CHAPTER II.

OF THE ARRANGEMENT OF THE MATERIALS WHICH COMPOSE THE CRUST OF THE EARTH.

SECTION I.--THE CLASSIFICATION OF ROCKS.

In the first place, we divide rocks into _stratified_ and _unstratified_. This division is one which will in general be easily recognized, even by the most inexperienced observer; and the distinction is important, because it separates the rocks of igneous origin from those which have been produced by deposition of sediment from water.

It will be shown hereafter that a part of the unstratified rocks have been formed at or near the surface of the earth; that is, they have taken their present form by passing from a state of fusion to a solid state above or between the stratified rocks, as in the case of lava (Fig. 2, A). The other unstratified rocks have cooled so as to take the solid form below the stratified rocks, as at B. The first are called _epigene_, or _volcanic rocks_; the last, _hypogene_, or _plutonic rocks_.

The lowest portion of the second division, the stratified rocks, are termed _non-fossiliferous_, from the fact that they contain no evidence of the existence of organic beings at the time when they were deposited. Their relation to the other rocks is shown at C. It is supposed that these rocks have been subjected to great changes by heat from the igneous rocks below them. On this account Mr. Lyell proposes to call them _metamorphic rocks_. The other portions of the stratified rocks are _fossiliferous_, containing the remains of organic beings which lived at the period when the rocks were deposited. They are represented at D. The division of the last-named rocks info groups will be given hereafter.

We have then four principal classes of rocks: _Plutonic Rocks_, _Volcanic Rocks_, _Non-fossiliferous Stratified Rocks_ and _Fossiliferous Rocks_.

SECTION II.--THE PLUTONIC ROCKS.

Granite is by far the most important of this class of rocks. Of its thickness no estimate can be made, as no mining operations have ever penetrated through it, and none of the most extensive displacements of rocks by natural causes has brought to the surface any other rock on which it rests. It may, therefore, be considered the foundation rock, the skeleton of the earth, upon which all the other formations are supported. The whole amount of granite in the earth's crust may be greater than that of all other rocks, but it comes up through the other formations so as to be exposed over only a comparatively small portion of the surface, and this is generally the central portion of mountain ranges, or the highest parts of broken, hill country. Still, it is not unfrequently found in the more level regions, in the form of slightly elevated ridges, with the stratified rocks reclining against it.

The structure of granite seems frequently to be a confused mixture of the minerals which compose it, without any approach to order in their arrangement; but in many cases it is found to split freely in certain directions, and to work with difficulty in any other. This may result from an arrangement of the integrant crystals, so that their cleavage planes approach more or less nearly to parallelism. When this is the case with the mica or felspar, it must diminish the cohesion in a direction perpendicular to these planes, and thus facilitate the cleavage of the mass.

Granite is found to penetrate the stratified rocks in the form of veins. The following section (Fig. 3) will show the relation of granite veins to the granitic mass below. The granite which is quarried for architectural purposes is often in comparatively small quantities, disappearing at the distance of a few hundred yards beneath the stratified rock; or else it exists in the form of isolated dome-shaped masses. It is probable that, if they could be followed sufficiently far, they would be found to be portions of dikes coming from the general mass of granite below. Even the granite nuclei of the great mountain ranges may be considered as injected dikes of enormous magnitude.

Granite is itself intersected with granite veins more frequently, perhaps, than any other rocks; but the vein is a coarser granite than the rock which it divides. It is not uncommon to find one set of dikes intercepted and cut off by a second set, and the second by a third. The substance of the dikes was, of course, in a liquid state when it was injected, and the first must have become solid before the second was thrown in; hence the dikes are of different ages. The dikes _a b c_, represented in Fig. 4, must have been injected in the order in which they are lettered.

It is probable that, by the process of cooling, the liquid mass from which these dikes have proceeded has been gradually solidifying from the surface downwards. If so, it would follow that the granite nearest the surface (1, Fig. 2) is the oldest, and the newest is that which is at the greatest distance below (4). It is possible that at great depths granite may be still forming, that is, taking the solid form, though of this there can be no direct proof. There is, however, proof that it has been liquid at periods of time very distant from each other; for the dikes sometimes reach to the top of the coal formation (for example), and then spread themselves out horizontally, as at _a_, showing that the rock above the coal had not then been deposited. Another dike will extend through the new red sandstone, as at _b_, and spread itself out horizontally as before. These horizontal layers of granite, by their position in strata whose ages are known, indicate the periods when granite has existed in a liquid state. Granite veins have been discovered in the Pyrenees as recent as the close of the cretaceous period, and in the Andes they have been found among the tertiary rocks.

There are several other rocks, of minor importance, often found in connection with granite. Hypersthene rock, in a few cases, forms the principal part of mountain masses. Greenstone is more frequently associated with the trappean rocks, but it sometimes passes imperceptibly into syenite and common granite. Limestone is found in considerable abundance, and serpentine in small quantities, as primary rocks, and have evidently been formed like granite, by solidifying from a state of fusion.

SECTION III.--THE VOLCANIC ROCKS.

The volcanic rocks consist of materials ejected from volcanoes. They are, however, ejected in very _different states_; sometimes as dust, sand, angular fragments of rock, cinders, &c., and sometimes as lava streams. In some instances, the lava has so little fluidity that it accumulates in a dome-shaped mass over the orifice of eruption, and perhaps in a few instances it has been thrust upward in a solid state.

There are _two principal varieties_ of lava, the trachytic, consisting mostly of felspar, and the basaltic, consisting of hornblende. When both kinds are products of the same eruption, the trachytic lava is thrown out first, and the basaltic last. The reason of this is, that felspar is lighter than hornblende, and probably rises to the surface of the lava mass at the volcanic focus, and the basaltic lava is therefore reserved till the trachytic has been thrown off.

These, like other rocks, have been produced at different epochs. There is, however, great difficulty in determining their age; There are some differences of structure and composition observed, in comparing the older and newer lavas; but the only method that can be relied on to determine their age is their relation to other rocks. When they occur between strata whose age is determined by imbedded fossils, they must be of intermediate age between the inferior and superior strata.

1. _Modern Volcanic Rocks._--Some of the volcanic rocks are of modern origin, and are produced by volcanoes now active. The total amount of these, and of all the other volcanic rocks, is probably less than that of either of the other principal divisions of rocks; yet they form no inconsiderable part of the earth's crust. The number of active volcanoes is not far from three hundred, and the number of eruptions annually is estimated at about twenty. In some cases, the lava consists of only a single stream, of but a few hundred yards in extent. It extends, however, not unfrequently twenty miles in length, and two or three hundred yards in breadth. The eruption of Mount Loa, on the island of Hawaii, in 1840, from the crater of Kilauea, covered an area of fifteen square miles to the depth of twelve feet; and another eruption of the same mountain, in 1843, covered an area of at least fifty square miles. The eruption in Iceland, in 1783, continued in almost incessant activity for a year, and sent off two streams in opposite directions, which reached a distance of fifty miles in one case, and of forty in the other, with a width varying from three to fifteen miles, and with an average depth of more than a hundred feet. The size of some of the volcanic mountains will also assist in forming an idea of the amount of volcanic rocks. Monte Nuovo, near Naples, which is a mile and a half in circumference and four hundred and forty feet high, was thrown up in a single day. Ætna, which is eleven thousand feet high, and eighty-seven miles in circumference at its base, has probably been produced wholly by its own eruptions. A large part of the chain of the Andes consists of volcanic rock, but the proportion we have not the means of estimating.

2. _Tertiary Lavas._--There is another class of volcanic products, which are so situated with reference to the tertiary strata that they must be referred to that period. The principal localities of these lavas, so far as yet known, are Italy, Spain, Central France, Hungary, and Germany. They are also found in South America. Those of Central France have been studied with the most care. They occur in several groups, but they were the seats of volcanic activity during the same epoch, and formed parts of one extensive volcanic region. Each of these minor areas, embracing a circle of twenty or thirty miles in diameter, is covered with hills two or three thousand feet in height, which are composed entirely of volcanic products, like the cone of Ætna. On many of them there are perfectly-formed craters still remaining. Numerous streams of lava have flowed from these craters, some of which can now be traced, throughout their whole extent, with as much certainty as if they were eruptions of the present century. Some of the lavas have accumulated around the orifices of eruption, forming rounded, dome-shaped eminences. These lavas generally consist of trachyte, and have therefore a low specific gravity, and imperfect fluidity. The basaltic lavas have often spread out over broad areas, and, when they have been confined in valleys, have reached a distance of fifteen miles or more from their source. There still remain indications of a current of lava which was thirty miles long, six broad, and in a part of its course from four to six hundred feet deep. The above sketch (Fig. 5) will give some idea of the highly volcanic aspect which the district of Auvergne, in France, presents.

The unimpaired state of some of the cones and craters, and of the lava currents, would lead to the impression that these regions have been the theatre of intense volcanic action within a very recent period. But there is good reason to believe that this has not been the case. "The high antiquity of the most modern of these volcanoes is indeed sufficiently obvious. Had any of them been in a state of activity in the age of Julius Cæsar, that general, who encamped upon the plains of Auvergne and laid siege to its principal city, could hardly have failed to notice them."

It is equally certain that the commencement of their activity was at a late period in the history of the earth. Lava currents are frequently found in France resting upon the early tertiary strata, but no lava current is found below them. The later tertiary strata contain pebbles of volcanic rocks, showing that lavas had been previously ejected, but none are found in the older strata of this formation. We must, therefore, conclude that these volcanic tracts assumed their volcanic character at some intermediate point in the tertiary period.

When we find that their activity commenced at so late a period and closed so long ago, we might be led to suppose that it was of very short duration. But a great number of facts, in the present condition of the country, require that we should assign to them a very prolonged activity. A single instance will be sufficient to show the nature of the evidence upon which this conclusion rests. The heavy line (Fig. 6) represents the present form of one of the valleys. A bed of lava forms the highest point of land represented, and a second bed is found in an intermediate part of the slope. The position of the upper bed must have been a valley, when the lava flowed there. We may represent this valley by the line _a b c_. The slow operation of natural denuding causes at length excavated the valley _d e h_, when another lava current flowed through it, covering its bed of pebbles, as before. The same denuding causes have at length produced the present valley, _f g h_. These remnants of lava-currents, as they have formed a very imperishable rock, have protected the subjacent strata from erosion, and furnish evidence of the position of the valley at different periods. When we consider with what extreme slowness denuding causes produce changes on the surface, and what extensive changes they have here nevertheless effected in the interval between the production of the different lava currents, we are compelled to feel that that interval was a very prolonged one. Yet this period, however long it may have been, was evidently less than the period of activity of these volcanoes.

3. _Volcanic rocks of an earlier date_ are also found, sometimes as distinct lavas, though generally as volcanic grits. They occur interstratified with the cretaceous rocks, and with every other formation of the fossiliferous series, showing that, from the earliest times, these rocks have been accumulating as they now are.

_The trappean rocks_ may, in a general classification, be considered as volcanic. It will be shown, hereafter, that they are the lavas of submarine volcanoes. They do not, however, occur in the form of lava currents, but in great tabular masses, generally between stratified rocks, or in the form of dikes. They are also entirely unconnected with cones or craters.

The trappean rocks occur more or less abundantly in all countries. One of the most noted localities of this rock is a region embracing the north of Ireland, and several of the islands on the western coast of Scotland. It contains the celebrated Giant's Causeway, which consists of a mass of columnar trap; also Fingal's Cave, which is produced by a portion of the trap being columnar, and thus disintegrating more rapidly than the rest, by the action of the waves. An immense mass of greenstone trap, which has generally been considered as a vast dike, though often a mile in thickness, is found extending from New Haven to Northampton, on the west side of the Connecticut river. It then crosses to the east side, and continues in a northerly direction to the Massachusetts line. Under different names, it constitutes a nearly continuous and precipitous mountain range for about one hundred miles. Dr. Hitchcock supposes this greenstone range to be, not an injected dike, but a tabular mass of ancient lava, which was spread out on the bed of the ocean during the period of the deposition of the Connecticut river sandstone. It was subsequently covered with a deposit of strata of great thickness, and then by subterranean forces thrown into its present inclined position.

There is a mass of basaltic rock in the valley of the Columbia river, in the Oregon Territory, which extends without interruption for a distance of four hundred miles. Its breadth and thickness is not known, but in some places the river has cut a channel in this rock to a depth of four hundred feet. Its age has not been determined, and it will, perhaps, be found to be a tertiary or modern production.

SECTION IV.--THE NON-FOSSILIFEROUS STRATIFIED (OR METAMORPHIC) ROCKS.

1. _Gneiss_ is the most abundant rock in this class, and is generally found reposing on granite. Its stratification is sometimes very distinct, but it is often so imperfect that it can scarcely be recognized. This is more frequently the case in the vicinity of granite on which it rests, and into which it insensibly passes. A large part of the material used for building purposes, under the name of granite, is obscurely marked gneiss. In all primary countries it is an abundant rock, occupying extensive districts, and sometimes forming mountain masses.

2. _Mica slate_ lies next above gneiss, and is a very abundant rock. As it differs from gneiss only in the proportion of mica which it contains, and as the quantity of mica in it is very different in different places, it is often difficult to make the distinction between them. It also passes by insensible degrees into the argillaceous rocks. Many of the argillaceous rocks are found, upon close examination, to contain mica in minute scales in such abundance as to make it doubtful whether they ought not to be regarded as mica slates; that is, the metamorphic action by which argillaceous slate is converted into mica slate had proceeded so far, before it was arrested, that it becomes impossible to say whether the argillaceous or micaceous characters predominate.

3. _Argillaceous slate._--The last rock of this series is a slaty rock, more or less highly argillaceous. It does not differ in lithological characters from the same rock in the higher strata. It is doubtful whether the roofing-slates should be considered as belonging to the metamorphic series or not. They have been subjected to a very high degree of metamorphic action, and yet strata intimately associated with them have, in occasional instances, contained fossils.

It is not easy to fix the exact upper limit of this series. The fossils are few, obscure, and seldom met with in the lowest fossiliferous series; and the transition is very gradual from the distinctly metamorphic to the fossiliferous rocks. This renders it impossible always to determine accurately the line of separation.

The gneiss, mica slate and argillaceous slate, have the order of superposition in which they are here named. They differ only in the amount of metamorphic action to which they have been subjected; and the gneiss which is most highly metamorphic has, by being the lowest, been most acted upon,--the mica slate less, and the argillaceous slate least. In a particular locality, however, the lowest rock which was subjected to these causes of change, instead of having been of such a character as to produce gneiss, may have been a limestone, and in that case the lowest metamorphic rock would be a saccharine marble. In another locality the lowest rock may have been a sandstone, which would be converted into quartz rock. Hence there may occur, in any part of the metamorphic series, crystalline limestone, quartz rock, hornblende slate, chlorite slate, and talcose slate; and any one of these rocks may be as abundant in any particular region, as gneiss, mica slate or argillaceous slate, is in another.

The metamorphic rocks occur in all countries where there has been any considerable amount of volcanic action, and their total amount is very great; but their stratification is so confused and contorted, their superposition so irregular, and denudations have been so extensive, that no estimate can be made of their thickness. They are, perhaps, equal to all the other stratified rocks.

SECTION V.--THE FOSSILIFEROUS ROCKS.

The fossiliferous rocks are divided into seven systems, which are readily distinguished by the order of superposition, lithological characters and organic remains. These systems are the Silurian, the Old Red Sandstone, the Carboniferous, the New Red Sandstone, the Oölitic, the Cretaceous, and the Tertiary systems. There is also an eighth system now in process of formation.

It is the opinion of some geologists that there is another system situated between the metamorphic rocks and the silurian system. It has been called by Dr. Emmons, who has studied it with much care, the "Taconic System," the Taconic Mountains, in the western part of Massachusetts, being composed of these rocks. It is the lower part of what has been called, in England and Wales, the _Cambrian system_.

The strata of this system have a nearly vertical position, and consist principally of black, greenish and purple slates, of great thickness. Granular quartz rock, however, occurs in considerable quantity, and in this country two thick and important beds of limestone are found. These limestones are occasionally white and crystalline. Generally, however, as a mass, they are a dark, nearly black rock, with a network of lines of a lighter color. All the clouded marbles for architectural and ornamental purposes are from these beds, and our roofing and writing slates are all obtained from the argillaceous portion of this system.

The number of species of organic remains contained in this system is very small, and these, so far as discovered, belong to the annelida, with a few doubtful cases of mollusca. This system of rocks is found coming to the surface in a large part of New England, and the eastern part of New York, also in the western part of England and Wales.

Those geologists who deny the existence of this system consider these rocks as parts of the silurian system which have been most disturbed by subterranean forces, and most altered by proximity to igneous rocks. The annexed sketch (Fig. 7) will exhibit the relations here referred to. Certain portions of the silurian rocks are supposed to have been thrown into folds by the upheaval of the primary rocks. The plications nearest to the intrusive granite would be most altered. That part of the figure below the line _a a_ represents the outcropping edges as they now appear, the upper portion of the folds having been removed by some abrading cause.

As it is yet uncertain which of these views is correct, convenience will justify us in retaining the name of Cambrian system till further investigations shall settle the question.

1. _The Silurian System._--The following tabular arrangement exhibits the divisions of the system as recognized in England, in New York, in Pennsylvania and Virginia, and in Ohio.

Key to Divisions ---------------- C - Cambrian Rocks. S - Silurian System. D - Devonian. Ch - Champlain Division. On - Ontario Division. He - Helderberg Division. Er - Erie Division.

Divisions as recognized Divisions as recognized Pennsylvania Ohio. by English Authors. by the New York and Virginia. Geologists. /-------------------\ /-----------------------\ /----------\ /--------\ { Upper Cambrian { { Potsdam Sandstone. } No. 1. { { { C { Rocks, of Sedgwick{ { Calciferous Sandrock. } { { { Birdseye Limestone. } No. 2. } { (probably). {Ch{ Trenton Limestone. } } Blue { { } Limestone { Llandeilo Flags. { { Utica Slate. } } and Marl. { { { } No. 3. } { { Hudson River Group. } { { { { { Gray Sandstone. } No. 4. { { { Oneida Conglomerate. } { {On{ { Caradoc Sandstone.{ { Medina Sandstone. } No. 5. { { { Clinton Group. } { { Niagara Group. { { } } { { { Oneida Salt Group. } : { { { Water-lime Group. } : { { { Pentamerus Limestone. } No. 6. : { { { Delthyris Shaly } : { { { Limestone. } : S { { { Encrinal Limestone. } : Cliff { Wenlock Rocks. {He{ Upper Pentamerus } : { { { Limestone. : Limestone. { { { Oriskany Sandstone. } : { { { } No. 7. : { { { Cauda-Galli Grit. } : { { { Schoharie Grit. } : { { { Onondaga Limestone. Wanting. : { { { Corniferous Limestone. Wanting. } { } { { { Marcellus Shales. } } Black { { { Hamilton Group. } } Slate. { Upper and Lower { { } No. 8. { { { Tully Limestone. } { Ludlow Rocks and {Er{ } : { { Genesee Slate. D : the Devonian { { } : { { Portage Group. } No. 9. } Waver : System. { { } } { { {Chemung Group. } } Sandstone.

This name, _Silurian_, was first used to designate the lowest well-characterized fossiliferous rocks in England. But it is now used to embrace the whole system as it occurs elsewhere. It is well exhibited in New York, both in consequence of its great development there, and because the whole system is only slightly acted upon by disturbing forces, so that the outcropping edge of each division extends over a large surface.

This system is of great thickness, amounting, in places where it is well developed, to twenty thousand feet.

The Champlain division commences with a quartzose sandstone, passing gradually into limestone, which is succeeded by a very thick argillaceous deposit, the Utica slate and Hudson River group. The Ontario division in the lower part is a mass of sandstone. Above this is the Clinton group, consisting of shales and sandstones. The most important part of this group, in an economical point of view, is a fossiliferous, argillaceous iron ore, coextensive with the group in this country, and is worked to supply a large number of furnaces. The last of the division is the Niagara group, which commences with a mass of shale, and becoming at length calcareous, it terminates in a firm compact limestone. This limestone has withstood the action of denuding causes better than the shales either above or below it. It therefore presents a bold escarpment at its outcrop, and occasions waterfalls wherever streams of water cross it. The falls of Niagara are formed by this rock. The Niagara limestone, in its extension westward, becomes the lead-bearing rock of Missouri, Iowa and Wisconsin. The Helderberg division is a succession of highly fossiliferous limestones, with the intervention of only occasional beds of grits and shales. One member of the series is the Onondaga Salt group. The water obtained from this group in New York annually furnishes immense quantities of salt. The Erie division consists of a thick mass of shales and sandstones.

The fossils of this system are very numerous, but consist mostly of the lower forms of animal life. _Corals_ (Figs. 8 and 9) are abundant, and constitute in some places a large proportion of the limestones. The _Crinoidea_, or lily-shaped animals, consist of a jointed stem permanently attached, and bearing at the free extremity of the stem an expanded portion, which is the pelvis, or digestive cavity. The mouth is surrounded with a series of leaf-like tentacula, which serve the purpose of seizing and holding food. Fig. 10 represents the pelvis of one of the silurian fossils. The general character of the animal is better represented by Fig. 30. The most abundant fossils of this period are the lowest orders of _bivalve mollusca_ (Fig. 11). The _Cephalopoda_ are characterized by having the organs of locomotion attached to the head. The shell of several species is peculiar in being divided into distinct cells, or chambers (Fig. 12, _b d_), perforated by a tube (siphuncle _a_). These fossil shells are sometimes straight, as the _Orthoceras_ (Fig. 13), or curved, as shown in the several forms of Fig. 14. The _Trilobite_ was an articulated, crustaceous animal, having two lines along the back dividing it into three lobes, from which circumstance its name is derived. It is found in great numbers in the Silurian rocks (Fig. 15). In a few instances remains of fishes have been found, but they by no means characterize the system.

The geographical range of this system is probably greater than that of any system of rocks above it. It is found occupying a large part of the territory west of the Alleghany Mountains, from Canada, through New York, and the other states, to Alabama; and extending westward to and beyond the Mississippi river. It occupies a large district in the west of England, and is found in great force in the north and east of Europe.

2. _The Old Red Sandstone._--This formation consists almost entirely of a sandstone of a red color. It admits of division into three parts, though the characters vary in different places. The lowest is a thin-bedded argillaceous sandstone, consisting of finely levigated material, and easily splitting into thin sheets. From this circumstance it has received the name of _tilestone_. The middle portion is composed of nodules or concretions of limestone imbedded in a paste of red sand and shale. This has been called by English geologists, _cornstone_, and though very partially developed in some regions where the system is found, it is yet a very persistent member. The highest member of this formation is a mass of red sandstone, often passing into a coarse _conglomerate_. In England the thickness of the Old Red Sandstone is not less than ten thousand feet. In this country it is scarcely three thousand feet.

The fossils of this system are a few shells, a small number of vegetable species, and in particular localities the remains of fishes in great abundance. The system is characterized principally by fossils of this last kind. The fishes of this system have a cartilaginous skeleton, but are covered with plates of bone, which were faced externally with enamel. The jaws, which consisted of solid bone, were not covered with integument. The exterior bony covering seems to have been the true skeleton, as is, in part, the case with the tortoise. In some of the fishes of this period there is a wing-like expansion on each side of the neck, which has given them the name of _Pterychthis_ (Fig. 17). In others, as the _Cephalaspis_, the plate of bone on the back is so large as to cover nearly the whole body, and make it resemble a trilobite (Fig. 18).

This system has an extensive geographical range. In England, it occupies a band of several miles in width, extending from the Welsh border northward through Scotland to the Orkney Islands. In this country, it forms the Catskill Mountains, in New York, and extends south and west so as to underlie the coal-fields of Pennsylvania and Virginia.

3. _The Carboniferous System._--This system consists of three parts, distinguished by lithological and fossil characters.

The _carboniferous limestone_ is a dark-colored, compact limestone, forming the base of the system, and reposing on the old red sandstone. Its thickness is from six hundred to one thousand feet, often with scarcely any intermixture of other rock; but it sometimes loses its character of a limestone, and becomes a sandstone, or conglomerate. It generally contains the ores of lead in considerable quantity, and from this circumstance has been called _metalliferous limestone_. In England it is the principal repository of these ores. In the Western States it is the upper portion of the lead-bearing strata.

The fossils are marine, and very numerous. Corals and crinoidea are very abundant. The crinoidea, in some localities, form so large a part of the rock as to have given to it the name of encrinal limestone. The orthoceras and trilobite are found, but become extinct with this formation. Several species of bivalves, such as Delthyris and Leptæna, are also common.

Next above the limestone lies the sandstone, sometimes called _millstone grit_. It is generally drab-colored, but occasionally red. Its thickness is often equal to that of the limestone. Sometimes it is fine-grained and compact; but generally it is coarse-grained, and often passes into a conglomerate. It contains but few fossils, and those of vegetable origin.

The highest part of the system is the _coal measures_. They consist of beds of sandstone, limestone, shale, clay, ironstone and coal, occurring without much uniformity in their order of superposition. The coal measures have a thickness of about three thousand feet. The sandstones and limestones are not distinguishable from the sandstones and limestones in the lower part of the system. The ironstone either occurs in concretionary nodules, often formed around some organic nucleus, or it is an argillaceous ore, having a slaty structure. In either case, it consists of subordinate beds in the shale. The coal consists of several beds distributed through the measures. The beds vary in thickness from a few lines or inches to several feet. In a few cases beds have been found measuring fifty or sixty feet in thickness. The workable beds are ordinarily from three to six feet thick.

The carboniferous formation is very much disturbed by dikes, faults (Fig. 18; see also Fig. 50), and other dislocations. The amount of change of position in the strata, by faults, is very various; frequently but a few feet. In one case in England there is a fault of nearly a thousand feet. There is a case of dislocation in Belgium where the strata are bent into the form of the letter Z, so that a perpendicular shaft would cut through the same bed of coal several times.

The characters and order of superposition which have now been given may be regarded as the general type of the carboniferous formation. There are, however, several important modifications. 1. Beds of coal sometimes alternate with beds of millstone grit. Thus, in Scotland and in the north of England, this intermediate member of the system disappears, or, rather, is incorporated with the coal measures. The same is true, to considerable extent, in this country. 2. Sometimes the carboniferous limestone also disappears as a distinct member of the system, partly by becoming arenaceous, and partly by the intercalation of beds of coal. In this last case, the whole formation from the old to the new red sandstone becomes a series of coal measures. In this country the carboniferous limestone is found very generally to underlie the coal strata. 3. The fractures and faults, which were formerly supposed to be characteristic of the coal formation, are seldom found in the great coal-fields of this country, except in those of the anthracite coal of Pennsylvania; and even there they are much less common than in the coal-fields of Europe.

There are three principal varieties of coal, distinguished by the different proportions of bitumen which they contain. The common bituminous coal kindles readily, emits much smoke, and throws out so much liquid bitumen that the whole soon cakes into a solid mass. It contains about forty per cent, of bitumen. The second kind, or cannel coal, contains twenty per cent., and inflames easily, but does not agglutinate. The stone-coal, or anthracite, contains scarcely any bitumen, ignites with difficulty, emits but little smoke, and produces a very intense heat. The bituminous varieties are always found in the least disturbed portions of the coal districts; and the anthracite is found in the more broken and convulsed portions, where we may suppose that the subterranean heat has been sufficient to drive off the volatile bituminous part, and reduce it to the anthracite form. Hence the eastern Pennsylvania coal-fields, which lie near the principal axes of elevation of the Appalachian Mountains, furnish only anthracite; while the same coal-seams, in their extension to the western part of the state, are bituminous.

Where coal is quarried in large quantity, a shaft is sunk through the overlying strata to the coal-beds, and the coal is raised to 'the surface by steam power. After the coal has been quarried to some distance from the shaft, pillars of unquarried coal are left to support the overlying strata. Fatal accidents have sometimes occurred by the giving away of these supports. Over a large part of the coal-fields of the United States it has not yet become necessary to sink shafts. The quarrying is commenced at the outcrop of the coal-bed; and, till the cover becomes of considerable thickness, it has been found economical to "strip" off the overlying rock, rather than to work a subterranean gallery.

Brine-springs are often found in the coal measures of sufficient strength to be used in the manufacture of salt. This is now done to considerable extent in Ohio. In the valley of the Kenhawa river, Kentucky, the rocks of which belong to the carboniferous system, the brine is nearly saturated with salt; and in some of the borings they have even discovered beds of rock-salt of great thickness and purity.

There is no other part of the geological series so obviously connected with national prosperity as the coal formation. While a country is new, the forests furnish an abundant supply of fuel; but in the course of a few years these are consumed. This country will soon be principally dependent upon its coal-mines for fuel, even for domestic purposes; and, in carrying on the great branches of national industry, such as the smelting and working of iron, and in the formation of steam for the purposes of manufacture and transportation, we are already mainly dependent upon mineral coal. A nation which does not possess an abundant supply of this mineral, _or which does not use it_, cannot long maintain a high degree of national prosperity.

In these inexhaustible masses of coal, accumulated ages before the existence of the human race, is a most obvious prospective arrangement for securing our happiness and improvement. And this arrangement embraces not only the accumulation of a combustible material in such abundance, but also its juxtaposition with an equally inexhaustible accumulation of iron ore, and the limestone which is necessary as a flux in the reduction of the ore. So bulky and heavy materials as coal and iron ore could neither of them have been transported to any considerable distance for the manufacture of iron; and without the manufacture of iron on a large scale, the present operations in manufactures and transportation could never have been entered upon. A large proportion of the iron furnaces in this country, and nearly all of them in Great Britain, employ mineral coal for fuel, and obtain their ore from the beds contained in the coal measures.

The fossils of the coal measures are almost entirely of vegetable origin, and are very abundant. They are seldom found in the coal-beds, but in the strata of shale immediately above or below the solid coal.

The _Stigmaria_ (Fig. 19) is found most abundantly, and in a large proportion of cases to the exclusion of every other form, in the lower shales. It consisted of a large dome-shaped mass, often three or four feet in diameter, with trailing branches, or roots, spreading off horizontally to a distance of twenty feet. In a few instances tree ferns have been found, petrified in a horizontal position, and being apparently a mere continuation of the stigmaria. Hence the stigmaria has been supposed to be the base of the tall tree ferns, the leaves of which so abound in the upper shales. If this is not the case, there are no forms of the existing flora of the earth analogous to the stigmaria. It is always found in connection with the coal-beds of the carboniferous formation, and never with the coal-beds which sometimes occur in the later formations.

The tree ferns (Fig. 20) attained a height of fifty or sixty feet, and a diameter of four feet. They have received the name of _Sigillaria_ in consequence of the seal-like impressions (Fig. 21) with which the surface is covered, and which are the scars left where the fronds have fallen off. These fronds (fern leaves) are the most abundant fossil of the series. They are distinguished by some peculiarity in form, as the Sphenopteris (wedge-shaped fern leaf), Pachypteris (thick fern leaf), &c. (Figs. 22 and 23.)

There was another kind of Sigillaria (Fig. 24), in which the surface was fluted, and the markings are superficial, and occur on the ridges. It reached as great a size as the tree ferns, but to what general class of plants it belonged is still doubtful.

The _Lepidodendron_ (scale-covered tree) (Fig. 25) is the fossil which most nearly resembled in general appearance our present forest trees. Specimens are found four feet in diameter and seventy feet in height. In botanical characters it resembled, in some respects, the trailing club-mosses, while in others it was very similar to the Norfolk Island pine.

The _Calamite_ (Fig. 26) was a plant resembling, in its jointed and striated surface, the equisetum (rush), but was sometimes twelve inches in diameter.

The carboniferous formation exists more or less abundantly in all the great divisions of the earth. It occurs in nearly all of the countries of Europe. The largest deposits known are, however, in the United States; especially in the States of Pennsylvania and Virginia, and in Ohio.

4. _The New Red Sandstone._--The lower division of this formation, called the Permian system, consists of a thick mass of sandstones, generally of a red color, with occasional alternations of argillaceous rock, succeeded by a series of magnesian limestones. The upper division, or Triassic system, is composed of a red conglomerate, a limestone which has received the name of Muschelkalk (shelly limestone), and a series of variegated marls and sandstones.

The ores of copper are found, to considerable extent, in this formation. The rich copper mines of Germany are in the magnesian limestone, or, as it is there called, Zechstein (minestone). The Lake Superior copper mines occur in a red sandstone formation, which will probably be found to belong to this system.

The salt-beds, salt springs, and beds of gypsum, are so, generally found in this rock in England, that it has been called by the English geologists the "saliferous system." It is, however, found that in other countries these minerals occur in equal abundance in formations of an earlier and later date.

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.

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 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.