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 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.

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.