124 LAND MAMMALS IN THE WESTERN HEMISPHERE 



the occurrence of similar fossils in the South Shetland Islands, 

 an Antarctic group. On the Chilian coast the Navidad forma- 

 tion, which is believed to be approximately contemporaneous 

 with the Patagonian, has so different a fauna as to point to 

 some kind of a barrier between the Atlantic and the Pacific, 

 and this barrier, Dr. von Ihering holds, was the land-extension 

 from South America to Antarctica. 



After some oscillations of retreat and advance, the sea with- 

 drew from Patagonia and the terrestrial accumulations of the 

 Santa Cruz stage were formed. These beds are partly com- 

 posed of river-deposits, but chiefly of more or less consolidated 

 volcanic ash or tuff, and have yielded a surprising number of 

 beautifully preserved mammals. No other assemblage of 

 South American Tertiary Mammalia is so well known and 

 understood as the Santa Cruz fauna, and the very large number 

 of all but complete skeletons which have been found strongly 

 suggests that many of the animals were buried alive in the 

 showers of volcanic ash. The Santa Cruz fauna is completely 

 and radically different from any of the North American as- 

 semblages, and at that time no immigrant from the north had 

 penetrated so far as Patagonia. 



In the upper Miocene the Andes stood at a much lower 

 level than they do now ; fossil plants, some of them collected 

 at a great height in the mountains, are the remains of a luxuri- 

 ant and purely tropical flora nearly identical with the vegeta- 

 tion of the modern forests of Bolivia and Brazil. Such a 

 vegetation could not exist at the altitudes where the fossils 

 occur and these demonstrate a great elevation of the mountains 

 since those leaves were embedded. The same mild climatic 

 conditions which prevailed in the northern hemisphere during 

 the Miocene must also have characterized Patagonia, sub- 

 tropical shells extending far to the south of their present 

 range. 



Whatever may have been true of the land-bridge connecting 

 South America with Africa during the early Tertiary epochs, 



