VERTEBRATE LIFE IN AMERICA. 673 



invertebrate fossils have thrown little light on the question, which is 

 essentially whether the great Lignite series of the West is uppermost 

 Cretaceous or lowest Eocene. The evidence of the numerous verte- 

 brate remains is, in my judgment, decisive, and in favor of the former 

 view. 



This brings up an important point in paleontology, one to which 

 my attention was drawn several years since, namely : the compara- 

 tive 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 subdivisfons 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 ex- 

 ample, are but slightly affected by changes that would destroy rep- 

 tiles or birds, and the higher mammals succumb under influences that 

 the lower forms pass through in safety. The more special applica- 

 tions 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 mam- 

 mals, henceforth the dominant type. 1 



The Tertiary of Western America comprises the most extensive 

 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 equiva- 

 lents 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 rep- 

 resentative 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 some- 

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

 if, as was probable, the main migrations took place from this conti- 

 nent. It is better at once to recognize this principle, rather than at- 

 tempt to bring into exact parallelism formations that were not strictly 

 contemporaneous. 



The fresh-water Eocene deposits of our Western Territories, which 

 are in the same region at least two miles in vertical thickness, may 

 be separated into three distinct subdivisions. The lowest of these, rest- 

 ing unconformably on the Cretaceous, has been termed the Vermilion 

 Creek, or Wahsatch, group. It contains a well-marked mammalian 



1 See Frontispiece Section, March number. 

 vol. xii. 43 



