THE AGE OF THE LIGNITIC FORMATION BY ITS FAUNA. 23 



sand by the waves. They there constitute the lower member of those heavy 

 beds of sandstone, which have been remarked already as forming the base of 

 the Lignitic, and which have in their remains, and also in their compounds, 

 the same characters as the lower flaggy or shaly sandstone overlying Creta- 

 ceous No. 4. No trace of animal Cretaceous remains has been found in 

 connection witli them, neither in the south nor in the north Lignitic field. 

 When Cretaceous No. 5 is tiot formed, the base of the Lignitic rests on No. 4. 



In 1857, an account of the Tertiary of Nebraska was published by 

 Messrs. Meek and Hayden, and the same year a map of the region bordering 

 the Missouri River, together with sections and explanatory notes illustrating 

 the geological structure of the country, was prepared by Dr. Hayden. The 

 authors had then collected a great quantity of specimens of animal and veg- 

 etable remains from the base to the summit of the Lignitic group. Speaking 

 of the animal remains which were studied by Dr. Leidy, of the vegetable 

 fossils by Dr. Newberry and myself, and of the invertebrate by himself and 

 Dr. Meek, Dr. Hayden says,* "None of us doubted even their Tertiary age^'; 

 an assertion forcibly resulting from the determination of the materials col- 

 lected in the exploration. For, considering merely the Mollusk, we see in the 

 volume of the Proceedings quoted above that of one hundred and fifty species 

 of MoUusks described from the Fort Union group, fifty-four are of the Ter- 

 tiary age, fifty strictly fresh-water species, and only four belong to genera 

 supposed to inhabit salt or brackish waters. The more prominent genera to 

 which these MoUusks are referred are Ostrea, Unio, Pisidium, Corbicula, 

 Potamomya, Melania, Melampus, Vivipa?-a, etc., all of Tertiary types. 



The section of the Tertiary measures as recognized by Dr. Hayden finds 

 its place here as elucidation of the distribution of the essential groups of ani- 

 mal fossils which characterize the formation. The Tertiary divisions indi- 

 cated by the distribution of fossil plants may present some differences. The 

 subject has to be examined after the descriptive part of the vegetable remains. 

 The section is cojiied from Dr. F. V. Hayden, Annual Report, 1874, p. 23: — 



* Proceedings of the Academy of Natural Sciences of Philadelphia, May, 1857. 



