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15. Volcanic Rocks.—These are the products of fire, or sub terraneous 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.
Tertiary System, ........ .2,000 feet.
....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 epochs have the same mineralogical character: if we except the coal, we will find the rest alternating with marls, clays, limestoner, and sandstones; each being formed from the ruins of
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 incompletels 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.
chronological, Arrangement of strat A.
ldeal section of the crust of the earth, showing the chronological arrangement of the strata.
L—Lyell. 1. Alluvial or modern deposits. a. 2. do. do. Overlaying the coal
2. Tertiary formations.
formation. 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 playry, gneiss, & mica schist. or blue clay, Shanklin sand. b. 3. do. do. Overlaying Grau
4. The Wealden. wacke. 5. The Oolite. c. Dyke of Trap, passing thro' 6. The Lias. grauwacke and the lamina7. The Saliferous, consisting of tions of marine limestones, New Red Sandstone, Mag- forming basaltic columns. nesian Limestone. c. 1. do. do. intersecting, and 8. The Carboniferous System, overlaying an older dyke of namely, the Coal measures, porphyry. the Mountain limestone. c. 2. do. do. overlaying the oolite 9. The Devonian, or Old Red system. Sandstone. c. 3. do. do. overlaying the chalk 10. The Silurian, consisting of being lava of the extinct marine limestones, shales, volcanoes of the Tertiary calcareous flags. period. 11. The Cambrian. d. Modern lava. 12. Mica Schist. e. Cave in magnesian limestone. 13. The Gneiss. f. Metallic veins in granite. 14. Granitic System. g. Extinct volcano. 15. Volcanic Rock. h. Artesian well.
a. 1. Dyke of Porphyry in gran
ite, passing through & over-
The order of succession which we have given has been determined by a series of the most patient investigations, and from an immense accumulation of facts, collected by able observers in all parts of the globe. On page 194 we have given a diagram showing their order. It will be perceived that some parts of the representation are necessarily exaggerated. Commencing at the left we have the hypogene, or underlying rocks, unstratified and stratified; then the primary and secondary fossiliferous, and also the tertiary, lastly the volcanic. The alluvium we have placed in various situations, overlaying the older rocks,