CHAPTER L 



GENERAL GEOLOGIC AND CLIMATIC HISTORY OF THE 



TERTIARY. 



Although, as observed in the introduction (p. 7), we still lack 

 exact knowledge, certain broad generalizations are beginning to 

 emerge from the facts collected chiefly by American paleontologists 

 since the pioneer studies of Hayden and Leidy in the middle of the 

 last century. 



Among the earlier contributors to our geologic and stratigraphic 

 know^ledge are Hayden, Leidy, Marsh, Cope, King, Scott, and Osborn. 

 Among the more recent contributors are Matthew, Hatcher, Wort- 

 man, Darton, Merriam, Peterson, Douglass, Gidley, Granger, and 

 Sinclair.^ 



The most central fact established is that there were during the 

 Tertiary period two grand natural divisions of geologic deposition 

 and of animal and plant habitat, similar to the two natural divisions 

 which exist to-day, namely, (1) the Mountain Region and (2) the 

 Plains Region. 



THE MOUNTAIN REGION. 



The mountain and high-plateau region, as a whole, stretched 

 north through British Columbia to its broad Asiatic land connection, 

 which was apparently interrupted and renewed more than once dur- 

 ing the Tertiary period. On the south it terminated, according to 

 Suess, in the mountains which form the northern boundary of the 

 southern Mexican State of Oaxaca. We have a few glimpses of the 

 life of limited areas of this vast region in Tertiary time. 



The Eocene Tertiaries of the Mountain Region, lying in and west 

 of the Rockies, in which the life is best known, were partly formed 

 by the post-Cretaceous or post-Laramie uplift, accompanied by 

 great volcanic activity, lava flows, and eruptions of volcanic dust, 

 and by the formation of a series of lake, river, and flood-plain basins, 

 filled with volcanic and erosion sediments. 



a Six of these observers either have been continuously or were for a time connected with the expedi- 

 tions sent out by the present writer from the department of vertebrate paleontology of the Amer can 

 Museum of Natural History, with instructions to combine very precise geologic and paleontologie 

 observations. Of the others, Hatcher's pioneer work for the United States Geological Survey and 

 for the Carnegie Museum, Merriam's and Sinclair's work in the John Day region (University of Cali- 

 fornia studies) , and Douglass's observations in Montana have been most important. Darton's report 

 on the central Great Plains (1905) is the latest and most comprehensive contribution. 



19 



