SECTION VII.--THE TIME NECESSARY FOR THE FORMATION OF THE STRATIFIED

ROCKS.

There are no means of which the geologist can avail himself to determine

the antiquity of the earth, or the amount of time since the sedimentary

deposits commenced. But a nigh degree of antiquity may yet be shown.

The materials for all the stratified rocks have been obtained by the

destruction of previously solidified igneous rocks. This destruction may

have been accomplished in part by the operation of volcanic forces, but

much of it is the result of slow disintegration, and of the eroding

power of running water; and we can scarcely conceive of a period

sufficiently protracted for such results.

This conclusion of the high antiquity of the earth is confirmed by

observing that the stratified rocks consist of layers often not thicker

than sheets of paper, and probably not averaging the tenth of an inch;

and yet each layer is separate from the rest, in consequence of some

change in the conditions under which it was deposited. Each layer was

probably produced by the deposition of all the sediment furnished at one

time, and hence only as many layers would be formed in a year as the

number of freshets in the rivers which furnished the materials. If we

consider the fossiliferous and metamorphic rocks to be each forty

thousand feet in thickness,--which is not too large an estimate,--we

must reckon the years by hundreds of thousands to make the time

sufficiently extended for the result.

All the formations of any considerable extent now above the surface of

the sea existed before the creation of man, for none of them contain any

evidence of the existence of human beings; and if they had existed while

these strata were forming, sufficient evidence would have been left of

the fact, either in the form of fossilized human bones, or of works of

human art. Hence, whatever be the estimate which we form of the

antiquity of the earth, from the slowness of denudation, or from the

thickness of the strata, we must now add to that estimate the period

elapsed since the creation of the human species.

We have seen that at different periods of the earth's history different

species of animals inhabited it. We are unable to fix with accuracy the

ordinary duration of species. But the species which are now extinct

probably had an existence as long-continued as will be enjoyed by

species now living. Many recent species are known to have existed at

least nearly six thousand years, without, in most cases, any indications

of their soon becoming extinct. Whatever period be assigned as the

ordinary duration of species, that period has been several times

repeated; for the earth has been several times re-peopled, and every

time by species which had not before existed.

Moreover, the _amount_ of organic matter in the strata must have

required long periods of time for its accumulation. The vegetable

deposits, now converted into coal, are generally several feet thick, and

often over a hundred feet, and are known to extend over several thousand

square miles, both in this country and in Europe. Many of the

sedimentary rocks consist almost entirely of animal remains. The

mountain limestone, for instance, is eight hundred feet or more in

thickness, and in some places consists of the exuviæ of encrinites and

testacea.

In other cases the length of time required is shown, not from the amount

of organic remains, but from the evidence that they were deposited very

slowly. The polishing stone called tripoli is found in beds of ten or

twelve feet in thickness, and is composed entirely of the siliceous

shells of animalcules, so minute that, according to the estimate of

Ehrenberg, the number in a cubic inch is forty-one billions. Several

other rocks, such as semi-opal and flint, are sometimes found to have a

similar constitution. The time necessary for the accumulation of beds

several feet thick by the shells of animalcules so minute must have been

very great.

Each of these facts carries us back to a period immeasurably anterior to

the creation of man, as the epoch when the sedimentary deposits

commenced. There are no facts in geology which point to a different

conclusion. It is of the utmost importance to the geological student to

familiarize himself with this principle. It will assist him in

comprehending the greatness of geological changes, and in applying other

principles in explanation of geological phenomena.

This principle, so obvious to any one who allows himself to reason from

the facts which geology presents, has sometimes been regarded as at

variance with the Mosaic account of the creation. And if this account

really assigns an antiquity to the earth of not more than six thousand

years, the difficulty exists.

The statements made by Moses are found, upon examination, to be of the

most general character. They assert, in the first place, simply that "In

the beginning God created the heaven and the earth." The time which

elapsed after this first act, and previously to the acts of creation

subsequently recorded, is not limited by the sacred narrative. It may

have been during this indefinite lapse of time that God gave existence

and enjoyment to a large number of animal species on the surface of the

earth, and at the same time effected most of those physical changes in

the crust of it which have rendered it a fit abode for intellectual and

moral beings.

But if the word _day_, in the first chapter of Genesis, be considered to

mean a prolonged period (and philologists regard such an interpretation

as admissible), then that chapter is a record of the most important

events in the history of the earth up to and including the introduction

of man. And the account, thus understood, coincides with the results of

geological examinations.

Instead, then, of discrepancy between the works and the word of God, we

have this remarkable fact, that a history of the earth, written long

before the science of geology was known, is not contradicted, but

confirmed, by the progress of science thus far.

CHAPTER III.

OF THE CHANGES TO WHICH THE CRUST OF THE EARTH HAS BEEN SUBJECTED.

SECTION I.--CHANGES WHICH HAVE TAKEN PLACE AT GREAT DEPTHS BELOW THE

SURFACE.

The lowest change of winch we can gain any information is the _formation

of granite_. It will be shown hereafter that it has been in a melted

state, and that it has taken its present form on cooling. But whether

any considerable portions of the granitic masses, or of the melted

masses now below the surface, have resulted from the fusion of

stratified rocks, we have not the means of determining. It is, however,

not improbable, that in the changes of level to which the crust of the

earth has been subjected, the stratified rocks may have gone down so far

as to become melted. At the same time, the melted rock which is thrown

to the surface by volcanoes is subjected to the various destroying

agencies by which it becomes sedimentary matter, to be deposited as

mechanical strata. Thus, as the igneous rocks from below are brought up

to furnish materials for mechanical strata, there must be an equal

amount of depression of the mechanical strata towards the seats of

igneous action. And if this change takes place more rapidly than the

thickness of crust increases, then portions of the sedimentary rocks

must be undergoing fusion.

Next above the granite an immense thickness of rock occurs, which

exhibits, from its stratification and from the water-worn fragments

which it contains, distinct evidence of its mechanical origin. And yet

it is very different from the later mechanical formations. It is more

highly crystalline; it has, to a great extent, assumed a cleavage

distinct from the planes of stratification, and chemical affinity has

been so far active as to produce new combinations, and give to them

their peculiar crystalline form, as in the case of garnets, iron

pyrites, &c. These strata also differ from those above them in

containing no organic remains. It is not certain that organic life

existed on the earth at the time when these rocks were deposited. Either

it did not, or the evidence of it in the strata of that period has been

obliterated. The changes have at least been sufficient to justify their

being characterized as _metamorphic rocks_.

SECTION II.--CHANGES IN THE MASS OF THE STRATIFIED ROCKS.

1. The stratified rocks were deposited as mud or sand, and were at first

in a yielding state. Most of these deposits have become _solidified

rock_, such as limestone, clay slate and sandstone. The chalk of England

is, however, but imperfectly consolidated, the great sandstone formation

of New Holland is a friable mass easily disintegrated, and occasionally

beds of clay in a plastic state are found as far down as the coal. Among

the later rocks the solidification is less general, though there is some

degree of hardening in all except the most superficial layers. The

_fissile structure_ results from the solidification of the particles

composing each layer separately.

2. Since the solidification of the strata, or perhaps in connection with

it, there has been something of movement among the particles, resulting

in mineral veins, conchoidal structure, &c. One of the most general

changes of this kind is that by which a mass becomes separable into thin

sheets, independent of the stratification, and not parallel with it.

This structure is represented by Fig. 48, in which the heavier lines are

those of stratification, and the lighter of _cleavage_.

3. The strata have been everywhere more or less broken, and the

_fractures_, nearly vertical, extend to groat depths. When a fracture

reaches the surface, it often becomes a channel for water. It is thus

widened by the erosion, the deepest parts become filled with debris, and

it becomes a _gorge_, _ravine_ or _valley_.

[Illustration: Fig. 48.]

If the fracture does not come to the surface, it becomes a _cavern_. In

limestone, caverns which are formed in this way are very frequent, and

extend for many miles. There is generally a stream of water running

through them, but not of sufficient volume to have produced the erosion

which has been effected.

When the sides of the fracture are but little separated, some mineral

often separates itself from the adjacent rock, and filling up the space,

reunites the broken parts. It is then called a _vein of segregation_

(Fig. 49, _a b_). But the fracture is more frequently filled with some

volcanic rock injected from below. It is then a _dike_ (_c_ _d_), and

may have a width of many rods, though it often diminishes in width till

it is a mere thread. A dike of which the injected material is a metallic

ore is a _mineral vein_.

[Illustration: Fig. 49.]

4. The uplifting force by which the fracture is produced has frequently

raised the rock on one side higher than it has on the other. This is

called a _fault_. (Fig. 50.) The unequal movements by which the fault is

produced seem in some instances to have been repeated several times, and

the grinding of the broken edges upon each other has polished and

striated the sides of the fracture.

[Illustration: Fig. 50.]

[Illustration: Fig. 51.]

[Illustration: Fig. 52.]

5. Sedimentary rocks are often found with the planes of their strata

more or less _inclined_. It is evident that they were not thus formed.

The depositions of sediment from water will always be horizontal, or, at

most, only slightly inclined. But there is often evidence in the rock

itself that its strata were once horizontal. It is frequently observed

that vertical strata contain pebbles with their longer axes in the plane

of the strata. (Fig. 51.) When these pebbles were deposited, the longer

axes would take, on an obvious mechanical principle, a horizontal

position. Their present vertical position must have resulted from a

change in the position of the strata in which they are enclosed. The

same thing is shown by the position of a petrified forest in the south

of England, known as the Portland dirt-bed. Some parts of it are

inclined at an angle of forty-five degrees. The position of the

vegetable remains (Fig. 52) shows that when they were growing the

surface was horizontal.

The line _b d_ (Fig. 53), on inclined strata which makes with the

horizon the greatest angle, is called the _direction of the dip_. The

angle thus formed (_a b d_) is the _angle of inclination_. When inclined

strata come to the surface, the exposed edge, _b c_, is the _outcrop_,

and the line of outcrop on a horizontal surface is called the _strike_

of the strata.

[Illustration: Fig. 53.]

When the inclined position is produced by an uplift of the strata, along

a given line, so that they dip in opposite directions, this line is

called an _anticlinal axis_, as at Fig. 54. If, however, the strata are

fractured along this line, as at _b_, the fracture becomes a _valley of

elevation_.

[Illustration: Fig. 54.]

If depression take place along a given line, as at _c_, the strata will

dip towards this line, and it will be a _synclinal axis_. The depression

will be a _valley of subsidence_. A synclinal axis would also be

produced by an elevation of the strata, as at _d_ and _e_, on each side

of it, and the valley thus produced is one of elevation.

When successive sets of strata, as _f_ and _d_, Fig. 53, are not

parallel, they are said to be _unconformable_.

[Illustration: Fig. 55.]

[Illustration: Fig. 56.]

6. When the strata are subjected to displacement, they do not always

take a merely inclined position, but are often _contorted_ (Fig. 55), or

folded together (Fig. 56). These _folded axes_ frequently succeed each

other for many miles. (See Figs. 7 and 82.) In the case represented by

Fig. 56, if the highest portion has been removed, so that the line _a b_

represents the actual surface, we shall have apparently a succession of

deposits, of which those at _b_ would be the newest, and the oldest

would be found at _a_, when in fact the strata at the extremities are

parts of the same layer.

It is probable that disturbances like those now mentioned have been

taking place continually, in different places, from the earliest times.

There have been no periods of universal disturbance, and none of

universal repose. On the contrary, the periods of disturbance in one

part of the world have been periods of repose in another. For example,

the coal measures of Europe were much broken and disturbed before the

deposition of the new red sandstone, and the close of the coal period

was at one time supposed to have been a period of general convulsion. It

is now ascertained that the principal coal-fields in this country were

not much disturbed at that period, and have not been since.

SECTION III.--CHANGES OF ELEVATION AND SUBSIDENCE.

The continents, if we except the more rugged and broken portions, rise

from the sea with an almost imperceptible ascent; and even the mountains

have a much gentler slope than we are apt to suppose, so that a section

of the earth parallel to the equator would be almost a perfect circle.

The slope of a mountain, from its base to its highest point, rarely

forms with the horizon an angle of as much as twelve degrees. In the

following figure (57), A represents the peak of Chimborazo, B of

Teneriffe, C of Ætna, and D of Mount Loa, the principal volcano of the

Sandwich Islands. The highest mountains would be represented on a

twelve-inch globe by an altitude of less than the one-hundredth of an

inch above the level of the sea. But the rising and sinking of these

masses, though so small compared with the dimensions of the earth, are

yet geological changes on the largest scale.

1. _The Elevation of Mountains._--Mountains have formerly been covered

with the waters of the ocean. This is evident, in the case of some

mountains, from the existence of stratified rocks reaching to the

summits. The stratification could have been produced only by deposition

from water. It is, moreover, evident from the existence of marine

fossils, distributed through these strata, so abundantly, that they

cannot be accounted for on any other hypothesis than that the animals

lived and died where the remains of them are now found. These strata

must therefore have formed the bed of the sea while the fossils were

accumulating.

[Illustration: Fig. 57.]

There is no _direct_ evidence that the granitic mountain peaks were ever

submerged. But there is reason for believing that the sedimentary strata

which now occupy the lower slopes were, at the time of their deposition,

continuous,--the igneous rock having subsequently broken through

them,--so that the waters of the ocean once rested on the whole area

which the mountain now occupies!

If the ocean could ever have been above its present level sufficiently

to have covered all the sedimentary rocks, we might assume that the

height of mountains has not been changed. But the level of the ocean

cannot be subject to much variation. The total amount of water on the

globe is always the same. If the continents and mountains were all

submerged at once, and the waters were expanded by the highest

temperature consistent with the liquid form, there would not be a change

of level of more than two hundred and fifty feet. We may assume, then,

that the ocean level has always been essentially the same that it now

is. We must therefore conclude that the sedimentary rocks, and the

mountains of which they form a part, have been elevated to their present

position from the bed of the sea.

Different mountain ranges have been elevated at different periods. The

silurian and carboniferous formations were deposited before the

Alleghany Mountains, which they contributed to form, were elevated;

while the new red sandstone and the cretaceous and tertiary formations

were deposited subsequently to the upheaval. They are accordingly found

at the base of the range, nearly horizontal, and have risen above the

level of the ocean only as the continent generally has risen. The

Pyrenees were elevated after the deposition of the cretaceous rocks, and

have carried them up so that they appear at a high angle, while the

tertiary rocks at the base are horizontal, as in the United States. The

Andes have carried up the tertiary rocks with them, and their elevation

must therefore belong to a recent period. It appears that they are even

yet rising.

It has recently been shown that the Alps have been subjected to upheaval

at several distinct periods. At the close of the silurian period they

formed a cluster of islands. At the commencement of the tertiary period

they became a mountain range, and at the close of that period they were

thrown up some two thousand feet higher, to their present position.

Nearly the same things will probably be found true of other mountain

ranges, when their structure has been minutely studied.

The elevation of contiguous parallel ridges will necessarily leave

intervening _valleys of elevation_. As mountain ranges generally consist

of several such ridges, valleys of this description are numerous, and

they are often of great extent.

It is obvious that there are mountains in the sea of as great height

above the lowest valleys as the mountains of continents are above the

level of the sea. If a new continent should hereafter be formed by the

elevation of a large area of the bed of the sea, the existing mountains,

now appearing in the form of islands, would partake of the general

movement, and the new continent would have the same general diversities

of surface as existing continents. The mountains would have existed long

before the continent. It is therefore to be supposed that the mountains

of the present continents were elevated before the continents, and that

they stood for long periods as islands, exposed to the action of waves,

tides, and marine currents.

2. _The Elevation of Continents._--Continents have been elevated by so

slow a movement that it has not generally been perceived, even when they

have been peopled by nations advanced in civilization. And yet

satisfactory evidence is always left of former sea-levels.

Almost every seaboard furnishes examples of beaches, evidently once

washed by the sea, but now elevated more or less above high water.

At Lubec, near the northern extremity of the coast of Maine,

barnacles[B] are found attached to the rocks eighteen feet above high

water. The pilots at that place, and for a hundred miles north and south

of it, speak of the ship-channels as diminishing in depth, though it is

certain that they are not filling up. Such facts are to be explained

only by supposing that the coast is rising.

[B] The barnacle is a marine animal, permanently fixed

to the rocks, and live but a short time without being

surrounded by sea-water.

Lakes are numerous throughout the northern portions of North America,

which are receiving annually large quantities of sediment, and must

ultimately become alluvial plains. Those of moderate depth, as Lake

Erie, cannot require periods very protracted to fill them. Their

continuance in such abundance indicates that the elevation of the

continent to its present height is comparatively recent. This conclusion

is confirmed by evidence of another kind. Throughout this region of

lakes, beds of clay containing the remains of existing species of marine

animals, are found at all elevations from the sea-coast, to the height

of about four hundred feet, but not higher. These clay beds are very

recent, and were deposited when the surface was four hundred or five

hundred feet lower than it now is; and this amount of elevation has left

the existing lakes scattered over the surface.[C]

[C] "It is remarkable that on the shores of the great

lakes there are certain plants the proper station of

which is the immediate neighborhood of the ocean, as if

they had constituted part of the early flora of those

regions when the lakes were filled with salt water, and

have survived the change that has taken place in the

physical conditions of their soil."--_Torrey's Flora of

the State of New York._

[Illustration: Fig. 58.]

The following (Fig. 58) exhibits Europe as it was during the Silurian

epoch, and Fig. 59 as it was at the commencement of the tertiary epoch.

The land, as it then existed, is represented by the white surface, the

present waters by the dark shading, and the land which has been

reclaimed from the ocean by elevation since those periods by the lighter

shading.

[Illustration: Fig. 59.]

The whole southern part of South America, embracing an area equal to

that of Europe, has been elevated within a very recent period; and some

parts of it, if not all of it, are still rising. The shells found on the

plains from Brazil to Terra del Fuego, and on the Pacific coast, at a

height of from one hundred to thirteen hundred feet, are identical with

those now inhabiting the adjacent seas. And "besides the organic

remains, there are, in very many parts, marks of erosion, caves, ancient

beaches, sand-dunes, and successive terraces of gravel," all which must

have resulted from the action of the waves at a period not remote. At

Lima, articles of human skill peculiar to the original inhabitants of

Peru were found imbedded in a mass of sea-shells eighty-three feet above

the present sea level. The elevation on the Pacific coast has been in

part by sudden uplifts of a few feet at a time; but it is found, from

time to time, that there has been a change of level, amounting to a

foot or more in a year, when there have been none of these sudden

movements.

A considerable portion of Europe, reaching from North Cape in Norway to

near the southern part of Sweden, more than a thousand miles, and from

the Atlantic to St. Petersburg, more than six hundred miles, has been

rising at the rate of about three feet in a century, for at least two

centuries, and probably much longer. This change is proved by the

occurrence, at considerable elevations above the sea, of shells now

found in the Baltic; by rocks once sunken, now raised above the surface

of the sea, and by ancient seaports having become inland towns. To

determine the truth by actual measurement, the Royal Academy of

Stockholm, about thirty-five years since, caused marks to be cut in the

rocks along the coast, to indicate the ordinary level of the water. This

is easily ascertained, as the Baltic is nearly a tideless sea. The

present level of the sea, compared with that indicated by the marks

before mentioned, leaves no doubt that the country is rising.

3. _The Subsidence of Land._--Elevations can be shown to have taken

place by fossils, and by other evidences of former sea levels which are

left on the surface; but depressions leave but few indications of change

of level. It is yet doubtful whether the depression is equal to the

elevation; that is, whether the amount of land remains nearly constant,

or whether there has been an augmentation of the dry land within the

tertiary and recent periods. We are certain that the augmentation, if

any, has not been equal to the elevation, for subsidences to a great

amount are known to have taken place.

There are occasional instances of submerged forests seen at low tide, at

some distance from the shore. There are several near the coast of

England and Scotland, and near the coast of Massachusetts. They are but

a few feet below low water, and do not indicate a subsidence of more

than about twenty feet.

Numerous instances are on record of the sinking down of wharfs and

buildings near the sea during earthquakes. Almost every violent

earthquake is accompanied by a change of level. The changes of this kind

which have been noticed are in seaport towns, because greater facilities

are there afforded for detecting them, and because loss of property

awakens attention to them; but there is every reason to suppose that

these changes of level extend to great distances both into the country

and into the sea.

[Illustration: Fig. 60.]

An immense area in the Indian and Pacific Oceans, probably ten millions

of square miles, is undergoing change of level. The lines A B and D G

(Fig. 60) represent nearly the axes of depression; while an intermediate

and two exterior parallel lines would represent axes of elevation. The

evidence of these changes is found principally in the peculiarities of

the wall of coral rock encircling the islands.

The following figures represent, in sections, modifications of form of

the same island. The coral wall built up around the island by the

polyps, from the depth of fifty, or at most of a hundred feet, is shown

at _c c_ (Fig. 61). If the island is elevated, this wall becomes a

_fringing reef_ (Fig. 62), _b'_ becoming the level of the sea, and the

animal begins a new wall at the same depth as before. But if the island

is gradually sinking, the wall is kept built up to the surface, and

becomes a _barrier reef_ (Fig. 63). A channel is thus left between the

island and the reef, which, though gradually filling up with broken

coral or other sediment, is generally deep enough for a ship-channel. If

the island continue to subside till it disappears, and the coral wall is

still kept at the surface, it then becomes an _atoll_, a circular coral

island (Fig. 64), often of many leagues in diameter, beaten by the surf

on the outer edge, but enclosing a quiet lake, which communicates only

by occasional channels with the ocean.

[Illustration: Fig. 61.]

[Illustration: Fig. 62.]

[Illustration: Fig. 63.]

[Illustration: Fig. 64.]

The islands contiguous to the lines A B and C D (Fig. 60) are uniformly

atolls, or are surrounded by barrier reefs, and are therefore subsiding;

while the islands at a distance from these lines are surrounded by

fringing reefs, which indicate that they are rising.

A well-authenticated instance of gradual subsidence is that of

Greenland. The entire western coast, from its southern extremity to

Disco Island, a distance of six hundred miles, has for the last two

centuries been slowly subsiding. The dwelling-houses and places of

worship built by the early European settlers are now in part or entirely

submerged. The natives are said to be aware of the subsidence, and never

build their huts near the sea.

4. We have thus seen that both elevation and depression may take place.

There is reason to believe that these changes of level have, in some

cases, been several times repeated. In one of the eastern ranges of the

Andes, opposite to Chili, there is a mass of marine strata of five

thousand feet in thickness. About the middle of the series there occurs

a silicified forest. In one place a clump of coniferous trees was found

of more than fifty in number, and a foot or more in diameter. The base

of the strata must have been twenty-five hundred feet below the surface

of the sea, in order to admit of the deposition of the first half of it.

It was then elevated, so that a forest grew upon its surface. It was

then depressed at least twenty-five hundred feet, more, to admit of the

deposition of the subsequent strata, and the whole is now uplifted to

form a mountain range of eight thousand feet in height.

[Illustration: Fig. 65.]

The temple of Jupiter Serapis, near Naples, in Italy, was built near the

sea, about eighteen hundred years ago. It was gradually submerged, and

finally lost by the deposition of sediment nearly to the top of the

columns. It was afterwards elevated, so as to be entirely above the

level of the sea. The remains of the temple (Fig. 65) were afterwards

discovered by the columns projecting a little above the ground. The

sediment was removed to the depth of forty-six feet, when the workmen

came to the base of the columns, and to a pavement seventy feet in

diameter. In 1807 an artist was employed to take drawings of the ruin.

The pavement was then above the level of the sea. Sixteen years

afterwards the same artist found the pavement covered with water, and

the depth has continued to increase since that time. It is considered

that for the last forty years the depression has been three-fourths of

an inch a year.

Instances enough have now been given to show how extensively the system

admits of change. They are sufficient to justify us in searching for

indications of great revolutions in past times, even where no such

indications have as yet been discovered. They will serve as a key to

many otherwise inexplicable phenomena, In order to the interpretation of

such phenomena readily, we must cease to look upon these as exceptional

cases, and regard them not only as facts, but as facts of frequent

occurrence.

From the examples which have now been given, as well as from

speculations upon the cause of these changes, it seems highly probable

that all the surface of the solid portion of the earth, whether land or

the bed of the sea, is undergoing changes of level. It may be so gradual

that in the life of an individual it would be imperceptible, even where

the best means of detecting it exist. These means are generally the

works of man, and they are themselves so liable to change, that it would

be scarcely possible to detect variations of level, which amount to but

a few inches in a century.

If we admit that the relations of land and water have always been

variable, it is impossible to arrive at any certain conclusion as to the

amount, position or form, of the dry land at any former period. We may

determine, with some degree of certainty, what portions of the present

continents were submerged at particular epochs. Thus, we may infer that

most of this country was submerged during the silurian period, from the

great extent of the Silurian rocks; and, from the limited extent of the

chalk formation in this country, we know that during the cretaceous

period most of the continent was above the surface of the sea. But we

have absolutely no data for determining what portions of the bed of the

sea were at any time dry land.

It is supposable that the land has been principally confined to the

equatorial regions at one period, and to the polar at another. At still

a different period the land may have existed as islands scattered

through a general ocean. These relations may, therefore, be assumed to

have existed, if there are geological phenomena which best accord with

such relations.

SECTION IV.--CHANGES ON THE SURFACE OF THE EARTH.

1. The principal changes of this class consist in the wearing down and

removing immense quantities of the surface rock. The form in which the

_igneous rocks_, of which the entire crust of the earth was originally

composed, now appear, furnishes no assistance in judging of the amount

of denudation which they have suffered. We can judge only from the

amount of rock for which they have furnished the materials, and these

are the whole sedimentary series which exist both as dry land and as the

bed of the sea.

2. The _sedimentary rocks_ have also been subject to great denudation;

and we often have, in what is left, some indications of how much has

been removed. One of these indications consists in the now level surface

of those portions of country in which large _faults_ exist. By the

excavations for coal, in England, faults have been discovered of five or

six hundred feet. At the time that they were formed, the surface must

have presented precipitous escarpments (as represented by the dotted

lines in Fig. 50) of a height equal to the dislocation; but the whole is

now reduced to a general level (_z z_), denuding causes having removed

the elevated portions.

The extent of _valleys_ will often give some idea of the amount of

denudation to which a region has been subjected. In the north-west of

Scotland there is a succession of hills of about three thousand feet in

elevation, consisting, for the upper two thousand feet, of horizontal

strata of old red sandstone. (Fig. 66.) We cannot conceive that these

mountain masses were deposited in their present isolated form. The whole

intervening spaces must have been filled with strata continuous with

those by which the elevations are formed.[D]

[D] "I entertain little doubt that when this loftier

portion of Scotland, including the entire Highlands,

first presented its broad back over the waves, the

upper surface consisted exclusively, from one

extremity to the other, of a continuous tract of old

red sandstone; though, ere the land finally emerged,

the ocean currents of ages had swept it away, all

except in the lower and last raised borders, and in

detached localities where it still remains, as in the

pyramidal hills of Western Rosshire, to show the

amazing depth to which it had once overlaid the

inferior rocks."--_Miller, Old Red Sandstone_,

_p. 22_.

[Illustration: Fig. 66.]

[Illustration: Fig. 67.]

A somewhat similar instance occurs in the Connecticut river sandstone,

in the central part of Massachusetts. The following figure (Fig. 67)

represents two mountains of 1 the sandstone, between which the

Connecticut river flows. The dotted lines indicate a depth of one

thousand feet of the rock which has been swept away. It is also thought

that a bed of equal depth has been removed from this section southward,

through the State of Connecticut, to the sea-coast.

3. _Valleys_, and even many of the larger valleys, are produced by the

wearing down of the surface. The lower portion of the Connecticut valley

is one of denudation, though in its upper part it is a valley of

elevation, resulting from the upheaval of the Green and White Mountains.

The water-courses from the mountains are transverse to the direction of

the ranges, and generally consist of valleys of denudation. These

valleys were no doubt originally fractures, produced while the mountains

were rising. The fractures have been subsequently widened by denudation

into valleys.

[Illustration: Fig. 68.]

4. The rocky surface, beyond the fortieth parallels of latitude, and in

the vicinity of glacier-producing mountains, is generally covered with

_grooves_ and _striæ_ (Fig. 68), varying from several inches in depth to

the finest perceptible lines. Rocks that are of a soft consistence, or

which have been long exposed to atmospheric agents, seldom exhibit these

marks, though there are probably few places, outside of the parallels

before mentioned, where the rocky surface, if it has been protected from

atmospheric decay, does not contain such grooving.

5. Another change at the surface consists in the formation of a _soil_;

that is, of a superficial layer, of no great thickness, of earthy

matter, a large proportion of which is always in a minutely divided

state. In some instances it is common sediment, unsolidified; in

others, it consists of the surface rock in a state of disintegration;

but a large part of the soil within the region where the grooved

surfaces are found consists of materials transported from a distance.

Soils are distinguished according to their predominant minerals, as

siliceous, aluminous and calcareous. If siliceous matter is in excess,

it will be a light, warm soil, and allow the water to pass through it

too freely. If the clay predominates, the soil is cold, stiff, and too

retentive of moisture. A proper admixture of these three ingredients

constitutes the best soils. There are some other mineral ingredients

essential to the productiveness of soils, but they are always in small

proportion. In addition to the inorganic part which is common to the

upper soil, and the subsoil, there is required, in order to render the

upper layer productive, a large admixture of decaying animal and

vegetable matter.

SECTION V.--CHANGES OF CLIMATE.

Our means of determining the climate of any former period consists in a

comparison of the fossils of such period with the existing forms of life

in warm and cold climates.

The earliest abundant _vegetation_ consisted principally of ferns,

rushes and mosses, and a larger growth was attained than is attained by

any of the allied forms of the present time. We may infer that the

circumstances under which these lower forms of vegetable life are now

produced in the largest proportion, compared with other forms, and under

which they grow to the largest size, are the circumstances approaching

most nearly those under which the early vegetation was produced. These

circumstances are found to be a position elevated but little above the

level of the sea, a humid atmosphere, and the highest terrestrial

temperature. Such facts favor the conclusion that during the coal period

an ultra-tropical climate prevailed, and that the land existed in the

form of low islands, thickly set in a general ocean.

The peculiar characters of some of the _animal fossils_, from the

earliest fossiliferous to the tertiary series, indicate that a warmer

climate prevailed during their formation than now exists. The remains of

marine animals, such as the cephalopoda, are found in great numbers and

in high latitudes, in a fossil state; but similar species, as the

nautilus, now abound only between the tropics. The same is true of the

crinoidea. Coralline limestone is also found in great abundance and in

high northern latitudes; but the stone-producing coral now exists only

in very warm seas. The remains of saurian reptiles are numerous in the

oölite and Wealden; but all the larger recent species of the lizard

tribe, such as the crocodile, are confined to the warmer regions of the

earth.

A former warm climate in Siberia is indicated by the occurrence there of

the remains of elephants. These animals were so abundant that their

tusks are now collected as an article of commerce. The abundance and

high state of preservation of these remains seem to preclude the

explanation that they were conveyed there, from the present tropical

regions, by any great geological convulsion. The species must therefore

have inhabited the country, though the elephant is now found only

between the tropics. The Siberian elephant was a different species from

any now existing, and, unlike the recent species, had a covering of

coarse hair. There is, however, no reason to conclude that it could

endure a continued low temperature; and its sustenance would have been

impossible, from the very stinted vegetation which that region now

affords. We must therefore suppose that Siberia enjoyed, at the period

when it supported these animals in such abundance, a tropical climate.

Most of the facts which go to prove a change of climate have been

observed in the northern hemisphere; but the explorations in South

America and New Holland furnish ground for believing that the geological

phenomena of the two hemispheres are essentially alike, and that the

indications of climate are the same for the same periods.

Such is, in general, the evidence in reference to climate; and it leads

to the conclusion that a highly tropical climate prevailed in the

temperate, and for some distance, at least, into the polar zones, in the

early geological periods; while there is no reason for supposing that

the tropical regions experienced a temperature too high for physical

life to endure it. The climate of the earth was characterized then by a

higher temperature than now, and by greater uniformity. This was the

climate, with perhaps a gradual reduction of temperature, till the later

portions of the tertiary period.

Before the close of the tertiary period, a change occurred, and probably

a rapid one, to a more rigorous climate than now exists. The destruction

of the elephant in Siberia was evidently sudden, and was followed by

extreme cold; for the animals are in some cases entirely preserved in

ice, and in so perfect a state that, when the ice which surrounds them

becomes melted, the flesh is devoured by carnivorous animals. There are

occasionally found, in the drift of the boulder period, shells similar

to those of the Arctic regions, and in a condition to show that they

have not been transported. The clay beds of the northern portion of the

United States and of Canada were deposited during the last depression of

that portion of the continent, and they contain the remains of marine

animals identical in several instances with species now living, but

confined to more northern regions. It must therefore be admitted that

the interval between the middle tertiary and the modern era was one of

great cold. It is generally referred to as the _Glacial period_.

Very considerable local changes of climate have also occurred within the

historical period. Thus the mean temperature of the Alps has been so

reduced that the ancient passes have in modern times become choked up

with snow, and other passes have been sought,--a result, perhaps, of

additional upheaval. It would seem that Siberia is now receiving a

milder climate. The ice in which elephants have for centuries been

imbedded has been slowly melting for at least thirty years.

SECTION VI.--ADVANTAGES RESULTING FROM GEOLOGICAL CHANGES.

1. The division of the general surface into land and water, as well as

the diversified form of the land, the existence of mountains and low

lands, and the consequent modifications of climate, the waterfalls, and

the river-systems, constituting the drainage of continents, are all

results of the process of upheaval.

2. A large part of the mineral substances employed for architectural and

economical purposes are oceanic deposits, such as the marbles, slates,

sandstones and mineral salt, and would have been inaccessible if they

had not been elevated from the position in which they were formed. And

the elevation of them above the bed of the sea would have exposed only

the superficial layer, if they had not been either irregularly uplifted,

as at _e c_ (Fig. 69), or unequally worn down, as at _b_.

[Illustration: Fig. 69.]

The granitic rocks, as they were formed below the aqueous rocks, must

have remained unknown and useless, if they had not been brought to the

surface, as at _c_, by the most convulsive efforts of nature of which we

have any knowledge. Thus, natural mechanical forces have effected for

man what the mechanical forces under his control would be entirely

insufficient to accomplish.

3. It is by changes of this kind that we become acquainted with the

geological structure of the crust of the earth. Mining operations have

never extended to a greater depth than three thousand feet, while the

inclined position of the strata exposes for examination, along their

outcropping edges, _e a c_, the whole series, even to the primary rocks.

The upheaval of the granitic rocks, and the removal by denudation of the

overlying deposits, shows us the crystalline character which the earthy

materials take, when subjected to pressure and cooled from fusion with

extreme slowness. Thus we have, exposed to observation, the process of

nature in the formation and modification of rocks for several miles in

depth. Of the central portions, however, including by far the largest

part of the mass of the earth, we have no knowledge whatever.

4. _Springs_, and the other means of obtaining water for domestic

purposes, depend in part upon the inclined position of strata, and the

broken and uneven condition of the surface, and in part upon the

alternation of permeable and impermeable strata. If all the strata were

porous, like the sandstones, the water which falls upon the surface

would gradually settle through them to the level of the sea; or, if they

were all impermeable, like the clays, the water would pass over the

surface, and be collected in lakes or the ocean. As it is, the porous

structure of the soil and of some rocks acts as a reservoir, from which

the water is gradually discharged, and the intervention of impermeable

strata prevents its taking a perpendicular direction downwards. Thus, if

the stratum _e b_ (Fig. 69) consists of porous rock, and the one below

is impermeable, the water which is absorbed at _e_ will appear at _b_ as

a spring. Or, if the line _a d_ is a fracture, the water received at c

may reappear as a spring at _a_. If the strata were perforated by boring

at _e_ till the porous stratum _a_ is reached, the water will rise to

the surface, constituting an _Artesian well_. An ordinary well consists

of an excavation continued till a stratum is reached which is

permanently saturated with water.

5. Most of the _metallic ores_ which occur in the stratified rocks, with

the exception of iron, are found in fractures or as dikes. Without

these disturbances of the strata, the ores would have remained either

sparingly diffused throughout the adjacent strata, or as a part of the

melted mass at the volcanic centres. The ores and metals which are found

in the primary rocks are accessible only by the bringing up of these

rocks to the surface.

The fracturing, displacement, and elevation of the strata, attended, as

is often the case, with the destruction of property and of the life both

of man and the inferior animals, might, at first view, be thought an

unnecessary, if not a wanton infringement upon arrangements already

established. But the results which we have noticed, though by no means a

full enumeration of the advantages resulting from geological changes,

are sufficient to show that even the more violent disturbances to which

the crust of the earth has been subjected constitute an important part

of that series of adjustments which has rendered it a suitable abode for

human beings. These changes are therefore neither useless nor

accidental, but are essential parts of a wise and beneficent system.

CHAPTER IV.

OF THE CAUSES OF GEOLOGICAL PHENOMENA.

An exhibition of the _composition and structure of the earth_, together

with an account, as far as there is reliable evidence, of the

modifications which they have undergone, has been the object of the

preceding chapters. They are mainly a collection and classification of

observed facts. No reference has been made to causes or modes of

operation, except in a few cases where it was necessary in order that a

statement or description, might be intelligible.

If the facts have been given with sufficient clearness and detail to

convey a correct general idea of the crust of the earth, we are prepared

to inquire what are the agencies employed, and how they have operated in

producing it. It is the province of the geologist to question every

known power in nature, and to ascertain what geological effects each one

is now producing; and, observing what effects are produced by given

causes, he is to judge of the causes which have produced like effects in

past geological periods.

Some of these causes are in their nature limited, and effects can be

referred to them only within those limits. Thus, the congelation of

water expands it by a certain proportion of its volume, and beyond that

it can have no effect. But the expansive power of steam varies with the

temperature; and hence the effects referred to it may be equally varied.

Thus, we are not to expect exact uniformity of results in all past

times, but _the results will vary only as the circumstances vary_ upon

which the operation of these causes depends.

Geological causes, in most instances, operate with extreme slowness; and

therefore it will require a series of observations, continued for a long

time, to ascertain what are the capabilities of these causes. But a

single instance of their effects proves their capabilities thus far.

Hence, one instance of the deposition of a stratum of salt in a salt

lake; of the filling of a fracture with fluid lava; of a volcanic

eruption, like that of Iceland in 1783; of the subsidence of a volcanic

mountain, as that of Papandayang in Java; or of the rising of a large

area of land, as in Sweden, as fully proves that natural causes exist

capable of producing these effects, as if the effects were produced

daily. As these effects increase in number, and careful observations are

made and authentic accounts preserved, the means of correctly explaining

geological phenomena will increase. The causes thus far known are

Atmospheric Causes, Chemical Action, Organic Agency, and Aqueous,

Aqueo-glacial and Igneous Action.

SECTION I.--ATMOSPHERIC CAUSES.

The oxygen of the atmosphere is capable of uniting with some of the

constituents of rocks, by which their cohesion is weakened or destroyed.

This is the cause of the rapid disintegration of some varieties of

granite. The protoxide of iron which they contain is converted, by

contact with the atmosphere, into the peroxide. Its volume is thus

increased, and portions of the rock are separated from the mass. When

granite or limestone contains sulphuret of iron, the oxygen of the

atmosphere, in connection with moisture, combines with the sulphur,

forming sulphuric acid, by which limestone and the felspar of granite

are rapidly decomposed. Hence, a rock which contains an oxide or

sulphuret of iron should not be used for architectural purposes.

Carbonic acid is another constituent of the atmosphere which operates as

a decomposing agent. The water that falls from the atmosphere is charged

with it, and thus becomes capable of dissolving calcareous rocks.

Carbonic acid is thus indirectly the means of the rapid destruction of

rocks of this class. It is also believed that carbonic acid enters into

direct combination with some of the constituents of rocks, and

particularly felspar; for it is found that in those countries where

carbonic acid issues in great quantities from the earth, the rocks,

especially those which contain felspar, disintegrate rapidly. Masses of

many tons' weight, which appear to be solid granite, after being broken

are found to be in such a state of decay that fragments may be reduced

to sand between the fingers.

The moisture of the atmosphere has some effect as a decomposing agent.

Rocks which are exposed to frequent alternations of moisture and dryness

soon crumble into fragments. Rain, falling upon the surface of rock,

produces, mechanically, a destroying effect, which is not to be

overlooked.

Variations of temperature, especially those alternations above and below

the freezing point, have greater influence than any other cause in the

destruction of rocks. When the water with which a rock is saturated

congeals, the resulting expansion tends to enlarge the interstices, and

thus to separate the particles of the rock. When the ice melts, the

particles fail to resume the closeness of arrangement with which they

were before packed. By frequent repetition of this action, the

superficial portion loses its cohesion, and disintegrates. It is also

found that in the region of perpetual snow the surface of the mountain

masses is covered with rock in a disintegrated or fragmentary state, in

greater abundance than below the snow line; but no explanation of this

fact has yet been found.

In mountainous regions, electrical discharges and violent storms have

some destroying effect. Winds have considerable power in changing the

place of earthy matter in a disintegrated state. In deserts, the sands

are carried in great quantities to great distances.

The causes now enumerated, when considered separately, and as acting for

only limited periods of time, seem hardly worthy of notice; but when

considered as operating conjointly, and for indefinite periods of time,

they must have produced important changes on the surface of the earth.

From these causes, the surface and ornaments of castles and other

ancient edifices, and of boulders, and all insulated rocks, are found to

be decayed, and often to a considerable depth. It is from these causes

that a soil is produced on every surface of rock which is not so exposed

to the action of currents that the debris is removed as fast as it is

formed. Hence it is, also, that a slope of detritus is formed at the

base of every declivity, so that the ledge appears only at the highest

points.

It is from a combination of these atmospheric causes that a large part

of the sediment is furnished which brooks and rivers carry away. And

when cohesion is not entirely overcome, it is so far weakened that other

causes are much more effectual than they would otherwise be, in

effecting the disintegration of rocks.

SECTION II.--CHEMICAL ACTION.

All those changes in which the action is molecular,--that is, between

the molecules as such, and not between the masses,--including the

effects of the imponderable substances, we regard as resulting from

chemical agency.

Under the control of these molecular forces the crystalline rocks have

taken their form; and if the crust of the earth could have remained in a

fixed condition, in which these forces would have been in equilibrium,

no further chemical action could have taken place. But, instead of being

in a fixed condition, the present system is one of perpetual change.

Various disturbances of this equilibrium of forces,--such, for instance,

as the diurnal and annual changes of temperature at the surface, and the

still greater secular changes of temperature at great depths,--will

bring the chemical forces into operation. The mechanical disintegration

of the crystalline rocks, and the deposition of them in strata

independently of the chemical affinity of their particles, will give

occasion for chemical changes,--that is, for a rearrangement of the

particles in accordance with their affinities,--whenever any movement of

the particles among themselves can take place. These movements take

place, to a very great extent, under the influence of electrical

currents, and of change of temperature, even while the masses retain

their solid form.

Chemical affinity has exhibited itself on the largest scale in the

formation of the various mineral species of which the crust of the earth

is composed; but we may also refer to the same cause the formation of

divisional planes in rocks, the concretionary arrangement, and mineral

veins.

1. _Divisional Planes._--It has before been stated, that the older

rocks, in many cases, cleave freely in planes not parallel with the

stratification. (See Fig. 48.) In some instances, in beds of lava, a

similar cleavage exists, sufficiently perfect to allow of its use as a

roofing material. In these cases, there must have been a rearrangement

of the particles, so that their axes of greatest attraction would lie in

parallel planes; the same arrangement which exists in mica and other

crystalline substances, which have one and but one free cleavage.

A similar arrangement has sometimes taken place under such circumstances

as to submit the process to more careful scrutiny. In the gold mines of

Chili, the powder from which the gold has been washed is "thrown into a

common heap. A great deal of chemical action then commences; salts of

various kinds effloresce on the surface, and the mass becomes hard, and

divides into fragments which possess _an even and well-defined slaty

structure_." When a portion of clay, worked into a paste with a very

weak acid, is submitted to a weak voltaic action for several months, and

then dried, it is found to have acquired a distinct though imperfect

cleavage structure.

It appears, then, that both electrical currents and ordinary chemical

action are capable of arranging the particles of an earthy mass into

separable layers. We may then regard this change in the older rocks as

an imperfect crystallization, and probably induced by electro-chemical

agency.

It is also found that all rocks are divided into huge blocks by seams

not parallel with the cleavage, and too regular to be considered as

fractures. These seams bear an analogy to the secondary faces of

crystals, which are never parallel to the cleavage.

2. _Concretionary Formations._--There exist in many rocks concretions

which differ from the mass of the rocks. In most of the tertiary clays

there are small concretionary nodules, which contain more calcareous

matter than the mass of clay around them. In the coal formation, the

nodular iron ore consists of concretionary masses. In the chalk

formation, nodules of flint abound, and generally in layers. In many of

these cases, particularly in the clays and coal, the nodules have an

organic nucleus, and, although concretionary, they retain the marks of

stratification of the adjacent rocks. Hence they could not have been

deposited in the form of nodules. There must therefore have been in the

rock, though in the solid state, such motion among the molecules that

particles of a particular mineral have separated from the mass and

rearranged themselves in concretionary layers, yet so gradually as not

to disturb the lines of original stratification.

[Illustration: Fig. 70.]

There are other instances, similar to the last in all respects, except

that the segregated portion does not take the concretionary form. When

gypsum is distributed in small proportion through a formation, there

seems very little reason to doubt but that it is, by a molecular action,

segregated from the strata in lenticular masses, as at a (Fig. 70). Many

of the limestone strata contain irregular aggregations of quartz. It is

presumed that the siliceous and calcareous matter was deposited together

as sediment, and that the aggregation has resulted from a movement among

the particles similar to that by which the concretionary structure is

produced.

The columnar structure of basalt seems to have resulted from a peculiar

molecular action, at first resembling a concretionary arrangement, while

the mass was cooling from a state of fusion. In experimenting to

ascertain the cause of this structure, Mr. Watt fused in a furnace seven

hundred pounds of basalt. When cooled, he found that "numerous spheroids

had been formed, and that when two of them came in contact, they did not

penetrate each other, but were mutually compressed and separated by a

well-defined plane, invested with a rusty coating. When several met,

they formed prisms." (Fig. 71.)

[Illustration: Fig. 71.]

3. _Mineral Veins._--The phenomena of veins are such that they cannot

all be referred to the same cause. In some, the vein-stuff has been

protruded as a dike, differing from ordinary dikes only in the

accidental circumstance that it contains a metal or a metallic ore.

Mineral veins are not, however, generally filled by injection from

below. It is found that those veins only are productive which have an

east and west direction. But injected dikes run in all directions. The

ore often varies in richness at different depths in the vein, or passes

into ore of some other metal. The ore also varies in kind and quality,

according to the character of the rock through which the vein passes.

These phenomena are best explained by supposing that the sediment of

which the strata were formed contained the mineral substances of these

veins in small proportion. After they were solidified, and fractures had

been formed, the mineral substance was transferred by molecular action

to the fissures, and deposited.

It was shown by the early experiments of Davy, that voltaic currents are

capable of taking up mineral substances from their solutions, and

removing them from one cup to another. It has been ascertained that in

most mineral veins a proper apparatus will detect the existence of

electric currents. It may be regarded as certain, that the unequal

heating of different parts of the surface at the same time, by the sun,

causes a vast current of feeble intensity to circulate around the earth

once in twenty-four hours. The unequal distribution of heat below the

surface may also produce currents subject to other laws. We should

expect that these currents would take up the mineral substances diffused

through rocks, and deposit them by themselves. It seems probable,

therefore, that the molecular action, from which the segregation of

metallic veins has resulted, was that of voltaic currents.

SECTION III.--ORGANIC CAUSES.

The effects of all organic causes in producing geological changes are

inconsiderable, compared with those of inorganic causes. With the

exception of the coral formation, the most important of these effects

are those produced by human agency. We find examples of this agency in

the distribution of animals and plants beyond the regions where they are

indigenous; in the increased numbers of certain species, and in the

diminution, if not extinction, of others; in the modifications of

climate, dependent on the destruction of the forests and the cultivation

of the soil; in controlling the course of rivers; in arresting by

embankments the encroachments of the sea; in breaking up and changing

the place of great quantities of rock by mining and engineering

operations; and in the increased quantity of sediment furnished to

streams by cultivating the surface, and thus preventing the protecting

influence which the matted roots of trees and the smaller vegetables

would otherwise have. Such effects, though attributable mainly to man,

are produced in some degree by all other animals.

Besides these general effects, it is the existence of organic forms that

has conferred on all the sedimentary rocks their fossiliferous

character. _The records_ of the climate of each geological period, of

the physical geography, of the vegetable productions, and of the animal

forms by which the earth was peopled, consist in the remains of the

living beings of these several periods, imbedded in the contemporaneous

rock formations. But in the sediment deposited since the human era

there must have been furnished both the remains of human beings and

works of art, such as implements of labor and war, pottery, coins,

fragments of ships, &c.

Moreover, the _quantity of material_ which has been furnished by organic

causes is by no means small. The coal-beds are the product of vegetable

growth exclusively. We not unfrequently find strata of great extent

consisting almost entirely of the shells of molluscous animals, of the

stems of encrinites, or of the shields of microscopic animalcules.

But the most abundant rock which can be regarded as the product of

animal organization is the _coral formation_. It consists of immense

walls of coral limestone, separating either an atoll or the land of an

island or continent from the open sea. The base of this wall has a width

varying from a hundred feet to a mile or more, and the outer edge of it

is at such a distance from the shore as to give a depth not much

exceeding a hundred feet. Over this area of the bed of the sea, which

forms the base of the wall, the coral polyp commenced its work.

Attaching itself in immense numbers over this area, it deposits

calcareous matter from its under surface, and thus, by degrees, elevates

itself towards the surface of the water, till it reaches a level a

little above low-water mark. The height of the wall would not, with

these conditions, exceed one hundred feet; but some hundreds of the

islands surrounded by coral walls are gradually subsiding. The

depositions of the polyps keep pace with the subsidence, so that this

wall has reached an elevation from its base of a thousand feet, and in

one instance of two thousand feet. (See Figs. 61, 62, 63, 64.)

Most of the islands of the torrid zone are thus surrounded with coral

reefs, except a few where the cold polar currents reduce the temperature

too low to admit of their growth. In one instance, along the north-east

coast of New Holland, there is a coral reef, some twenty-five miles from

the land, which has a continuous extension, excepting occasional inlets

of no great depth, of a thousand miles. The reef along the island of New

Caledonia is four hundred miles long. A large number of other reefs have

a nearly equal extension. There is thus an area of several thousands of

square miles covered to a great depth with this coralline limestone.

Some limestone formations of great extent among the older rocks were the

work of similar animals. These lower forms of organization have,

therefore, always been important geological agents, both in collecting

the carbonate of lime from its solution in the waters of the ocean, and

in depositing it as solid rock.

SECTION IV.--AQUEOUS CAUSES.

Water is, next to heat, the most important geological agent. All the

stratified rocks are aqueous deposits, and their total amount is in some

respects a measure of the influence which this agent has exerted. The

materials have been obtained from the destruction of preëxisting rocks,

transported by water, and deposited in layers.

When the first strata were formed, the sediment must have been obtained

entirely from igneous rocks, because only those rocks existed; but now

it is obtained from every kind of rock which is exposed to abrading or

decomposing agencies. Hence, many of the later formations contain

fragments, and sometimes within the fragments well-characterized

fossils, of earlier formations.

The sediment which is ultimately to become stratified rock is deposited

on the beds of the ocean, and other great reservoirs of water. The

formation of most of the aqueous rocks, therefore, as well as of the

igneous rocks, is deep below the surface; and neither of these

operations, on the large scale, is directly exposed to our observation.

We may, however, learn by observation, how the sediment is furnished to

the waters and transported by them, and we can form some correct ideas

of the manner in which it will be laid down on the bed of the ocean, and

solidified.

I. _The Furnishing of Sediment._

1. Almost all the minerals which occur in the geological formations are,

to some slight extent, _soluble in water_. Hence, rain water, by

passing through a stratum of earth or rock and reäppearing as a spring,

loses the insipidity which it had as pure water, and becomes palatable.

It is then found to hold in solution some small proportion of earthy

substances, upon which this change of taste depends. Although the

proportion of dissolved matter is very small, yet the surface of earth

upon which this distilled water is shed is one-fourth of the surface of

the globe, and solution below all that surface is constantly taking

place. No inconsiderable amount must thus have been furnished, from the

existing rocks of each period, towards the formation of the strata of a

later period.

There are some substances which are soluble in water, in large

quantities. _Rock-salt_ is an example. It is not found in any very large

proportion in rocks generally, but a very large aggregate amount has

been taken up by the waters which have filtered through the strata. The

ocean gathers into itself, by degrees, all the soluble substances which

are thus taken up. It receives supplies of water charged with these

substances from springs, rivers and lakes. It returns as much water as

it receives; but it is always in the form of vapor, and is therefore

pure water. Hence the saline properties of the ocean, and of those

inland seas which have no outlets. There is thus gathered the materials

for the rock-salt deposits.

But many substances which are not considered soluble in water become so

by some modification of the water. Water of a high temperature is

capable of dissolving silex. In Iceland and other volcanic regions, the

hot springs are charged with silex, which is deposited as the water

cools. Thus, siliceous formations accumulate around springs of this

kind. The various agates may have been deposited from such solutions.

In the decomposition of mica, felspar and volcanic rocks, a considerable

amount of potassa is set free. Potassa or soda renders the water in

which it is dissolved capable of dissolving silex in large quantity. In

these ways water removes, with some degree of rapidity, one of the most

insoluble minerals which rocks contain.

In volcanic countries, and in coal districts, carbonic acid is abundant,

both in spring-water and in the gaseous form. Water charged with this

gas becomes capable of dissolving limestone. Where the water is exposed

to the air, the gas gradually escapes, and the calcareous matter is

deposited. Many accumulations of this kind are now taking place. Some

have already extended several miles in length, and they are often of

great thickness, in one instance, in Italy, two hundred feet (Fig. 72).

It is also probable that many calcareous springs issue below the surface

of lakes and seas, and thus, both fresh-water and marine deposits would

now be forming. These formations are distinctly stratified, and are

white and crystalline, and become solid at the time of deposition.

[Illustration: Fig. 72.]

These dissolved materials are less observed than others, because they do

not render the water turbid; but there is reason to believe that several

of the aqueous formations, particularly the limestones, have been built

up chiefly from them.

2. The _abrading action of rivers_ furnishes considerable detrital

matter. The general form of the river courses is determined by other

causes than the agency of the river itself, yet a river which has a

rapid current is continually deepening its channel. We have proof of

this by observing, when the water is low, that irregularity of surface

which running water always produces, by wearing away the softer parts of

the rock, and leaving the harder in relief. Hence, a river will have its

rapids either where the hardest strata occur, and which therefore wear

down least rapidly, or where the rock has been hardened by the intrusion

or near proximity of dikes.

The abrading power of rivers occasionally becomes greatly increased by

waterfalls. The force which the water acquires in its descent is such as

to excavate a deep cavity at the foot of the fall, reaching back under

the ledge from which the water descends. The ledge is therefore

constantly being undermined. The cataract of Niagara is peculiar, in

having the rock at its base of a soft and friable texture, so that it is

rapidly worn away, while the upper rock is a compact siliceous

limestone. If the order of superposition had been the reverse, the falls

would have been converted into a series of rapids. It is now preserved

as a single fall, and as such it has probably cut the gorge, about two

hundred feet deep and seven miles in length, through which its waters

now reach Lake Ontario. A few years since, a large mass, perhaps half an

acre in area, fell from the centre of the horse-shoe fall. Another mass

of equal size has recently fallen from the western extremity of the

ledge. Thus the fall is gradually receding.

But the foreign substances, such as drift-wood, ice, sand and gravel,

with which the waters of a river are occasionally charged, contribute

more than everything else to its abrading power. At such times its

volume is generally greatest, and its current the most rapid. Its bed is

then sometimes perceptibly deepened and widened in a few hours.

Much the greater part, however, of the earthy matter which rivers convey

in such quantity to the ocean, is furnished by other means than the

eroding action of the river itself. It is the loose material, the soil

and alluvium, to which the solid rocks have been reduced by the

imperceptible but incessantly operating atmospheric agencies, from which

most of the sediment of rivers is obtained. After a rain, every

tributary rivulet is turbid with suspended earthy matter, and it is from

these sources that the larger streams receive the most of their

sediment.

Some observations have been made for the purpose of ascertaining the

quantity of sediment which rivers annually carry into the sea. The

Kennebec furnishes materials which, if spread evenly on an area of one

mile square, and consolidated into rock of the specific gravity of

granite, would have a thickness of six inches. The Merrimac furnishes

about two-thirds as much, the Ganges about two hundred and fifty times

as much, and the Mississippi two thousand times as much.

Thus, the tendency is, to reduce the highest parts of the land, and to

fill up the depressions of the sea; and though we have not data enough

to form any reliable estimate of the total annual discharge of sediment

into the ocean by rivers, yet they are sufficient to show that the

effects of this kind are on a large scale, and to relieve us from any

impression that existing agencies are inadequate to the production of

the stratified rocks.

3. _The action of waves_ is another means by which detrital matter is

furnished. Wherever the shore consists of loose materials, and is

favorably situated to be acted upon by the waves, there is annually a

sensible encroachment of the sea. Such encroachments are rapidly making

in many places; and thus a large amount of sediment is delivered to the

waters of the ocean.

The waves also encroach upon the coast when it consists of rocks, even

of the most indestructible kinds. They continually beat upon it,

undermine the cliffs, and precipitate them into the sea. The tides

increase the power of the waves, by varying the place of their action,

so as to present the same surface of rock alternately to the action of

water and of the air, frost and sun. During storms, the waves have

sufficient force to break off fragments of rock from the escarpment,

sometimes in masses weighing twenty tons or more, and remove them many

rods inland.

A bold, rocky coast always exhibits evidence of a great amount of

erosion. The steep escarpments and the high rugged shafts of rock (Fig.

73) against which the waves now beat are the remnants of masses of rock

which once extended further into the sea, but have been worn away by the

waves. It is by such agency that the deep inlets and harbors of the

coast of New England and Nova Scotia have been excavated.

[Illustration: Fig. 73.]

This more violent action of the waves is only occasional; but when of

less power, they are incessantly rolling the loosened fragments of rock

upon each other, and thus wearing them down to particles small enough to

be carried away by the water.

4. The action of waves is confined to the coast, and never extends to

great depths. But _marine currents_ act principally on the bed of the

sea. The temperature of the mass of the ocean is much higher in the

equatorial than in the polar regions. At the surface, the difference

amounts to sixty degrees. The waters of the torrid zone are thus

expanded, and flow over the colder waters of the north and south; while

these colder waters of the polar seas flow back, in an under current,

towards the equator.

For the same reason,--a difference of temperature,--there will be, in

the higher regions of the atmosphere, a current of warm and moist air

flowing from the equator north and south, while the cold and dry air

conies in from the polar regions towards the equator. In this way the

equatorial waters are carried, in a state of vapor, towards the poles,

where they are condensed, and go to increase the currents of water

moving towards the equator.

Such are the general causes of the oceanic movements in a north and

south direction; but these currents at once become deflected westward,

by the diurnal revolution of the earth, as the trade winds do. Hence

there results a Pacific equatorial current, which has a motion of about

thirty miles a day, and an Atlantic equatorial current, moving from

sixty to seventy miles a day. The principal marine currents are shown in

Fig. 74.

The currents moving towards the poles are superficial, and therefore do

not produce any marked geological effects. But the polar currents, and

those which are produced from them, are of great depth, and there is no

reason to suppose that they do not move, from their commencement, along

the bed of the ocean. There is also reason to suppose that they exist at

great depths, where the opposing superficial currents entirely conceal

them.

Wherever these currents come to the surface, their motion is undoubtedly

greater than it is at the bottom, where it is retarded by the friction

which the moving waters encounter, and by the irregularities of the bed

of the ocean. It should, however, be remembered, that they move with the

weight of the whole superior body of water; and therefore, though the

motion be very slow, it will still possess great power.

Any irregularities in the bed of the ocean beneath such a current must

be subject to very rapid abrasion. We shall sea hereafter, that

earthquake vibrations often shiver the rocks at the solid surface; and

if any of these ridges at the bottom of the ocean were thus acted upon,

the loosened portions would be swept away by the current and deposited

at lower levels, or where the current subsides. If, in any instance

during an earthquake convulsion, a fault should be produced across one

of these marine currents, like the great fault of over five hundred feet

in England, the abutment thus thrown up would soon be worn down; and if

it consisted of unconsolidated matter, it would be swept away almost

bodily.

[Illustration: Fig. 74.]

The effect of such currents will be greatest where they are deflected by

a continent or island. Thus, a marine current sets from near New Holland

in a direct line to the north of the island of Madagascar, where it is

arrested by the African coast, and deflected into the narrow Mozambique

channel, and there acquires a velocity of four or five miles an hour. It

is impossible that any kind of rock should receive the constant force of

such a body of water without being rapidly worn away; and, if there

should be any difference of texture in this rocky barrier, the softer

portions would yield the most rapidly, and thus valleys might be formed.

It is not improbable that the deep indentation on the western coast of

Africa may have been due, in a great measure, to the coast current from

the Cape of Good Hope; and that the Caribbean Sea and the Gulf of Mexico

may have been excavated by the force of the Atlantic equatorial current

being thrown into this angle.

We may regard these currents as oceanic rivers; and it is obvious that

the volume of the terrestrial rivers would bear no comparison with that

of these currents, and their effects would be equally small in the

comparison. The Gulf Stream, and the Mozambique and other similar

currents, must be wearing down the valleys through which they flow, to

such an extent as to furnish an immense amount of detrital matter for

the formation of new rocks.

It is principally to the agency of these deep marine currents that we

are to refer those extensive denudations, so abundant on the present

continents, such as the wearing out of the intermediate masses of rock

between the hills already referred to (Fig. 66), the denudation of the

Connecticut river sandstone, and, perhaps, the excavations which have

formed Lake Erie and Lake Ontario.

II. _The Transportation of Sediment._

The detrital matter obtained in these several ways is swept away by

running water. The specific gravity of rocks does not, in general,

exceed two and a half. Hence, to keep them suspended in water, will

require a force of only three-fifths of what would be necessary to

suspend them in the atmosphere. In the case of river currents, the

velocity and irregularity of motion are generally sufficient to keep all

the finer sediment equally distributed.

There will, however, be a division of the sediment according to the

strength of the current. Hence, the bed of a mountain stream, if there

is any loose material, always consists of pebbles. As it approaches the

alluvial region, the bed is sandy; and when the current becomes very

sluggish, it consists of a fine mud.

Rivers never deposit all their sediment, some of them none of it, along

their course. Large rivers continue partially distinct from the ocean

water to a considerable distance beyond their mouths. The waters of the

Amazon have been recognized at a distance of three hundred miles. This

depends in part upon the volume and velocity of the river; more,

however, upon the fact that river water is lighter than sea water. This

extension of a river will, in most cases, be sufficient to deliver a

part of its sediment into a marine current. When such a current sweeps

very near the mouth of a river, as it does to that of the Niger, the

Amazon, or the Mississippi, it is probable that most of its sediment is

carried away by it.

The transporting power of a marine current is greater than that of a

river, in consequence of the greater specific gravity of its water; but

it has scarcely any of that irregular motion of rapid rivers, upon which

their transporting power in a great degree depends. The force of the

current alone, when it reaches the bottom, is, however, sufficient to

remove every form of loose earthy matter. Thus it may be presumed that

the Gulf Stream sweeps all the sediment from its bed until it reaches

the latitude of Cape Hatteras, where the cold waters from the north

begin to underlie it, and it takes the character of a surface stream.

But the transporting power of marine currents depends mostly upon the

depth of water. It is found, by experiment, that ordinary river sediment

will sink in water about one foot in an hour. A current, therefore, of

a thousand feet in depth, which moves a mile in an hour, would carry its

sediment a thousand miles. It is obvious, then, that there is no part of

the bed of the sea which may not be receiving sediment.

III. _The Deposition of Sediment._

From what has been said of the weight of sediment, it follows that it

will be deposited whenever the water in which it is suspended is at

rest. Hence, when a river increases in breadth so as to form a lake, the

waters at the outlet are seldom turbid. The earthy matters with which

the principal and tributary streams were charged all settle to the

bottom, and go to lessen the capacity of the reservoir. Thus lakes are

continually diminishing in depth and area. In many instances, they are

already filled with sediment, and are thus converted into alluvial

plains, through which the river flows in a narrow channel.

It is frequently the case that a river, as it approaches the sea, has so

slow a motion that its sediment is deposited on the bed of the stream.

Thus the bed will be raised, and the banks will also be raised, by the

deposition of sediment upon them at periods of overflow. The river will

then be raised above the adjacent country. The river Po, for the last

part of its course, is from ten to twenty feet above the adjacent lands.

The same is true of the Mississippi, and many other rivers. The streets

of New Orleans are several feet below the surface of the river. In an

uninhabited country, such a river would soon seek a new and lower

channel; but in a populous country, it becomes a matter of interest and

safety to confine the river in its old channel, by artificial

embankments.

But the principal part of the sediment of rivers is conveyed to the sea.

It here mingles with the debris which the waves have furnished, and a

part of it is deposited to form deltas. The remaining part is taken up

by marine currents, mingled with the debris which they have furnished,

and is spread out on the bed of the ocean.

Of the extent of these deposits we can form no estimate. Those of rivers

and lakes are comparatively unimportant, as they are in the older

formations. Some of the delta deposits are already of great extent. That

of the Ganges contains an area of twenty-six thousand square miles, that

of the Niger twenty-five thousand, and that of the Nile twelve thousand.

The delta of the Rhone has increased its area by three hundred square

miles in the last thousand years. The Po has encroached upon the

Adriatic two thousand square miles in the last two thousand years, and

the Mississippi has enlarged its delta by one hundred square miles in

the last hundred years. In the deep valleys of the ocean accumulations

may be taking place on as large a scale as they ever have been in former

times.

IV. _Character of the Formations thus produced._

Sedimentary matter thus deposited would take the form of _strata_. Thus,

a delta deposit may receive at one time from a river a layer of coarse

gravel and pebbles, and in the course of a few hours the current may be

so reduced that it will convey to the same place only fine sand and

silt. Or, if a depositing current receive its sediment only at

intervals, the heaviest particles would be thrown down first, and the

more finely levigated particles would continue to fall, till the water

became transparent. Another supply would furnish another similar

stratum, and so on. The same arrangement might result from the sediment

being furnished by different rivers. Thus, if sediment were furnished to

the Gulf Stream by the Merrimac river, and the streams emptying into the

Bay of Fundy, the freshets would occur earlier in the season in the

Merrimac, and it would furnish a supply of sediment from a region of

primary rocks. A later supply would come from the red sandstone region

of Nova Scotia, and the stratification would be indicated by the

different kinds of rock produced. Thus stratification will result from

difference in the color, composition, or size of the particles of which

rocks consist. A great variety of causes, both general and local, may

therefore give to a deposit this character. Hence, as stratified rocks

are produced by the sediment now laid down from water, we may conclude

that the older stratified rocks are the sediment deposited in like

manner, in former times.

The occurrence of layers of different composition, as one way in which

the stratification is indicated, is produced by local and frequently

recurring causes. There are, however, other alternations of much greater

extent; those, for example, nearly twenty in number, distinguished by

striking differences in lithological character, into which the New York

system of rocks is divided. These alternations have resulted from more

general causes. The physical geography of a wide region must have been

so different, at the different periods during which these several

formations were deposited, as to change, at each period, the kind of

sediment furnished to the forming currents, and modify the types of

animal life.

We have seen that the same causes that determined the stratified

arrangement will determine the alternations of strata of coarse and fine

materials.

It is obvious that the stratification of the marine deposits will be

nearly horizontal. If the surface were very irregular upon which the

deposition commenced, the irregularity would constantly diminish; for

the movement of the water over this surface, however slow, would tend to

remove the accumulations from the highest points, and leave them at the

lowest (Fig. 75). Delta and lake deposits will, however, dip somewhat,

though never at a high angle, towards the deep water. In certain

situations, where a river and a tidal wave, coming in conflict, cause,

in succession, eddies and currents in opposite directions, we should

expect to find the stratification very irregular (Fig. 76); sometimes

false stratifications (_a b_), sometimes the strata cut off abruptly,

and at other times contorted or dipping in opposite directions within

short distances.

[Illustration: Fig. 75.]

Wherever sediment is deposited, it will entomb whatever of the remains

of animal or vegetable life may be mingled with it. They will be at once

protected against the influence of all the ordinary decomposing

agencies, and will continue for ages to retain their peculiar markings,

and even their colors. They will thus constitute, in all future time, a

record of the present condition of the organic world. The lacustrine

deposits can contain only fresh-water species of animals, marine

deposits only marine animals, while deltas may contain the remains of

marine life mingled with those which have been washed down by rivers.

The remains of birds, insects, and terrestrial animals, may occasionally

occur, in every kind of deposit. Sediment deposited in deep water will

never contain fossils in abundance, the deep parts of the ocean being

almost wholly destitute of animal or vegetable life. It is only in water

of a few fathoms that the greater number of species and of individuals

occur. In all these particulars the deposits now forming sustain a close

resemblance to the older formations.

[Illustration: Fig. 76.]

There are certain formations, as that of the coal, which required

conditions for their formation different from those of ordinary

sedimentary deposits. Coal consists of mineralized vegetable matter. Its

vegetable origin is proved by the uniform occurrence of vegetable

fossils almost exclusively in the coal measures. When reduced to thin

slices and examined under a high magnifying power, a structure very

similar to the ligneous tissue of existing coniferæ is sometimes found

to exist. There are probably vegetable deposits now taking place not

altogether unlike those which produced the coal measures.

We know that many rivers--the Mississippi, for example--now carry into

the sea great quantities of ligneous matter. Before the country was

inhabited by man, the quantity was undoubtedly much greater than it now

is. It floats for a time; but the ligneous tissue itself is heavier than

water, and as soon as the air is excluded from the pores, and they are

filled with water, it will sink. The woody and earthy matters are swept

into the sea together; but, as they sink under different circumstances,

they will be deposited separately. _Thus wood may continue to accumulate

in particular places in the sea_ for long periods, with but little

intermixture of earthy substances.

It is, however, to be expected that, in the progress of geological

changes, the places which at one time receive deposits of wood will at

another receive detrital matter, and thus _the wood will become deeply

buried_ beneath sedimentary strata.

Wood thus situated will become converted into coal. Trees which had been

covered to considerable depth with earth have been found near the

Mississippi river changed to lignite, a substance resembling charcoal.

In this case, the wood had been exposed to no greater heat than is

common to the crust of the earth at the depth where it was found; and

yet it had undergone this change since the country has been known to

Europeans, as it retained the marks of the axe when it was discovered.

It has also been found by experiment that vegetable matter, by long

submersion in water, passes into the state of lignite. This is the first

step in the conversion of wood into mineral coal.

When lignite is exposed to moderate heat and great pressure, it loses

the characters of lignite, and becomes mineral coal. This is shown by

facts observed in Germany, Ireland and Iceland, where beds of lignite

have been overspread by basalt. The upper portions of the lignite are

changed to mineral coal. The lower portions, which the heat did not

reach, retain the characters of lignite.

Beds of vegetable matter, with a great thickness of rock deposited above

them, would therefore be subject to all the conditions necessary to

convert them into coal, namely, pressure from the superincumbent mass,

and the heat which the strata uniformly assume at great depths.

It is not improbable, therefore, that coal-beds are now forming, and

that they have been formed at every geological period since an abundant

terrestrial vegetation commenced. Accordingly, there occurs in Virginia

an extensive coal-field in the oölite formation. Coal-fields also occur

in England, of less extent, in the same formation. In France, and other

parts of Europe, there are extensive beds of lignite in the tertiary

formation.

We have therefore no difficulty in accounting, in a general way, for the

formations of the carboniferous period. The vegetables were probably

less woody than those of the present time of equal size, and were

therefore more easily prostrated and committed to the waters. They grew

rapidly in moist ground, and perhaps in shoal-water, and required an

atmosphere charged with moisture and of a high temperature. Thus much is

inferred from the conditions most favorable for the growth of recent

species analogous to the coal-plants. These recent species are tropical

plants, and grow in moist insular situations, conditions which would

have existed at the carboniferous period, if the present coal-fields

were then an archipelago dotted with low islands.

Such being regarded as the origin of the coal-beds, the _alternations_

of the earthy and carbonaceous strata may be referred, provisionally, to

those great changes in physical geography upon which the other

alternations of strata on a large scale depend. But the regularity with

which the coal-seams and sandstone succeed each other presents some

difficulties which, in the present state of knowledge, we cannot

satisfactorily account for.

_Beds of salt_ occur, interstratified with other rocks, in nearly all

countries. Still, it is not a sedimentary deposit, and its formation

must depend upon peculiar circumstances. In New York, saline, together

with earthy matter, constitutes the Onondaga limestone, one of the

formations of the New York system. In Kentucky, the strata of rock-salt

are in the coal formation; in England, they are in the new red

sandstone; in Spain, they are in the greensand, and in Poland they are

in tertiary strata. The conditions of its formation have therefore

existed in connection with the deposition of every fossiliferous rock.

It has been shown that the ocean is the principal reservoir of the

saline matters which are taken up whenever water percolates through

rocks. It must happen not unfrequently, in the course of submarine

elevations, that a basin of sea-water will be cut off from its

communication with the sea; and from this basin the evaporation might be

more rapid than the supply of water. The great salt-lake of Utah is

undoubtedly a basin of this kind. The Mediterranean Sea is another such

basin, not yet wholly separated from the ocean. The evaporation exceeds

the supply of water from the rivers, and a powerful stream is therefore

continually thrown in from the ocean, through the Strait of Gibraltar.

The waters of the Mediterranean are already more highly charged with

salt than ordinary sea-water. This sea may ultimately become a saturated

solution, and begin to deposit salt. But whether it does, or not, it

indicates the way in which salt-beds may be formed.

V. _Solidification of Aqueous Deposits._

Sediment is generally deposited as a soft mud, but in nearly all the

older formations it has become solidified. When rocks are deposited from

a chemical solution, they take at once the solid form. Such is the case

with rock-salt and with limestone, when the material has been held in

solution. Solidification takes place in nearly the same way when water

which holds carbonate of lime or oxide of iron in solution filters

through beds of sand or gravel. The substance held in solution is

deposited in the interstices till they become filled, and the whole is

changed to solid rock.

Some rocks are composed of such materials that they _set_, like

hydraulic cement, when they are deposited. Other rocks become solid

simply by drying. Thus a deposit now forming in Lake Superior becomes,

by drying, nearly as hard as granite. Such a deposit will therefore

become solid whenever it shall be elevated above the water.

The pressure to which all but the upper layers are subjected is probably

sufficient to reduce most rocks to the solid state. Dry and pulverized

clay is reduced by artificial pressure, for a moment, almost to stone.

The pressure upon the deep-seated rocks is constant, and greater than

any artificial pressure can be.

In addition to these causes, all the older rocks have been subjected to

a high temperature, some of them nearly to that of fusion. By this means

the solidification of every kind of rock would be promoted, and probably

some may have been reduced by it to the solid state, which would

otherwise have remained as an incoherent mass.

SECTION V.--AQUEO-GLACIAL ACTION.

1. _Glaciers._--A glacier is a mass of ice occupying the bed of a

mountain valley, having a slow progressive motion, and reaching somewhat

lower in the valley than the line of constant snow. (Fig. 77.) The

Glacier des Bois, which may be regarded as a specimen of the Alpine

glaciers, covers an area of about seventeen square miles. In its

lowest portion, when all its branches have become united into one

stream, it has an average width of half a mile, and is five miles long.

It is estimated that the glaciers of the Alps cover an area of fourteen

hundred square miles. These have been the most carefully studied, though

glaciers are found in the valleys of various other ranges of mountains.

In the higher valleys, the snow, which falls at all seasons of the year,

accumulates in immense quantities, and the steep mountain sides

contribute, by frequent avalanches, to this accumulation. The snow, when

thus increased, does not become a compact, adhesive mass; but, changing

into particles of solid ice, it resembles sand rather than snow. It is

this _névé_ which constitutes the upper part of every _glacier_, and

which, in a modified form, constitutes the lower part.

The valleys descend rapidly towards the base of the mountains; and this

snow-ice, having no cohesion between its particles, _moves slowly down

the slope of the valley_, like a very imperfect liquid. After descending

below the line of perpetual snow, the surface will melt during the day;

and the water, sinking into the porous mass, becomes frozen, and

converts the whole into more or less compact ice, yet never into a rigid

mass. Influenced by its own weight, and by the pressure of the snow-ice

behind, it still continues its motion, and conforms itself to the shape

and curves of the valley through which it passes. The average movement

per annum may be stated at about five hundred feet.

The temperature of the rocky bed of the valley will be a little, and but

a little, higher than thirty-two degrees. There will therefore be but

little melting at the bed of this river of ice. As it receives continual

accessions from the atmosphere, it will therefore increase in volume

till it descends to the level of perpetual snow. Below this line _the

waste exceeds the addition_; and as it approaches the lower and

cultivated portions of the valley, it rapidly diminishes, till it

finally loses the solid form, and becomes a rivulet. The terminus of the

glacier is determined principally by the general climate of the country.

Any considerable variation of climate will cause it to recede, or

descend lower down the valley. The terminus varies, however, somewhat

with the seasons, being lower in winter than in summer, though the

motion is much less in the cold season than in the warm; and it descends

many rods further some seasons than it does others.

The glacier consists principally of snow, more or less modified in

structure; but it also contains whatever else may have been thrown upon

its surface, or into the snows by which it is fed. Tributary glaciers

extend up through all the gorges into which the irregular surface of the

mountain-top is divided. On these rough peaks there are always fragments

of rock, varying in size from fine sand to masses weighing many tons;

some of them loosened when the mountain was upheaved, some by subsequent

earthquake vibrations, and others still by tempests, lightnings, and

changes of temperature. When the snow has accumulated to a certain

extent on the steep slopes, it falls in avalanches into the valleys,

carrying with it loosened masses of rock, and often breaking off large

fragments from the rocky escarpments against which it strikes. These

avalanches are almost constantly descending, and hence a glacier always

contains considerable _earthy matter distributed through it_.

[Illustration: Fig. 77.]

The friction of the glacier, at its edges and along its bed, separates

more or less of the rock over which it moves; and hence there is always

a layer of mud and pebbles under the glacier, and a line of loose

fragments, called a _lateral moraine_, at the sides. When two glaciers

unite, the two lateral moraines, thus brought together, come to the

surface, forming a medial moraine, and show the line of junction

sometimes for miles.

The friction of the glacier on the bed of rock, assisted by the layer of

pebbles, will wear down the prominent portions, and everywhere polish

the surface. Fragments of rocks may be frozen into the glacier at all

depths. Those which lie near the lower surface of the glacier would, by

slight melting of that surface, project downward so as to act as a

graver's tool on the rock over which it passes. Hence, when the

extremity of the glacier has receded beyond its ordinary limit, the

surface of rock exposed is found, upon examination, to be _polished_,

_striated_, and _occasionally grooved_ an inch or two deep.

Since the waste is almost wholly superficial, earthy matter, which was

at first concealed in the mass of the glacier, is continually coming to

view, as the surface melts and runs off. Thus, none of the freight of

the glacier is left along its course, but all is carried to its terminus

and discharged there. Hence, at the lower extremity of the glacier there

is always an embankment of earth, pebbles, and boulders. If the glacier

recedes a few yards at one season of the year, and leaves its earthy

fragments scattered over this surface, they will be pushed forward into

a ridge, as the glacier again advances. This ridge is called a _terminal

moraine_, and consists wholly of substances which have been separated

from the mountain mass, often at the highest beginnings of the glacier.

At the terminus of all the Alpine glaciers, there is a series of these

moraines (_a a a_, Fig. 77) marking the successive limits of the glacier

in former times.

There is a ridge of boulders on the north side of the Swiss valley, near

the base of the Jura Mountains, resembling a terminal moraine. These

boulders consist of several groups, distinguished by peculiarities of

structure and composition; and each group lies opposite to the

particular Alpine valley which now furnishes the same kind of

fragments. It has been thought that, at a former period of more severe

climate, the Swiss valley was filled in part with ice, and that the

present glaciers extended across it to the Jura Mountains.

It is found that the polished and striated surfaces of the rocks in the

Alpine valleys are precisely like the surface of the rock, which has not

been exposed to atmospheric influences, in the north of Europe and

America. It has been proposed to extend the glacier theory, and account

for these phenomena by supposing that the north polar regions were, at

the ice period, capped with a glacier-mass, extending as far south as

the drift phenomena appear.

It is not to be doubted that the phenomena of polished surfaces and

transported materials in the immediate vicinity of the Alps, and near

other high mountains, are correctly referred to glacial action. This

theory has therefore solved, in part, one of the most difficult problems

in geology; but there is great difficulty in extending it so as to

account for the drift phenomena in general. If the motion depends upon

gravitation only, the origin must have a much greater elevation than the

terminus, which would not be the case in the great glacier supposed to

extend southward from the Arctic regions. Elevation of temperature, it

has been thought, might account for the movement of the mass southward.

2. _Icebergs._--In very high latitudes, the ice, which makes out from

the land into the sea during the cold season, suffers but little waste

at any time. This sheet of ice continues to increase in breadth and

thickness, by congelation, from year to year. The spray and the snows of

each succeeding year will also add to the mass. It thus accumulates to

the height of several hundred yards. It will also reach down a good many

feet below the surface of the sea, and will extend back on the land, or

lie heaped up against a precipitous escarpment, and firmly frozen to it.

After a certain amount of extension over the sea, the accumulated weight

of the ice and snow would tend to depress it, and break it loose from

the shore. The waves would tend to the same result, and would act at

greater mechanical advantage, as its extension from the shore becomes

greater. _Hence, it would ultimately become separated from the shore_,

and float in the water.

At its commencement, the earth, pebbles and rocks, which may lie along

the shore, and as far down into the sea as the congelation extends, are

frozen into it. In many situations its mass would be increased by

avalanches while it remained attached to the land, and these would

supply also masses of earth and rocks, as they do to glaciers. When it

becomes loosened from the shore, it will break off, and carry with it

some of the earthy portions of the coast, or the less firmly fixed

masses of rock from the escarpment against which it formed. Thus every

iceberg becomes _freighted_, more or less, _with earth and rocks_. This

has almost uniformly been found to be the case, when they have been

landed upon by ships' crews and examined.

We have seen that the general tendency of the waters of the ocean, and

of the lower stratum of the atmosphere, is to a motion from the poles

towards the equator. However irregular, therefore, the course of an

iceberg may be, its general _movement_, influenced both by the

prevailing winds and by ocean currents, will be _towards the equator_.

[Illustration: Fig. 78.]

These floating ice-mountains (Fig. 78) are formed in _great numbers, and

of vast size_. The relative specific gravity of ice and water are such

that nine cubic feet of ice, below the surface of water, will support

one cubic foot above it. As icebergs are often one or two hundred feet

high, their vertical depth must be a thousand feet at least; and their

area is equal to a square mile, and sometimes it is much greater. In

1840, the United States Exploring Expedition, in the extreme southern

ocean, coasted for eighty miles along a single iceberg. They are never

absent from the polar seas; and at certain seasons they are so abundant

along the usual course of vessels from New York to Liverpool, as greatly

to obstruct and endanger navigation.

An iceberg may continue for some time to increase in size, while

floating in the polar seas, but will at length reach a latitude where

the waste will exceed the additions, in consequence of the temperature

both of the air and of the water. It will, therefore, drop gradually the

earthy matters which it contains, upon the bed of the ocean.

It is not improbable that icebergs may often reach down so far as to

strike the highest points of the bed of the sea. The ice would be

lifted, and glide over the elevation, without suffering any perceptible

deviation from its general course. It would thus affect the surface of

rocks exactly like a glacier. If, however, the iceberg becomes

permanently stranded, and melts in one place, its earthy matters will be

thrown down upon the elevation which first arrested it.

If the bed of the sea, between the fortieth and sixtieth degrees of

latitude, could be exposed for examination, the rocky surface would be

found to be polished and striated by the icebergs which have passed over

it, and the whole surface would be strewed with boulders and drifted

materials brought from Arctic and Antarctic lands. Sometimes it would be

accumulated in heaps, and sometimes spread nearly over the surface.

We have seen that very recently, probably about the close of the

tertiary period, the portion of Europe and America over which the

northern drift is found, has been depressed several hundred feet. It may

be presumed that at that time icebergs floated over it, polished the

surface of the rocks, and distributed the boulders and other drift which

is now found upon it.

SECTION II.--IGNEOUS CAUSES.

I. _Of the Temperature of the Mass of the Earth._--Heat has been the

most efficient agent in determining and modifying the structure of the

earth; and, in order that the explanations of the phenomena referable to

this cause may be intelligible, some idea must be formed of the actual

present condition of the mass of the earth with respect to heat.

At any point of the surface there are variations of temperature,

depending on external causes. But these variations are found to extend

only a little way below the surface,--never more than a hundred feet. At

greater depths, it is found that the temperature invariably increases

with the depth. Deep mines have always a temperature above the mean

annual temperature at the surface. The water obtained by deep boring is

always tepid when it comes to the surface. The thermal springs, so

abundant in this country and in Europe, are so situated as to justify

the impression that their waters come from great depths. To make these

general observations of any value, we must determine the law by which

the temperature increases. The result of all the observations yet made,

in mines and upon wells and springs, is that, below the first hundred

feet, the temperature increases by one degree of Fahrenheit's scale for

every forty-five feet.

Regarding this law of increment as applicable to all depths, at ten

miles below the surface we should have a temperature above that produced

by the combustion of wood; and at twenty-five miles, a temperature of

three thousand degrees, by which nearly all mineral substances would be

reduced to a state of fusion.

The general conclusion of a temperature sufficient to melt the mineral

substances of which rocks are composed, at no considerable distance

below the surface, is confirmed by the fact that portions of the

interior of the earth--at least, at the volcanic centres--are in a

melted state. The intimate connection between some volcanoes situated a

hundred miles or more apart, so that they are alternately in a state of

activity and rest, indicates that these centres are connected,--that

subterranean melted lava extends from one to the other, so that when one

is active, the elastic force is relieved at the other. These deep-seated

lakes of lava must therefore underlie large areas.

We are justified, then, in concluding that the mass of the earth, with

the exception of a comparatively thin superficial layer, has a very high

temperature.

By way of accounting for this temperature, it is now generally assumed

that the earth was originally in a state of fusion; that it was a mass

of liquid lava (if, indeed, it had not a temperature sufficient to

reduce it to the aëriform state). Starting with this assumption, there

must necessarily be a gradual reduction of temperature by radiation, and

a time must arrive when the surface would be crusted over with

solidified lava; and this crust would increase in thickness as the

cooling advanced, the interior still retaining its heat and liquidity.

The present condition of the crust of the earth, its form, that of an

oblate spheroid, with the exact difference of the equatorial and polar

diameters which is found to exist, as well as the phenomena of volcanic

eruptions, will all admit of explanation on this hypothesis.

It has, however, been rejected by some; and, to account for the heat of

the interior of the earth, it is suggested that, if the bases of the

earths and alkalies, particularly potassium, sodium and calcium, exist

in their metallic state beneath the surface, the rapid oxidation of them

by the access of water would generate heat of sufficient intensity to

melt the oxidized materials, and thus account for the phenomena

attributable to heat.

Either of these hypotheses may be adopted; but it is not necessary to

account at all for the existence of this temperature. The fact is

susceptible of proof; and, though we may not be able to frame any

hypothesis to account for its existence, we may yet employ the fact in

the explanation of other phenomena.

II. _The Action of Internal Heat in producing Volcanoes._

The phenomena of volcanoes and earthquakes are evidently produced by

some force operating from below. The effect of heat alone would be to

reduce the rock to a liquid state. There is no reason to suppose that it

is ever sufficient to reduce them to the aëriform state. The elastic

force must therefore depend upon some other substance associated with

the lava, and this substance is water.

This will be shown by an examination of lavas. At the time of their

ejection, they are in a fluid or semi-fluid state; but it is not a

complete fusion. Even the most fluid lavas contain particles of minerals

in a solid state. The liquidity depends upon the fusion of the more

fusible portions, and upon the steam of water at a high temperature,

which fills the interstices between the solid particles. The porous

character of cooled lavas is produced by the steam which filled the

cavities previous to solidification. Steam always escapes from the

surface of a lava current while it is cooling, and it is always

discharged in immense volumes from the orifice of eruption, in

connection with the lava, and especially at the close of an eruption.

The geographical position of volcanoes, also, leads to the conclusion

that water is essential to their activity. There are five principal

lines of volcanic activity. One, commencing at the southern extremity of

South America, extends northward along the Andes and Cordilleras to

California or Oregon. The second has a north-east and south-west

direction, from the Aleutian Islands through the Kurule, Japanese, and

Philippine islands, till it meets the third line, lying in a nearly east

and west direction, embracing Sumatra, Java, and most of the Pacific

volcanic islands. A fourth band commences in the Grecian islands, and

extends westward so as to include the volcanoes of Italy and the

adjacent islands, and the Azores. The fifth band embraces the volcanic

islands of the West Indies, crosses Mexico in about the latitude of the

city of Mexico, and extends into the Pacific. There are also some

isolated centres of volcanic activity, such as Iceland. These volcanic

bands embrace about three hundred volcanoes. It will be seen that they

must nearly all be in close proximity to the ocean, or to large seas.

About two-thirds of them are on islands. Moreover, the volcanic vents

which are wholly submarine are probably very numerous.

This circumstance of the position of volcanoes establishes a presumption

that they cannot exist at a distance from some large body of water; and,

taking it in connection with the constant presence of aqueous vapor in

lava, we are justified in the conclusion that _the presence of water is

an essential condition of volcanic activity_.

Knowing that heat and water exist at the volcanic centres, it is not

difficult to form an idea of _their mode of operation_. The water,

diffused through the interstices of the lava, and subjected to a

temperature sufficient to melt the lava, would possess an _elastic

power_, which, though never computed, we may well suppose capable of

overcoming any resistance which the crust of the earth might present.

The repressing force will be the tenacity and weight of the

superincumbent strata. Whenever the elasticity is superior to this

repressing force, it will manifest itself in the fracture of the strata,

and often in the ejection or lava to the surface.

This fracturing of the strata, produced by an uplifting subterranean

force, is believed to be the cause of the noise and the vibratory motion

which are the chief phenomena of earthquakes. The elastic force may

raise lava to the surface, and thus the fracture would become a volcano.

But the force may expend itself by the discharge of vapor into the

fissure, or by merely filling it with lava. In either case, the only

evidence of the existence of the volcanic force would be the noise and

the wave-like motion experienced at the surface. The cause of the

volcano and earthquake is therefore the same, though the phenomena which

characterize them are different.

When the strata are is thus fractured, lava may for a time be discharged

along the whole line. By the cooling of lava in the fracture, it would

become partially reunited. Still, this would be the line of least

resistance. It would therefore be again burst through in certain places,

which would long continue to be orifices of discharge, and thus the

original fracture would determine _a line of volcanic activity_.

The repressing force may become greater at an orifice of eruption than

at some other point, either by the great accumulation of ejected

materials around the opening, or by the dormancy of the volcano long

enough for the complete solidification of the lava with which the

channel was filled. The least resistance may then be far from any

previous vent, when a new orifice of discharge will be opened, and _a

new volcano make its appearance_. It seems probable, also, that

volcanoes may become extinct by the reduction of temperature at the

volcanic centre, and that new volcanic centres may be formed; but the

cause of this change of temperature is not yet well understood. New

volcanoes have broken out in the sea, near Iceland, in several

instances; others in the volcanic line east of Asia. Graham Island,

situated between Sicily and Africa, was formed by an eruption which

broke out in the bed of the sea where the soundings were more than one

hundred fathoms. The island was at one time two hundred feet 'above the

sea, and three miles in circumference. It was, however, gradually

destroyed by the action of the waves, and now remains a dangerous reef,

covered by less than two fathoms water. The volcano of Jorullo, in

Mexico, was formed in this way. Previous to the formation of the

mountain, the region where it now is was a cultivated table-land. During

the year 1759 volcanic action commenced and continued, until, at the

expiration of twelve months, a cone had been formed having an elevation

of sixteen hundred feet above the adjacent plain.

An orifice of eruption is at first but little elevated above the general

surface; but, by the accumulation of ejected matter, a cone is at length

formed around the vent. The upper portion of a cone always consists of

these materials, but there may also be in progress a general elevation

of that part of the earth's crust, and the cone will partake of that

general elevation. The cones of the Andes owe their height, in a great

measure, to a general movement of elevation; those of Ætna and

Vesuvius, in a greater degree, to accumulation of ejected matter.

In either way, the height may become so great that the force necessary

to raise a column of lava to the top would be greater than the sides of

the cone, weakened as they always are by fractures in all directions,

can sustain. Hence, the highest craters of Ætna and South America have

long been closed, and the lava escapes through fissures at a lower

level, and _lateral cones_ are produced.

[Illustration: Fig. 79.]

The form which the materials have, when ejected from volcanoes, depends

mainly upon the degree of liquidity of the lavas at the volcanic foci.

If the liquidity is very perfect, the aqueous vapor will readily rise

through the lava. The steam thus separated will drive before it whatever

rocks, or previous lavas, may obstruct it. In their progress they would

be reduced to sand and powder, and ejected as _volcanic cinders_. (Fig.

79.) If the lava possess considerable viscidity, the aqueous vapor will

separate with more difficulty, and the lava and vapor will ascend the

channel together. Large bubbles of vapor will, however, collect with

more or less of frequency; and, as they rise through the lava, will

drive forward a portion of it, and cause the overflow to take place by

pulsations. As the bubbles reach the surface, their bursting causes the

loud reports, which are compared to the discharge of heavy artillery.

With each explosion some of the lava will be projected violently into

the air, and, cooling, will fall to the surface as scoriæ,--or, if the

lava be highly vitreous, it will be drawn out into fibres, and descend

as volcanic glass.

III. _Geological Phenomena referable to Volcanic Action._

Volcanic agency has probably never been less than it is now, and we

ought therefore to find its effects very general and important.

1. The most obvious of these effects are the _fractures_ with which the

crust of the earth is everywhere intersected. The uplifting force upon

which all volcanic phenomena depend would necessarily fracture the

crust, and the wave-like motion resulting from the fracture would cause

numerous secondary fractures, having a parallel direction. They are

often of such extent, during earthquakes, as to endanger life. During

the great earthquake at Lisbon, in 1755, a fracture opened of sufficient

width to swallow up the quay, and several thousands of persons who had

fled there for safety. The chasm remained permanently open to the depth

of six hundred feet. The earthquakes with which the valley of the

Mississippi was visited in 1811 so often fissured the surface, that the

inhabitants protected themselves by clinging to the trunks of trees,

which they felled transversely to the direction of the fissures.

[Illustration: Fig. 80.]

The first fracture which is produced by the upheaving force will open

upwards, and scarcely reach down to the seat of the force. But there

will be other parallel fractures, dependent upon the first, and opening

downward. Thus, the primary fracture at _a_ (Fig. 80) will be at once

followed by the fracture _b_, opening toward the lava, which will be

injected into it, and which, on cooling, will form a _dike_. Their

formation is mostly concealed from observation, but not always. During

the eruption of Ætna, in 1669, numerous fissures opened, one of which

was six feet wide and twelve miles in length; and the light emitted from

it indicated that it was filled with lava to near the surface. The

process was as perfectly seen as from the nature of the case it could

be.

2. The conversion of the lower sedimentary strata into _metamorphic

rocks_ has been effected by volcanic heat. The material of which dikes

consist has been injected in a highly-heated state; and, by observing

the effect which they have had upon the adjacent rocks, we may judge of

the effect which subterranean heat must have upon the lower mechanical

strata. Wherever the dikes are of considerable thickness, they have

converted the adjacent shales into primary slate, the sandstones into

quartz rock, and the dark and friable limestones into granular marble,

and destroyed the organic impressions. In the southern extremity of

Norway there is a district in which granite protrudes in a large mass

through fossiliferous strata. These strata are invariably _altered_ to a

distance of from fifty to four hundred yards from the granite. The

shales have become flinty, and resemble jasper; and near the granite

they contain hornblende. The siliceous matter of the shales has become

quartz rock, which sometimes contains hornblende and mica, and therefore

constitutes a kind of granite. The limestone, which at points remote

from the injected rock is an earthy, blue, coralline limestone, has

become a white, granular marble, near the granite, and the corals are

obliterated. The altered shales and limestones in many places contain

garnets, ores of iron, lead, &c. The annexed (Fig. 81) is a plan of this

granite and altered rock.

[Illustration: Fig. 81.]

One of the most instructive examples of metamorphic action in this

country is found in the White Mountains of New Hampshire. These

mountains have, till recently, been thought to consist principally of

granite; but it is now ascertained that this supposed granite is an

altered rock of the silurian period. It is represented as "intersected

by veins of felspathic granite; and the general mass is itself in many

parts converted into a near approximation to a binary granite, composed

of distinctly developed quartz and white felspar, with a few sparsely

scattered specks of mica. In its weathered surfaces it wears a close

resemblance to some fine-grained granites; but, upon inspecting a fresh

fracture with a magnifier, we instantly perceive many rounded grains of

quartzose sand, and the felspar is imperfectly formed, though the mica

has more nearly reached the condition which it has in granite. In some

of the coarse varieties of this white rock, small rounded pebbles of

quartz are to be seen, giving unequivocal evidence, even to the naked

eye, of its being an altered sandstone. We feel no hesitation in

deciding it to have been a silico-argillaceous white sandstone, now

almost granitized by extensive metamorphic action."

Similar illustrations, on a small scale, may be seen in every country

where the strata have been cut through by intrusive dikes. Sir James

Hall has shown the same by actual experiment. He exposed pulverized

chalk to heat sufficient to melt it, and under sufficient pressure to

prevent the escape of the carbonic acid. After cooling, the chalk was

found to have taken the form of crystallized limestone. But instances

enough have been given to show what changes should be looked for

wherever the sedimentary rocks have been exposed to a high temperature.

The lower strata must have been exposed, for long periods of time, to

such a temperature. We do not know at what depth below the surface of

the earth the rocks become liquid; but above the line of actual fusion

there must be a mass of rock not melted, yet scarcely retaining the

solid form. For a great thickness, perhaps for several miles, it would

be in a more or less yielding state. As there is not actual fusion, the

stratification is not destroyed, but such a degree of mobility among

the particles exists, that some degree of crystallization takes place,

and the elastic forces below easily bend, throw into folds, compress,

and in every way contort these strata. At the same time, any organic

matters which they may contain are decomposed, and the impressions of

them are obliterated. And such is the condition in which the metamorphic

strata are actually found.