The Elements of Geology; Adapted to the Use of Schools and Colleges Part 5

[Ill.u.s.tration: Fig. 43.]

The _Mammoth_ was a species of elephant, now extinct, of which remains are found with those of the mastodon, but in the greatest abundance in Europe and Asia. A large number of skeletons, many of them imperfect, have been discovered in the low grounds in the south-east of England. It was this animal which was found encased in ice and sand in Siberia, in 1804.

Contemporaneously with the existence of these huge animals, a near approach was made to the present fauna of the earth, by the introduction of ruminant animals resembling the ox and deer, and especially by the existence of the cla.s.s of animals which in anatomical characters stands next to man, the apes and monkeys.

The tertiary system, though not generally so continuous over extended areas as the older formations, yet const.i.tutes the surface of a very large part of Europe. (See Fig. 59.) In the United States the earlier portion is found along the seaboard, from New Jersey to Louisiana, and extending back towards the mountains to a distance varying from ten to one hundred miles. The later deposits are found in detached portions throughout the Eastern and Middle States. It covers a large surface in South America, and is found in India.

8. _The Recent Formation._--It is intended to embrace in this term strata which have been formed since the creation of man. It is, however, impossible to separate them by any well-defined characters from those of the tertiary period. The recent formation consists of land which is forming by the filling up of lakes, and by the increase of deltas from the acc.u.mulated sediment which rivers have furnished.

There is, however, no doubt but that formations on a large scale have continued in progress over extensive areas of the bed of the sea; and they have been no less rapid, we may presume, than they were in earlier periods. But, though they are preserving the records of the present era, they will probably remain in a great measure inaccessible for many ages.

These deposits, so far as they are accessible, are found to contain the remains of plants and animals (including man) now living in the vicinity where the deposits are forming.

SECTION VI.--FOSSILS.

Any organic substance imbedded in a geological formation, or any product of organic life, as a coprolite or a coin, or any marking which an organic substance has given to a rock, is regarded as a _fossil_. The study of fossils, as a branch of practical geology, requires an acquaintance with the principles and the minute details of botany and zoology. Without this knowledge, however, many of the general conclusions to which the study of fossils has led may be understood.

1. _Fossils are preserved in different ways._--When any organic substance is imbedded in a forming rock, it may itself remain; or it may be removed by the infiltration of water, or other causes, so gradually as to leave its form, and even its most delicate markings, in the rock; or some mineral substance may have been subst.i.tuted, and fill the s.p.a.ce which the organic substance once occupied; that is, it may be an organic substance preserved, it may be an impression of it, or it may be a cast of it.

2. The process by which the subst.i.tution in this last case is effected is called _mineralization_. The mineralizing ingredient is generally derived from the contiguous rock. In siliceous rocks it is silex. In calcareous rocks it is carbonate of lime. When iron is diffused through a rock, it often becomes the mineralizer. The subst.i.tuted mineral is generally a very perfect representation of the original fossil. We cannot therefore suppose that the original substance was entirely removed before any of the mineral matter was deposited. The subst.i.tution must have taken place particle by particle, as the organic matter was removed. Fossils are, in fact, often found, in which the mineralization has been arrested after it had commenced, so that the fossil is in part an organic and in part a mineral substance. It has been proved, by direct experiment, that these changes of removal and subst.i.tution are simultaneous. Pieces of wood were placed in a solution of sulphate of iron. After a few days, the wood was found to be partially mineralized, and after the remaining ligneous matter had been removed by exposing it to a red heat, "oxide of iron was found to have taken the form of the wood so exactly, that even the dotted vessels, peculiar to the species employed, were distinctly visible under the microscope."

3. As the fossiliferous strata are generally of marine origin, it is to be presumed that only a small proportion of terrestrial animals are preserved; and our knowledge of the organic remains which are preserved is yet so imperfect, that discoveries are constantly making, as examinations are extended. Still, enough is known to enable us to draw some satisfactory conclusions as to _the order in which living beings were created upon the earth_.

Though most of the earlier organic forms which have been preserved are of animal origin, yet vegetable remains occasionally occur in connection with them, and we must suppose vegetables to have been produced abundantly. For all animal food consists of vegetable substances, or of animal substances which have once existed in the vegetable form. No animal is capable of effecting those combinations of inorganic matter upon which its growth and sustenance depend. We may therefore conclude that _the introduction of animals and vegetables was contemporaneous_.

The greatest development of vegetable life was, however, during the carboniferous period. The design of this abundant growth was prospective. It was not produced for the support of animal life, but for fuel, and stored till man should be introduced, and so far advanced in civilization as to make this supply of carbonaceous matter subservient to his wants and happiness.

In the earlier periods, the lower forms of animal life were, beyond all comparison, the most abundant; yet the four great divisions of the animal kingdom, Radiated, Articulated, Molluscous, and Vertebrated animals, were all represented. There is, however, no evidence that any vertebrated animals, except fishes, were created till after the carboniferous period. In the next formation, the new red sandstone, we find the tracks of reptiles and birds, and probably of marsupial animals. The first evidence of the existence of mammalia in great numbers is in the tertiary period, when the pachydermata and edentata were so much more abundant than they have ever been since, and when the bimana first appear.

But there is no evidence from geology that man existed till after the close of the tertiary period. The grounds upon which contrary statements have sometimes been made are untenable. In Ohio a very perfect impression of a human foot was found on a slab of limestone of the silurian age. But it was subsequently ascertained to have been common for the aborigines, in the vicinity of their encampments, to cut in the rocks, with surprising accuracy, the forms of the tracks of man and other animals.

There is a human skeleton in the British Museum imbedded in solid limestone, and another in Paris, both taken from Guadaloupe. It was at one time supposed, from the degree of solidification of the limestone, that it must have been formed at an early geological period; but it is found that the beach-sand of that island now solidifies rapidly, from the carbonate of lime which the waters there hold in solution. It is rendered probable that the skeletons found there have not been buried more than a century and a half.

4. As many parts of the bed of the present seas, which are probably receiving detrital matter constantly, are unfavorable for the development of animal life, while other parts are highly favorable, it might be presumed that animal life would be equally scanty in particular localities while the earlier rocks were forming, and in other localities very abundant. Hence some strata, for hundreds of feet in thickness, are composed almost entirely of fossils, while other strata are nearly or quite dest.i.tute of them. The same member of a formation may in one place be full of fossils, and in another without them. _The distribution of fossils_ is therefore subject to no general law; at least, none of which we can avail ourselves, in the search for them.

5. The value of fossils in geology consists in the use which is made of them in determining _the origin and age of strata_.

As the animal species which inhabit bodies of fresh water are always different from those found in the sea, their remains const.i.tute the best means of determining whether a formation is of _fresh water_ or _marine origin_. In order to decide this point, it may, in some cases, be necessary to be acquainted with the habits of particular species. In most cases, however, it will be sufficient to remember that in fresh-water formations, first, there are no sponges, corals, or chambered sh.e.l.ls; second, the univalves all have entire mouths (Fig.

44). Third, the bivalves are all bimuscular (Fig. 47). If, therefore, a formation is found to contain sponge, coral, a chambered sh.e.l.l, a univalve with a deeply notched mouth (Fig. 45), or a unimuscular bivalve (Fig. 46), it must be considered a marine formation.

[Ill.u.s.tration: Fig. 44.]

[Ill.u.s.tration: Fig. 45.]

[Ill.u.s.tration: Fig. 46.]

[Ill.u.s.tration: Fig. 47.]

We have seen that the same formation, as exhibited in different places, differs in its thickness, composition and degree of solidification. If we could trace the strata through all the intermediate s.p.a.ce, we might be certain of their being the same formation, notwithstanding the change in lithological characters. But this can seldom be done, even for a few miles in extent. Sections of the strata are obtained only occasionally, where rivers have cut through them, or where, over limited areas, the soil has been removed from the outcropping edges. It is also frequently the case that the strata are so much disturbed that their position will furnish no aid in determining their age. When folded axes occur (as here represented), the older strata are often the uppermost. There is an instance in the Alps in which strata of vast thickness have been inverted during the process of upheaval, and now rest on a bed of rock formed from the debris which they had supplied.

[Ill.u.s.tration]

And yet it is important to determine what formations are of the same age, notwithstanding their displacements, difference in lithological characters, and separation by great distances and by mountains or oceans. This determination can be made only by a comparison of the imbedded fossils. It is found that every formation, and every important member of a formation, contains an a.s.semblage of fossils peculiar to itself. When very widely separated, the species of fossils may not be identical, but so very similar that they are regarded as equivalent species. _The identification of formations_ consists in the identification of fossils. It is for this purpose mainly that fossils are regarded as of so great importance.

6. If each formation is characterized by the presence of new species, it follows that _the work of creation was a progressive one_, continued through long periods of time. The latest creation of which we have any geological evidence is that of man. And if the leading design of the existence of this earth was as a theatre for the development of moral character, it is to be presumed that the work of creation ceased when a species possessing moral capacities had been introduced.

It follows also, from what has been said, that there has been a constant disappearance, a death, of species. It would seem that each species has a life a.s.signed to it, which is to be completed and surrendered. Though its continuance is many times longer than the life of any individual of the species, yet _it is the course of nature that species should disappear_.

There may be something in the const.i.tution of each species by which its continuance is limited, making an old age and death necessary, as it is in individuals. But there are other causes by which the duration of species may often be terminated. The subsidence of New Holland would cause the destruction of a large number of species. The preservation of the human species was at one time effected only by a special and miraculous interference. Slowly operating causes are now at work, by which many species, such as the elephant, wolf and tiger, will at length become extinct. Their existence in a natural state cannot long be continued in a civilized country. The forest, their natural abode, disappears, and some are intentionally destroyed, because they render life and property unsafe. Under the operation of these causes, the Irish elk (cervus giganteus) has become extinct, probably within the human era. The Dodo, a gallinaceous bird, found living when maritime communication between Europe and the East Indies was first established, is now extinct. The Apteryx, a bird belonging to New Zealand, has probably become extinct since the commencement of the present century.

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 a.s.signed 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 acc.u.mulation. 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 exuviae 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 polis.h.i.+ng stone called tripoli is found in beds of ten or twelve feet in thickness, and is composed entirely of the siliceous sh.e.l.ls 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 const.i.tution. The time necessary for the acc.u.mulation of beds several feet thick by the sh.e.l.ls 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 a.s.sist 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 a.s.signs 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 a.s.sert, in the first place, simply that "In the beginning G.o.d 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 G.o.d 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 G.o.d, 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.