102 LAND MAMMALS IN THE WESTERN HEMISPHERE 



and the other near the bottom of the series, which have yielded 

 a very considerable number of fossil mammals, and of these 

 the lower is the Puerco, the upper the Torrejon, The Fort 

 Union is quite different in character and is composed of great 

 areas of sandstone and clay rocks, with a maximum thickness 

 of 2000 feet, in eastern Wyoming, South Dakota, Montana 

 and the adjoining parts of Canada. The modes of formation 

 of these beds have not yet been fully determined ; that they 

 may have been partly laid down in shallow lakes is indicated 

 by the masses of fresh-water shells in certain localities. In 

 others are preserved multitudes of leaves, which have given a 

 very full conception of the plants of the time, and great swamps 

 and bogs have left the traces of their presence in beds of lignite, 

 or imperfectly formed coal. Deposits made on the flood- 

 plains of rivers and wind accumulations are probably also 

 represented. ''Vast stretches of subtropical and more hardy 

 trees were interspersed with swamps where the vegetation was 

 rank and accumulated rapidly enough to form great beds of 

 lignite. Here were bogs in which bog iron was formed. Amid 

 the glades of these forests there wandered swamp turtles, alli- 

 gators, and large lizards of the characteristic genus Champ- 

 sosaurus'" (Osborn, p. 100). 



Fort Union mammals are relatively rare and most of those 

 that have been found are very fragmentary ; they are amply 

 sufficient, however, to demonstrate the Paleocene date of the 

 beds and to make it probable that they include both the Puerco 

 and the Torrejon faunas. 



The climate, as shown by the plants, was much milder and 

 more uniform than that of the Recent epoch, though some in- 

 dication of climatic zones may already be noted. The vegeta- 

 tion was essentially modern in character ; nearly all our modern 

 types of forest-trees, such as willows, poplars, sycamores, oaks, 

 elms, maples, walnuts and many others, were abundantly 

 represented in the vast forests which would seem to have covered 

 nearly the entire continent from ocean to ocean and extended 



