Page:A Treatise on Geology, volume 2.djvu/55

 CHAP. VI.

"So similar are the generic types of these mollusca to those of tertiary freshwater strata and those now existing, that had we only such fossils before us and no evidence of the infra position of the rocks in which they are found, we should be wholly unable to assign them a definite geological epoch." In the lapse of time during the deposition of these Purbeck strata, there was no great physical disturbance there, nor were the sediments much varied in mineral character, nor were the generic forms changed, and these forms are yet continued in other species which are in existence at the present day in the same physical region. The scale of lacustrine life, if formed on the mollusca, would not be marked by generic steps, as the contemporaneous scale of marine life is. Perhaps we may admit a similar result in the case of aquatic and land insecta, as compared with marine Crustacea.

The freshwater sediments of the Paris basin, studied in connection with those of Auvergne, Velay, and Cantal, offer a very complete view of the eocene lake deposits, and lead to the conclusion that the marine and freshwater strata of that basin are to be considered as marking sometimes the independent action of the sea and land floods in one basin, and sometimes their periodical alternation; the land floods always coming from the south, and the marine sediments from the north or west.

The gypseous deposit of the Paris basin is a repository of many extinct species of quadrupeds, while of birds 10 species, and several fishes and reptiles, also extinct, remain to augment the value, and complete the evidence presented by these precious relics. Four fifths of the quadrupeds belong to the division of pachydermata; and nearly all the species are such as might be supposed