OF WESTERN NORTH AMERICA. 



31 



geologic stages, but which have not been discovered, owing to the 

 accidents of deposition or to occurrence remote from centers of 

 deposition. We are too apt to assume the absence of a mammal from 

 the entire continent because it has not been found in what was form- 

 erly a very restricted region. Many forms previously considered 

 absent from the American Eocene have very recently been dis- 

 covered; for example, the animals related to the armadillos or 

 Dasypodidse, found in the Bridger. 



Second, the different contemporary conditions of environment and 

 habitat in different parts of the Mountain Region a^id of the Great 

 Plains Region; that is, local differences of habitat, differences of 

 longitude or of east and west distribution, differences of latitude 

 or of north and south distribution, differences of altitude or vertical 

 distribution — in short, such differences as exist to-day — render correla- 

 tion somewhat uncertain. Thus, the arboreal primates are very 

 common in the Eocene Mountain deposits, but no trace of them is 

 found in the Oligocene Plains deposits. 



PRELIMINARY CORRELATION OF THE EOCENE AND 

 OLIGOCENE MOUNTAIN DEPOSITS. 



Correlation of the Mountain Region basins of the Eocene is of 

 necessity almost exclusively paleontologic owing to the uniform 

 repetition of similar geologic and petrographic conditions in the 

 successive basins. 



These deposits, as above noted, fortunately overlapped each other 

 in time — that is, before one ceased, its more or less distant neighbor 

 began. The writer and others are now collecting exact data for 

 estimating these overlaps. Fig. 1 presents the writer's preliminary 

 correlation of the Eocene and Oligocene deposits, based on personal 

 studies, with the able cooperation of Matthew, Granger, Sinclair, 

 Loomis, and others. 



The chief omissions, which will soon be supplied, are (1) the Fort 

 Union section of south-central Montana (Douglass, 1902), with its 

 fauna equivalent to the Torrejon; (2) the typical Wasatch (Cory- 

 l)liodon zone) section, Evanston, Wyo. (Veatch, 1907); and (3) the 

 typical Wind River section (Loomis) . 



PRELIMINARY CORRELATION OF THE OLIGOCENE TO 

 LOWER PLEISTOCENE MOUNTAIN AND PLAINS DE- 

 POSITS. 



The geologic and paleontologic correlation of the Great Plains 

 deposits above the lower Miocene is even more difficult owing (1) 

 to the irregular nature of these deposits and (2) to our present lack 

 of exact analysis of the mammalian fauna. 



The accompanying preliminary correlation (fig. 10), based on the 

 researches of Hayden, Leidy, Cope, wScott, Wortman, Merriam, 



