The fossils of this system are not abundant. In the Permian portion, impressions of fishes are found, always with the peculiarity that the tail is heterocercal (Fig. 27); that is, with the spine continued into the upper lobe. The same peculiarity prevails in the carboniferous and all the earlier formations. Fishes with the tail homocercal begin to appear in the Triassic portion of this system, and are found in all the subsequent formations. The remains of saurians also occur in this formation.
The red sandstones seem to have been better adapted to retain the forms which were impressed upon them than to preserve the organic remains which were deposited in them. Hence, while they contain but few fossils, the strata are often covered with ripple marks, with sun cracks, occasioned by contraction while drying, or with depressions produced by rain-drops, and the pits are sometimes so perfect as to show the direction of the wind when the drops fell. (Fig. 28. The tracks of animals are also well preserved. Some of them were produced by reptiles (Fig. 29, c), and some probably by marsupial animals, but most of them by birds (a, b). President Hitchcock has distinguished the tracks of more than thirty species in the sandstones of the Connecticut valley. Birds, reptiles and marsupial animals, seem to have been first introduced during this period.
The new red sandstone is well developed in all its members on the continent of Europe. In England, all the members are present, except the Muschelkalk. The Triassic portion of it occurs in North America. It is found in detached portions, probably as parts of a continuous formation, in Nova Scotia, the eastern part of Maine, the Connecticut valley, and from New Jersey southward through Pennsylvania, Maryland, &c., to South Carolina.
5. The Oölitic System.—The lower portion of this system is the Lias, and consists of a series of fissile, argillaceous limestone, marl, and clays. The Oölite forms the intermediate member of the system, and consists of alternations of clay, arenaceous rock and limestone. Some of the limestones have an oölitic structure, and the whole system takes its name from this circumstance, though this structure is not found in all parts of it, and is often found in other formations. The central part of the oölite, the coral rag, is principally a mass of corals and comminuted shells. The Wealden, the highest member of the oölitic system, is an estuary deposit, consisting of calcareous beds, followed by sandstone, and terminated by the Wealden clay.
This system is throughout highly calcareous, and furnishes, wherever it is developed, valuable materials for architectural and ornamental purposes.
This system is distinguished for the great amount and variety of its organic remains. The vegetable productions were intermediate between those of the coal period and those of the present time. The upper oölite, in the south of England, contains the stumps of trees and other plants, rooted in a black carbonaceous layer, evidently the soil from which they grew. These stumps and prostrate trunks are the remains of coniferous trees of large growth. (Fig. 30.)
Corals occur in great abundance; also encrinites (Fig. 31), mollusks (Fig. 32), and cephalopoda.
But this system is specially characterized by the remains of saurian reptiles. The Ichthyosaurus (Fig. 33, a) was a marine animal, having the general form of a fish, while its head, and especially its teeth, resemble those of the crocodile. It was an air-breathing animal like the cetacea, and was furnished with similar paddles. It was carnivorous, and was undoubtedly the largest and most formidable animal existing in the earlier part of the oölitic period. Its length could not have been less than thirty or forty feet.
The Plesiosaurus (Fig. 33, b) was also a marine animal, and in ninny respects similar to the Ichthyosaurus; but its general form was more slender, its head was small, and its neck was of great length, the cervical vertebræ exceeding in number those of the swan.
The Pterodactyle (Fig. 34) was a small saurian, of the size, probably, of our largest eagle. The finger-bones, which in the other saurians form the paddles, are in the Pterodactyle very much lengthened, so as to support a membranous expansion, like that of the bat. These wings were of sufficient size to enable it to sustain itself in the air, and to make a rapid and easy flight.
The Iguanodon is a Wealden fossil, remarkable for its great magnitude. It is estimated that its length was seventy feet. It was a lizard, adapted for motion on land, and was herbivorous.
This formation is well developed in England, and, with the exception of the Wealden, on the continent of Europe. It has been supposed that no part of the oölitic series was to be found in this country; but there is a highly arenaceous rock occupying the valley of the James river, in the vicinity of Richmond, Virginia, of considerable extent, and a thousand feet in thickness, containing a bed of coal of forty feet in thickness, which, from its fossils, must be referred to the oölitic series.
6. The Cretaceous Formation.—The lower part of this formation consists of greensand, interstratified with beds of clay. The intermediate portion is a mixture of argillaceous greensand and impure chalk. The upper part is composed of chalk, which is a friable, nearly pure carbonate of lime. The strata of chalk are separated, at intervals of from three to six feet, by layers of flint, either in the form of nodules or of continuous strata.
These characters, by which the cretaceous system is known in England, are but partially recognized elsewhere. Thus, in the Alps, the “Neocomian System,” consisting of crystalline limestones, is the equivalent of the English greensand; while the greensand of this country is the equivalent of the white chalk of England.
The fossils of the cretaceous formation are very different from those of the oölite, and are such as to show that it was deposited in deep seas. Microscopic shells are often so abundant as to constitute a large proportion of the mass. Zoöphytes are very numerous, such as sponges, corals, star-fishes (Fig. 35, d e), and a few crinoidea (b). Mollusks were also abundant, and cephalopoda, consisting of chamber-shells and belemnites (Fig. 36). The belemnite probably resembled the existing cuttle-fish; but the remains consist, in most cases, of a partially hollow calcareous substance (b), which was contained within the animal, and formed its skeleton.
The chalk and greensand are largely developed in England; and the same formation, with different lithological characters, is found in great force flanking the principal mountain ranges of southern Europe, and extending into Asia. In this country the system commences with the greensand and friable limestones of New Jersey, and following the Alleghany range to its southern termination, it bends around into a north-western direction, and is continued into Missouri.
7. The Tertiary System.—The tertiary strata embrace the formations from the cretaceous to the human era. They consist of clay, sand, sandstone, marl and limestone, and are distinguished from the lower rocks by being less consolidated; though the limestones are in some instances solidified, and resemble the strata of earlier origin. The tertiary strata are generally of less thickness than the older formations, and less continuous, being local deposits formed in lakes and estuaries. In a few instances they have been thrown into inclined positions, though in most cases they have been but slightly disturbed, and raised but a few hundred feet above the present level of the sea.
The late tertiary strata seldom overlap the older, so as to indicate their relative ages by superposition. They have therefore been separated into groups according to the proportions of living and extinct species of shells which they are found to contain. The oldest tertiary or Eocene formation[A] contains only four per cent, of living species, the Miocene contains seventeen per cent., the Pleiocene forty per cent., and the Pleistocene ninety per cent.
[A] Eös, dawn, and kainos, recent. The formation which commenced at the dawn of the recent period, containing but a small number of living species. Miocene (meion, less), less recent than the Pleiocene (pleion, more). Pleistocene (pleistos, most), most recent.
During the pleistocene period, peculiar conditions existed, by which a great amount of loose material, known by the name of drift, was spread over the northern portions of both hemispheres. In America it is found from Nova Scotia nearly to the Rocky Mountains, and extending as far south as Pennsylvania and the Ohio river. In Europe, it is found from the Atlantic to the Ural Mountains, and reaching south into Germany and Poland. It is also found in the colder portions of South America, and in the vicinity of several mountains, as the Alps.
It consists of irregular accumulations of earthy substances of different degrees of fineness, but characterized by containing masses of rock of considerable size, often of many tons weight, called boulders. Rocks having the same lithological characters exist in situ north of where the boulders and other drift are now found, though at a distance often of one or two hundred miles. There can be no doubt but that the drift has been transported from these northern localities; and the polished, striated and grooved condition of the rocky surface, wherever the drift is distributed, has obviously been produced by the passage of the drift materials over it.
Towards the close of this period, while the land was a few hundred feet below its present level, there were deposited in the valleys of the drift region beds of blue and gray clay, materials which are used in making bricks and coarse pottery; also beds of sand, sometimes evenly spread out, but often thrown into irregular mounds and ridges.
In regions which are not covered with drift,—as the south of Europe and the United States,—the pleistocene deposits are succeeded, without apparent change of conditions, by those which are now taking place.
The formations of the tertiary period are distinguished from those of the cretaceous period by the absence of deep-sea fossils, and from the oölite by the absence of its characteristic saurians. The mollusks are also very different, such genera as the cerethium (Fig. 37), murex (Fig. 38), and conus (Fig. 39), which abound in the present seas, first appearing in the tertiary period. The nummulite (Fig. 40), a peculiar form of chambered shell, is so abundant as to constitute in some places almost the entire rock.
The period is however characterized by the existence of a large number of pachydermatous animals, of which the tapir, hog, horse and elephant, are examples of living species.
The Paleotherium (Fig. 41) resembled, in most respects, the tapir. It was furnished with a short proboscis, and the foot was divided into three toes. The length of the largest species was about that of the horse; but its body was larger, and it was of less height.
The Anoplotherium (Fig. 42) was a more slender animal, and resembled in size and general form the gazelle.
The Megatherium, an animal of the late tertiary epoch, was larger than the existing species of elephant, and in its general structure and habits resembled the sloth.
The Mastodon (Fig. 43) lived during the latest portion of the tertiary epoch. Its remains are found most abundantly where the animal seems to have perished by sinking into the soft marshy ground near the brackish springs of New York and Kentucky. But they are found also in Europe and Asia. It was larger than any existing land animal, and was nearly allied in structure and habits to the elephant.
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 class 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 constitutes 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 accumulated 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.
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 substituted, and fill the space 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 substitution 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 substituted 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 substitution 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 substitution 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 destitute 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 constitute 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 shells; 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 shell, a univalve with a deeply notched mouth (Fig. 45), or a unimuscular bivalve (Fig. 46), it must be considered a marine formation.
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 space, 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.
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 assemblage 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 assigned 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 constitution 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 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.
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.
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.
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.
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.