hundred and

fifty thousand feet of beds has been accumulated in that part of

geologic time during which strata were being laid down in the fields

that are subjected to our study. Although in these rocks there are

many sets of beds which were rapidly formed, the greater part of them

have been accumulated with exceeding slowness. Many fine shales, such

as those which plentifully occur in the Devonian beds of this country,

must have required a thousand years or more for the deposition of the

materials that now occupy an inch in depth. In those sections a single

foot of the rock may well represent a period of ten thousand years. In

many of the limestones the rate of accumulation could hardly have been

more speedy. The reckoning has to be rough, but the impression which

such studies make upon the mind of the unprejudiced observer is to the

effect that the thirty miles or so of sedimentary deposits could not

have been formed in less than a hundred million years. In this

reckoning it should be noted that no account is taken of those great

intervals of unrecorded time, such as elapsed between the close of the

Laurentian and the beginning of the Cambrian periods.

There is a third way in which we may seek an interpretation of

duration from the rocks. In each successive stage of the earth's

history, in different measure in the various ages, mountains were

formed which in time, during their exposure to the conditions of the

land, were worn down to their roots and covered by deposits

accumulated during the succeeding ages. A score or more of these

successively constructed series of elevations may readily be observed.

Of old, it was believed that mountain ranges were suddenly formed, but

there is, however, ample evidence to prove that these disturbed

portions of the strata were very gradually dislocated, the rate of the

mountainous growth having been, in general, no greater in the past

than it is at the present day, when, as we know full well, the

movements are going on so slowly that they escape observation. Only

here and there, as an attendant on earthquake shocks or other related

movements of the crust, do we find any trace of the upward march which

produces these elevations. Although not a subject for exact

measurements, these features of mountain growth indicate a vast lapse

of time, during which the elevations were formed and worn away.

Yet another and very different method by which we may obtain some

gauge of the depths of the past is to be found in the steps which have

led organic life from its lowest and earliest known forms to the

present state of advancement. Taking the changes of species which have

occurred since the beginning of the last ice epoch, we find that the

changes which have been made in the organic life have been very small;

no naturalist who has obtained a clear idea of the facts will question

the statement that they are not a thousandth part of the alterations

which have occurred since the Laurentian time. The writer is of the

opinion that they do not represent the ten thousandth part of those

vast changes. These changes are limited in the main to the

disappearance of a few forms, and to slight modifications in those

previously in existence which have survived to the present day. So far

as we can judge, no considerable step in the organic series has taken

place in this last great period of the earth's history, although it

has been a period when, as before noted, all the conditions have

combined to induce rapid modifications in both animals and plants. If,

then, we can determine the duration of this period, we may obtain a

gauge of some general value.

Although we can not measure in any accurate way the duration of the

events which have taken place since the last Glacial period began to

wane, a study of the facts seems to show that less than a hundred

thousand years can not well be assumed for this interval. Some of the

students who have approached the subject are disposed to allow a

period of at least twice this length as necessary for the perspective

which the train of events exhibits. Reckoning on the lowest estimate,

and counting the organic changes which take place during the age as

amounting to the thousandth part of the organic changes since the

Laurentian age, we find ourselves in face once again of that

inconceivable sum which was indicated by the physical record.

Here, again, the critics assert that there may have been periods in

the history of the earth when the changes of organic life occurred in

a far swifter manner than in this last section of the earth's history.

This supposition is inadmissible, for it rests on no kind of proof; it

is, moreover, contraindicated by the evident fact that the advance in

the organic series has been more rapid in recent time than at any

stage of the past. In a word, all the facts with which the geologist

deals are decidedly against the assumption that terrestrial changes in

the organic or the inorganic world ever proceed in a spasmodic manner.

Here and there, and from time to time, local revolutions of a violent

nature undoubtedly occur, but, so far as we may judge from the aspect

of the present or the records of the past, these accidents are

strictly local; the earth has gone forward in its changes much as it

is now advancing. Its revolutions have been those of order rather than

those of accident.

The first duty of the naturalist is to take Nature as he finds it. He

must avoid supposing any methods of action which are not clearly

indicated in the facts that he observes. The history of his own and of

all other sciences clearly shows that danger is always incurred where

suppositions as to peculiar methods of action are introduced into the

interpretation. It required many centuries of labour before the

students of the earth came to adopt the principle of explaining the

problems with which they had to deal by the evidence that the earth

submitted to them. Wherever they trusted to their imaginations for

guidance, they fell into error. Those who endeavour to abbreviate our

conception of geologic time by supposing that in the olden days the

order of events was other than that we now behold are going counter to

the best traditions of the science.

Although the aspect of the record of life since the beginning of the

Cambrian time indicates a period of at least a hundred million years,

it must not be supposed that this is the limit of the time required

for the development of the organic series. All the important types of

animals were already in existence in that ancient period with the

exception of the vertebrates, the remains of which have apparently now

been traced down to near the Cambrian level. In other words, at the

stage where we first find evidence of living beings the series to

which they belong had already climbed very far above the level of

lifeless matter. Few naturalists will question the statement that half

the work of organic advance had been accomplished at the beginning of

the Cambrian rocks. The writer is of the opinion that the development

which took place before that age must have required a much longer

period than has elapsed from that epoch to the present day. We thus

come to the conclusion that the measurement of duration afforded by

organic life indicates a yet more lengthened claim of events, and

demands more time than appears to be required for the formation of the

stratified rocks.

The index of duration afforded by the organic series is probably more

trustworthy than that which is found in the sedimentary strata, and

this for the reason that the records of those strata have been

subjected to numerous and immeasurable breaks, while the development

of organic life has of necessity been perfectly continuous. The one

record can at any point be broken without interrupting the sequences;

the other does not admit of any breaches in the continuity.

THE MOON.

Set over against the earth--related to, yet contrasted with it in many

ways--the moon offers a most profitable object to the student of

geology. He should often turn to it for those lessons which will be

briefly noted.

In the beginning of their mutual history the materials of earth and

moon doubtless formed one vaporous body which had been parted from the

concentrating mass of the sun in the manner noted in the sketch of

the history of the solar system. After the earth-moon body had

gathered into a nebulous sphere, it is most likely that a ring

resembling that still existing about Saturn was formed about the

earth, which in time consolidated into the satellite. Thenceforth the

two bodies were parted, except for the gravitative attraction which

impelled them to revolve about their common centre of gravity, and

except for the light and heat they might exchange with one another.

The first stages after the parting of the spheres of earth and moon

appear to have been essentially the same in each body. Concentrating

upon their centres, they became in time fluid by heat; further on,

they entered the rigid state--in a word, they froze--at least in their

outer parts. At this point in their existence their histories utterly

diverge; or rather, we may say, the development of the earth continued

in a vast unfolding, while that of the moon appears to have been

absolutely arrested in ways which we will now describe.

With the naked eye we see on the moon a considerable variation in the

light of different parts of its surface; we discern that the darker

patches appear to be rudely circular, and that they run together on

their margins. Seeing this little, the ancients fancied that our

satellite had seas and lands like the earth. The first telescopes did

not dispel their fancies; even down to the early part of this century

there were astronomers who believed the moon to be habitable; indeed,

they thought to find evidence that it was the dwelling place of

intelligent beings who built cities, and who tried to signal their

intellectual kindred of this planet. When, however, strong glasses

were applied to the exploration, these pleasing fancies were rudely

dispelled.

Seen with a telescope of the better sort, the moon reveals itself to

be in large part made up of circular depressions, each surrounded by a

ringlike wall, with nearly level but rough places between. The

largest of these walled areas is some four hundred miles in diameter;

thence they grade down to the smallest pits which the glass can

disclose, which are probably not over as many feet across. The writer,

from a careful study of these pits, has come to the conclusion that

the wider are the older and the smaller the last formed. The rude

elevations about these pits--some of which rise to the height of ten

thousand feet or more--constitute the principal topographic reliefs of

the lunar surface. Besides the pits above mentioned, there are

numerous fractures in the surface of the plains and ringlike ridges;

on the most of these the walls have separated, forming trenches not

unlike what we find in the case of some terrestrial breaks such as

have been noted about volcanoes and elsewhere. It may be that the

so-called canals of Mars are of the same nature.

[Illustration: Fig. 23.--Lunar mountains near the Gulf of Iris.]

The most curious feature on the moon's surface are the bands of

lighter colour, which, radiating from certain of the volcanolike

pits--those of lesser size and probably of latest origin--extend in

some cases for five hundred miles or more across the surface. These

light bands have never been adequately explained. It seems most likely

that they are stains along the sides of cracks, such as are sometimes

observed about volcanoes.

The eminent peculiarity of the moon is that it is destitute of any

kind of gaseous or aqueous envelope. That there is no distinct

atmosphere is clearly shown by the perfectly sharp and sudden way in

which the light of a star disappears when it goes behind the moon and

the clear lines of the edge of the satellite in a solar eclipse. The

same evidence shows that there is no vapour of water; moreover, a

careful search which the writer has made shows that the surface has

none of those continuous down grades which mark the work of water

flowing over the land. Nearly all of the surface consists of shallow

or deep pits, such as could not have been formed by water action. We

therefore have not only to conclude that the moon is waterless, but

that it has been in this condition ever since the part that is turned

toward us was shaped.