« PreviousContinue »
8. THE CARBONIFERous, or CoAL SystEM, which is formed of sandstones, grits, shales, layers of ironstone, and clay; with immense beds of coal; fresh water limestone sparingly, and marine limestone abundantly. This system is characterized by innumerable remains of land and aquatic plants, of a tropical character, and belonging to extinct species and genera, with fishes, reptiles, and insects. The series includes the coal measures, which are sandstone, and shale, with numerous layers of coal, containing land plants in profusion. Limestones, with fresh water and marine shells, Millstone grit, which consists of sandstone and shale, with thin seams of coal, and quartose conglomerates sometimes used for millstones. The carboniferous or mountain limestone, consisting of limestone and flagstone, abounding in crinoidea, and marine shells, yielding several varieties of black, blueish grey, and variegated marbles. The coal bearing strata of this country differ some from the European. The seams of coal appear however, even in Europe, to be very unequally distributed; although the great coal formation belongs in the order where we have placed it, yet seams of anthracite coal are found in almost every rock from the lias, to the upper metamorphic rocks, showing that the coal beds have occurred at very unequal intervals, hence their formation may be of any date between the new and the old red sandstone. 9. THE DEvoNIAN, or OLD RED Sandstone System.—This name is derived from the English locality, where it is most largely developed, viz: Devonshire. It is a marine deposit, chiefly remarkable for its extraordinary forms of fossil fish. This system is composed of various strata, flagstones, conglomerates, quartose grits, sandstones, marls, and limestones; the prevailing color of all these is a dark red. But few fossils are found in the sandstone and conglomerates, but in the marls, and concretionary limestones, sometimes called corn-stones, peculiar genera of fish, and many species of marine shells are found. This system lies immediately below the mountain limestone. The sandstones are in various stages of induration, and when slaty are employed for roofing. The red color is derived from peroxide of iron. The formation of these rocks, has manifestly resulted from the waste
SUCCESSION OF STRATA. 191
of slate recks, their detritus being cemented together by red sand, or marl, into coarse conglomerates.
Primary Fossiliferous Period.
10. THE SILURIAN SystEM.—This name is derived from silures, the name of the ancient Britons who inhabited that part of England where it is most developed, viz: the border counties of England and Wales, and south Wales. It is a marine deposit of vast extent and importance, containing a great abundance of organic remains. It is principally composed of marine limestone, shales, sandstones, and calcareous flags, abounding in shells, corals, trilobites, and crinoidea of peculiar types; but few vegetable remains are found below the old red sandstone.
11. THE CAMERIAN, or GRAUwacKE SystEM.—Grauwacke is a coarse slaty rock, containing fragments of other rocks, sometimes passing into the common clay slate, and sometimes, when the fragments are very numerous and small, into sandstones and grits; it contains a few shells and corals, and occasionally impressions of fuci; with this system all traces of organic remains disappear. The fineness of grain, general aspect, and character of these rocks, are well known from the universal employment of slate for economic purposes.
II. HYPOGENE OR METAMORPHIC ROCKS.
12. THE Mica Schist.—This formation is supposed from certain traces of stratification, to have been sedimentary in its origin. but subsequently altered by the influence of heat. It consists of mica slate, granite rock, crystaline limestone, or white marble, and hornblende schist, exhibiting no traces of organic remains.
13. The GNEiss SystEM.—Layers of gneiss, sienite, and quartz rock, alternating with clay slate, and mica schist, but still exhibiting marks of former stratification.
14. GRANITIC System—Consisting of porphyry, serpentine,
and trap rock in shapeless masses, and in dykes and veins.
15. Volcanic Rocks.—These are the products of fire, or subterraneous heat, ejected from beneath the surface, through fissures in the earth’s crust, both in ancient and modern times. The erupted materials of the ancient volcanoes being trap, basalt, toadstone and tuff, and the products of recent sub-aerial volcanoes, lava, scoriae, pumice and ashes.
The general proportionate thickness of each of these several deposits has been estimated as under, but the statement must be regarded as a mere approximation.
Tertiary System, ... . . . . . . . . . . . . .2,000 feet.
Carboniferous, ... . . . . . . . . . . . . . . 10,000 “ Old Red Sandstone, ... . . . . . . ... 10,000 “ Silurian, . . . . . . . . . . . . . . . . . . . . . . .7,500- “ Cambrian, ... . . . . . . ............ 30,000 “ Mica Schist, and Gneiss, not ascertained, but far exceeding that of any of the superposed deposits. We have now given a connected view of the order of succession of the several strata, each characterised by its peculiar animals and plants. All these are marine deposits except one, the fourth, called the Wealden. This is a fresh water formation, and is the deposit of a mighty ancient river, or of several of them, and its organic remains are such as might be expected to result from the sediment of such a river, consisting of plants, shells, fish, and reptiles, imbedded in the mud together. It is almost the only evidence which remains of the ancient land, showing that while the immense deposits were going on in the bed of the ocean, here were bodies of fresh water, rolling over a vast extent of land, bearing upon their waters the remains of trees, and huge reptiles. In the following chapters we shall consider each of these formations more fully, and describe more particularly some of the fossils found in them. It will be observed by the careful reader, that most of the marine deposits of the several opochs have the same mineralogical character: if we except the coal, we will find the rest alternating with marls, clays, limestones, and sandstones; each being formed from the ruins of
GEological. NoMENcLATURE. 193
more ancient formations, and each, imbedding in its sediment the characteristic shells, fishes, reptiles, and plants, which were either washed into, or once lived in the ancient sea, of which it formed the bed. - The names which have been given to the different geological formations must be received with some caution, for they are not always indicative of formations identical with those from which the name was derived. Many of these names are borrowed from places; thus, we read of the Jura limestone, the Kimmeredge clay, Oxford clay, Purbeck marble, Portland rock, and Potsdam sandstone. These names, referring to the stratum of a known locality, were good so far as an identity with that stratum can be traced, but from the nature of the case, this is often incompletely done, and hence the names necessarily cease to be definite. Many of the English provincial names are still retained, though very uncouth and harsh sounding, thus Geologists often employ the terms Cornbrash, Lias, Gault, Coral Rag, and many others which have no systematic signification. Descriptive names applied in Geology are also defective, and when employed, no scrupulous regard must be had to their appropriateness. “The Green Sand may be white, brown, or red; the Mountain Limestone may occur only in valleys; the Oolite may have no roe-like structure; and yet these may be excellent geological names, if they be applied to formations, geologically identical with those which the phrases originally designated.” The term Oolite is an instance where a descriptive word has become permanent, and in like manner the term proposed by Mr. Murchison, for the transition series of rocks, which, from being distinctly.marked in South Wales, he calls Silurian, from the name of the ancient inhabitants, is in many respects excellent. The terms employed by Mr. Lyell, before mentioned, as divisions of the Tertiary formation, viz: Pliocene, Miocene, and Eocene, according to the percentage of recent shells, being founded upon a more natural distinction will undoubtedly come into general use, but even these are to be used with caution, and not allowed to set aside the indications drawn from the natural relations of the strata.