SUCCESSION OF STRATA.

191

of slate rocks, 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 CAMBRIAN, 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.

Destitute of Organic Remains.

STRATIFIED.

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.

UNSTRATIFIED.

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-ærial 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.

Cambrian,.

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 epochs 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.

66

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.

CHRONOLOGICAL ARRANGEMENT OF STRATA.

195

EXPLANATION.

Ideal section of the crust of the earth, showing the chronological arrangement of the strata.

M-Mantell.
B-Buckland.

L-Lyell.

1. Alluvial or modern deposits. a. 2. do. do. Overlaying the coal 2. Tertiary formations. formation.

3. Cretaceous system, compris- b. 1. Granite veins in Granite. ing the chalk, with & with- b. 2. do. do. passing through porout flints, chalk marl, galt or blue clay, Shanklin sand. 4. The Wealden.

5. The Oolite.

6. The Lias.

7. The Saliferous, consisting of
New Red Sandstone, Mag-
nesian Limestone.

8. The Carboniferous System,
namely, the Coal measures,
the Mountain limestone.
9. The Devonian, or Old Red
Sandstone.

10. The Silurian, consisting of

marine limestones, shales,
calcareous flags.

11. The Cambrian.

12. Mica Schist.

13. The Gneiss.

14. Granitic System. 15. Volcanic Rock.

a. 1. Dyke of Porphyry in gran

ite, passing through & over-
laying gneiss.

phyry, gneiss, & mica schist. b. 3. do. do. Overlaying Grau

wacke.