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Lignite series of the West is uppermost Cretaceous, or lowest 

 Eocene. The evidence of the numerous vertebrate remains is, 

 in my judgment, decisive, and in favor of the former view. 



This brings up an important point in Palaeontology, on6 to 

 which my attention was drawn several years since, namely : 

 the comparative value of different groups of fossils in marking 

 geological time. In examining the subject with some care, I 

 found that, for this purpose, plants, as their nature indicates, 

 are most unsatisfactory witnesses; that invertebrate animals 

 are much better ; and that vertebrates afford the most reliable 

 evidence of climatic and other geological changes. The sub- 

 divisions of the latter group, moreover, and in fact all forms 

 of animal life, are of value in this respect, mainly according 

 to the perfection of their organization, or zoological rank. 

 Fishes, for example, are but slightly affected by changes that 

 would destroy Reptiles or Birds, and the higher Mammals 

 succumb under influences that the lower forms pass through 

 in safety. The more special applications of this general law, 

 and its value in geology, will readily suggest themselves. 



The evidence offered by fossil remains is, in the light of this 

 law, conclusive, that the line, if line there be, separating our 

 Cretaceous from the Tertiary, must at present be drawn where 

 the Dinosaurs and other Mesozoic vertebrates disappear, and 

 are replaced by the Mammals, henceforth the dominant type. 



The Tertiary of Western America comprises the most exten- 

 sive series of deposits of this age known to geologists, and 

 important breaks in both the rocks and the fossils separate it 

 into three well-marked divisions. These natural divisions are 

 not the exact equivalents of the Eocene, Miocene, and Pliocene 

 of Europe, although usually so considered, and known by the 

 same names ; but, in general, the fauna of each appears to be 

 older than that of its corresponding representative in the other 

 hemisphere; an important fact, not hitherto recognized. This 

 partial resemblance of our extinct faunas to others in regions 

 widely separated, where the formations are doubtless somewhat 

 different in geological age, is precisely what we might expect, 



