84 THE AGE OF MAMMALS 



ditions of deposition in western North America were entirely different ; here 

 we find continuous depositions of uniform character. In France there is a 

 lack of stability, due to weak or unstable coast outlines, to the alternation 

 of land and sea deposits. IVIoreover, the relations of these land masses to 

 each other and to the Asiatic continent were continually changing. The 

 proximity of the sea favored a warm, moist, and uniform climate. 



PalcBogeography of North America 



The most important thing to note, as pointed out by Suess, is that North 

 America has been a relatively stable continent since the close of the Creta- 

 ceous; its great land surfaces are older, more prominent, and more extensive 

 than those of Europe. The land surfaces of Africa, however, are far older 

 than either. 



It is the West which best repays interest, and the most central fact estab- 

 lished is that there were during the CaBnozoic Period two grand natural divi- 

 sions of geologic deposition and of animal and plant habitat, similar to the two 

 natural divisions which exist* to-day, namely, the Mountain and the Plains 

 regions. The Atlantic Border region becomes of importance only in late 

 Cisnozoic times. The Pacific Coast region becomes of importance in Pliocene 

 and Pleistocene times. 



For the purposes of our study, the palseogeography of the United States 

 previous to the Pleistocene may therefore be divided into the following great 

 regions : 



1. The Atlantic Border Region. 



2. The Plains Region. 



3. The Mountain Region. 



4. The Pacific Coast Region. 



1. Atlantic border region. — More unstable conditions, somewhat similar 

 to those of western Europe, may have prevailed along the eastern and Atlan- 

 tic seaboard in Eocene times; that is, the north and south Atlantic borders 

 were rising and falling. Only toward the end of the Eocene were portions 

 of Florida raised out of the sea. The continental shoreline appears to have 

 bordered the Atlantic in a general northeast to southwest direction from the 

 region of southern New York to northern Florida. There was a well-marked 

 indentation in southeast Georgia: from the vicinity of the Chattahoochee 

 River the shorelines rounded to the west, northwest, and north, forming the 

 eastern coast of a greater Gulf of Mexico which extended to the meeting of the 

 Ohio and Mississippi Rivers. From this the Gulf extended in a southwesterly 

 direction. At the culmination of the Eocene a movement of elevation took 

 place. During the entire Eocene epoch the country stretching to the Missis- 

 sippi and eastward to the Appalachians and Atlantic coast is, with a few 

 exceptions, a terra incognita so far as its terrestrial mammalian life is con- 

 cerned. Glimpses only of its marine or seashore mammalian life are afforded 



