GEOLOGY
802
GEOLOGY
Section of the Earth's Crust, showing the Relative Position of its Strata. N.B.— The above distribution of the successive layers consist merely of the titles and systems adopted by different authors. Rome of the strata are found onlv in certain localities of limited extent, and there are modifications peculiar to other places. The " rocks" (a general term in geology for all strata, of "whatever character) are represented in the cut as tilted up by an adjacent mountain. Several such changes of level, and not seldom even by drpression, are sometimes exhibited as having successively taken lilace. The layers are frequently corrugated or otherwise disturbed, either singly or throughout the whole or various parts of the series. Any one of tliem may appear at the surface, in consequence of derangement or tlie absence of those properly belonging above it. Seldom or never are they all found together as here. They exist in endless variety of combination, both chemical and mechanical, and are sometimes thrown into almost promiscuous confusion by volcanic and other causes; but when present and in their natural site, they observe the arrangement indicated on the diagram.
feartilaginous, and clad in strong integuments of bone composed of enamelled plates, instead of the horny scales which form the covering of the fish of the pres- ent day ; and it lias been suggested by Dr. Buckland that this hard coating may have formed a defence against the injurious effects of water of a liigli temper- ature. Tlie first traces of land vegetation liavc lieen found at the top of the Silurian, where the Old Red Sandstone rests on it." "The fossil remains of a small reptile, which is stated to have been found in a roclt at the top of the Old Red Sandstone, have been supposed to be the first traces of terrestrial life upon the globe ; but professor Owen is of opinion that tlie rock in question does not belong to the Old Red Sand- stone formation, but to another long subsequent — the Trias.
"The sj'steni that succeeded the Devonian is tlie Carboniferous, which is one of importance and interest to mankind, as having been the perio'd of the forma- tion of coal, iron, and the mountain limestone — a com-
bination of products that have contributed so largely in these latter days to the comfort and convenience of the human race. The coal-measures, it is well as- certained, are the product of profuse and extensive vegetation, and the nature of tlie phmts of which it lias been formed is easily discoverable by a close ex- amination of the mineral itself, which, on inspection, discloses them to have been almost entirely of the cryptogamic order, and such as would be produced in abundance in positions of shade, heaf, and hnnikUty, Ferns, calamites, and esquisitaceous plants preponder- ate, and wood of hard and ligneous tissue, which is, in a great measure, dependant on the unshaded light of tlie sunbeam, is of rare occurrence in tliis formation, while season rings, which result from the impact of tlic direct rays of sunlight on the tree, are not found at all ill the fossil woods of this or the previous forma- tion, though they appear in those of the succeeding systems."
"In confirmation of these views, it is remarkable
GEOLOGY
803
GEOLOGY
that other geological phenomena, besides that of the absence of the season rings in the trees, indicate that there was no variation of seasons on our earth before the close of the carboniferous sera. Temperature ap- pears, up to that period, to have been tropical and uni- form in all latitudes; for the fossil remains testify that the animals and plants that lived and grew in the carboniferous and precedin.; nei as at the equator were cf the same species as those that lived and grew at the same period in the arctic region? — and the coal-meas- ures arc as abundant in the high latitudes as in the temperate and tropical zones. These phenomena can only be accounted for by the continued prevalence of the central heat, and the consequent neutralization of the effect of the sun's rays, the influence of which now operates to produce the variety of seasons. The cli- matal condition of the earth in those ages must have been similar to those of a vast humid hothouse shaded from the direct radiance of the sun, and which would be eminentl}' conducive to the production of a prolific vegetation, such as that which has been stored up in our extensive coal-measures.
" The zoology of this asra furnishes us with the first undoubted traces of terrestrial animal life, in the form of insects of the beetle and cockroach tribes, scorpions, and reptiles of the batrachi m order — creatures which were adapted bj' nature to live in the dull, hazy, tepid atmosphere that overspread our planet at this time.
"At the close of the carboniferous iEra another com- menced, during wliicli the system of rocks, which has been denominated 'the Permian' system, was formed, the fossil remains of which indicate that great changes must have taken place in the phj-sical constitution and aspect of the earth. The exuberant vegetation which had supplied the material of the coal-measures of the preceding formation had died away, and a vegetation of a liigher order succeeded." "The animals, too, which inhabited the Permian earth disclose an advance in organic life. The Saurian, or true reptile, here made its first appearance ; and the earliest traces of birds present themselves in the New Ked Sandstone, a member of this sj'stem. The foot-tracks of these birds, of immense magnitude, which stalked on the Permian sands and mud, are found impressed on the now hardened slabs of sandstone and shales of that formation both in Scotland and in America.