OCR Text |
Show llG NEWER PLIOCENE PERIOD· [Ch. IX. of the land beinD' often carried across the ocean, and the aquatic b tribes over great continental spaces*. It is obviously expedient that the terrestrial and fluviatile species should not only be fitted for the rivers, valleys, plains, and mountains which exist at the era of their creation, but for others that are destined to be formed before the species shall become extinct; and, in like manner, the marine species are not only made for the deep or shallow regions of the ocean at the time when they are called into being, but for tracts that may be submerged or variously altered in depth during the time that is allotted for their continuance on the globe. Recapitulation.-We . may now briefly recapitulate some of the most striking results which we have deduced fwm our investigation of a single district where the newer Pliocene strata are largely developed. In the first place, we have seen that a stratified mass of solid limestone, attaining sometimes a thickness of eight hundred feet and upwards, has been gradually deposited at the bottom of the sea, the imbedded fossil shells and corallines being almost all of recent species. Yet these fossils are frequently in the state of mere casts, so that in appearance they correspond very closely to organic remains found in limestones of very ancient date. 2dly. In some localities the limestone above-mentioned alternates with volcanic rocks such as have been formed by submarine eruptions, recurring again and again at distant inter-vals of time. 3dly. Argi11aceous and sandy deposits have also been pro-duced during the same period, and their accumulation bas also been accompanied by submarine eruptions. Masses of mixed sedimentary and igneous origin, at least two thousand feet in thickness, can thus be shown to have accumulated since the sea was peopled with the greater number of the aquatic species now living. 4thly. These masses of submarine ongm have, since theiL· formation, been raised to the height of two thousand or three ·t. See vol. ii., chapters v., vi., and vii. Ch.JX.] RECAPITULATION, 117 th.o usand feet above the level of the sea , and tl li·S e1 e vat·w n ·I m-phes an extraordinary modification in the state of the earth's crust at s~me unknown depth beneath the tract so upheaved. 5t~ly. fhe m.ost probable hypothesis in regard to the nature of th1s change, Is the successive generation and forcible intrusion into the inferior parts of the earth's crust of lava which, after c~oling down, may have assumed the form of crystalline unstratified rock, such as is frequently exhibited in those mountainous parts of the globe where the greatest alterations of level have taken place. 6thly. Great inequalities must have been caused on the surface of. the new-raised lands during the emergence of the n.ewer Pliocene strata, by the action of tides, currents, and r1vers, combined with the disturbing and dislocating force of the elevatory movements. 7th_)~· There are no features in the forms of the valleys and s~a-chfts thus recently produced, which indicate the sudden rise of ~he strata .to the whole or the greater part of their present altitude, while there are some proofs of distinct elevations at successive periods. Sthly. 'Ve may infer that the species of teJTestrial and fl.u via. tile animals and plants which now 1·nhabI' t ex t ens1. ve distncts, formed durin(Jb' . the newer Pliocerle era , were 1.1 1 exl. st-enc~ not o~ly before the new strata were raised, but before their materials were brought together at the bottom of the sea. |