OCR Text |
Show • 44 WIDE RANGE OF MARINI<; TESTACEA. [Ch.IV. contemporaneous origin of formations in distant countries. As the same species of organic beings usually extend over wider areas than deposits of a homogeneous composition, they are more valuable in geological classification than mineral peculiarities; but it fortunately happens, that where the one criterion fails, we can often avail ourselves of the other. Thus, for example, sedimentary strata are as likely to preserve the same colour and composition in a part of the ocean reaching from the borders of the tropics to the temperate zone, as in any other quarter of the globe; but in such spaces the variation of species is always most considerable. In regard to the habitations of species, the marine tribes are of more importance than the terrestrial, not only because they are liable to be fossilized in subaqueous deposits in the greatest abundance, but because they ha~e, for the most part, a wider geographical range. Sometimes, however, it may happen, as we have shown, that the remains of species of some one province of terrestrial ·plants and animals may be carried down into two seas inhabited by distinct marine species; and here again we have an illustration of the principle, that when one means of identification fails, another is often at hand to assist us. In conclusion, we may observe, that in endeavouring to prove the contemporaneous origin of strata in remote countries by organic remains, we must form our conclusions from a great number of species, since a single species may be enabled to survive vicissitudes in the earth's surface, whereby thousands of others are exterminated. When a change of climate takes place, some may migrate and become denizens of other latitudes, and so abound there, as to characterize strata of a subsequent era. In the last volume we have stated our reasons for in~ ferring that such migrations are never sufficiently general to interfere seriously with geological conclusions, provided we do not found our theories on the occurrence of a small number of fossil species. ( 45 ) CHAPTER V. Classification of tertiary formations in chronological order-Comparative value of diff~:rent classes of organic remains-Fossil remains of testacca the most important- Necessity of accurately determining species-Tables of shells by M. Deshayes-Four subdivisions of the Tertiary epoch-Recent formatiousNew~~ r Pliocene period-OlderPlioceue period-Miocene period-Eocene period -The distinct zoological characters of these periods may not imply sudden chauges in the animate c1·eation-The recent strata form a common point of departure in distant regions-Numerical proportion of recent species of shells in different tertiary periods-Mammiferous remains of the successive tertiary eras-Synoptical Table of Recent and Tertiary formations. CLASSIFICATION OF TERTIARY FORMATIONS .IN CHRONOLOGICAL ORDER. WE explained in the last chapter the principles on which the relative ages of different formations may be ascertained, and we found the character to be chiefly derivable from superposition, mineral structure, and organic remains. It is by combining the evidence deducible from all these sources, that we determine the chronological succession of distinct formations, and this principle is well illustrated by the investigation of those European tertiary strata to the discovery of which we have already alluded. It will be seen, that in proportion as we have extended our inquiries over a larger area, it has become necessary to intercalate new groups of an age intermediate between those first exa~ mined, and we have every reason to expect that, as the science advances, new links in the chain will be supplied, and that the passage from one period to another will become less abrupt. We may even hope, without travelling to distant regions,without even transgressing the limits of western Europe, to render the series far more complete. 'l,he fossil shells, for example, of many of the Subalpine formations, on the northern limits of the plain of the Po, have not yet been carefully col- |