Page:A Treatise on Geology, volume 2.djvu/54

 40 chapter xi. of the Principles of Geology), for supracretaceous eocene, and by uniting the meiocene and older pleiocene periods. Upon this basis it appears worth while to inquire how far the shells found in lacustrine sediments support the inferences of the change of organic life, since the age of the chalk, which have been drawn from marine remains and bones of terrestrial quadrupeds, though there is reason to regret the neglect which this important subject of research has experienced. Contemporaneous with the marsupials of Stonesfield, and with the extinct dinosaurians of Sussex and Yorkshire, we have freshwater shells in the oolitic coal series of Whitby (Unionidæ) and others of like affinities in the Wealden beds. A valuable addition to our knowledge of the lacustrine deposits of Purbeck has lately been given by Professor E. Forbes. These truly lacustrine beds rest without gradation on the truly marine beds of the Portland oolite. These lowest freshwater beds contain modern genera, viz., cyprides, valvata, limnæa; above them are the well-known dirt beds with the bases of cycadeæ in situ; above the dirt beds are cypridiferous shales, covered by a varied series deposited in brackish water, and containing rissoæ and protocardia, and serpulites. Over these come again purely freshwater beds marked by cypris, valvata, and limnæa. Then a thin marine band,—followed by another group of freshwater beds with cypris, valvata, paludina, planorbis, limnæa, physa, cyclas, all different from their conveners in the beds below. With them are some vesicles of chara (gyrogonites). Marine beds cover these, and are followed by beds of freshwater and brackish origin, with the same cyprides as below, some fishes, &c. Again marine beds and brackish beds, and a third series of freshwater strata with a new series of fossils, cyprides, paludina, physa, limnæa, planorbis, valvata, cyclas, unio,—all modern genera. Marine strata come on above.