The Elements of Geology; Adapted to the Use of Schools and Colleges Part 4

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.

[Ill.u.s.tration: Fig. 27.]

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 _h.o.m.ocercal_ begin to appear in the Tria.s.sic portion of this system, and are found in all the subsequent formations. The remains of saurians also occur in this formation.

[Ill.u.s.tration: Fig. 28.]

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 Hitchc.o.c.k 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.

[Ill.u.s.tration: Fig. 29.]

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 Tria.s.sic 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 Oolitic System._--The lower portion of this system is the Lias, and consists of a series of fissile, argillaceous limestone, marl, and clays. The _Oolite_ forms the intermediate member of the system, and consists of alternations of clay, arenaceous rock and limestone. Some of the limestones have an oolitic structure, and the whole system takes its name from this circ.u.mstance, though this structure is not found in all parts of it, and is often found in other formations. The central part of the oolite, the coral rag, is princ.i.p.ally a ma.s.s of corals and comminuted sh.e.l.ls. The _Wealden_, the highest member of the oolitic 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 oolite, 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.

[Ill.u.s.tration: Fig. 30.]

[Ill.u.s.tration: Fig. 31.]

[Ill.u.s.tration: Fig. 32.]

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 oolitic period. Its length could not have been less than thirty or forty feet.

[Ill.u.s.tration: Fig. 33.]

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 vertebrae exceeding in number those of the swan.

[Ill.u.s.tration: Fig. 34.]

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 oolitic 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 oolitic 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.

[Ill.u.s.tration: Fig. 35.]

[Ill.u.s.tration: Fig. 36.]

The fossils of the cretaceous formation are very different from those of the oolite, and are such as to show that it was deposited in deep seas.

_Microscopic sh.e.l.ls_ are often so abundant as to const.i.tute a large proportion of the ma.s.s. _Zoophytes_ 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-sh.e.l.ls 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 princ.i.p.al 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 sh.e.l.ls 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] _Eos_, 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 acc.u.mulations of earthy substances of different degrees of fineness, but characterized by containing ma.s.ses 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 pa.s.sage 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 coa.r.s.e 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 oolite 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 sh.e.l.l, is so abundant as to const.i.tute 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.

[Ill.u.s.tration: Fig. 37.]

[Ill.u.s.tration: Fig. 38.]

[Ill.u.s.tration: Fig. 39.]

[Ill.u.s.tration: Fig. 40.]

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.

[Ill.u.s.tration: Fig. 41.]

[Ill.u.s.tration: Fig. 42.]

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.