GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS 117 



concerning the Oligocene vegetation of the region, but the 

 reptiles indicate diminished warmth. 



South America. — Marine OUgocene strata have great 

 extent around the Gulf of Mexico and the Caribbean Sea, and 

 the distribution of these shows that Antillia was broken up 

 by great submergences, the islands of the Greater Antilles 

 being much smaller than they are to-day. The greater part 

 of Central America and the Isthmus were under water, a 

 broad sea, broken only by scattered islands, separating North 

 and South America. Very little is known of the Oligocene in 

 the latter continent save a non-marine formation in northern 

 Patagonia, the Deseado stage (or Pyrotherium Beds), which, 

 like the Eocene of the same region, occupies depressions in 

 the worn and irregular surface of the Cretaceous rocks. The 

 attribution of the Deseado to the Oligocene is open to some 

 doubt, because of the entire absence in its mammalian fauna 

 of any elements which are also found in the northern hemi- 

 sphere. Hence, there are no means of direct comparison. 



4. Miocene Epoch 



North America. — The Atlantic and Gulf coasts, which 

 had been raised in the Oligocene, were again depressed, almost 

 restoring the Eocene coast-line, the chief differences being 

 the presence of the Florida islands and the nearly complete 

 closing of the Mississippi Embayment. There was a remark- 

 able change in the marine fauna from that of OUgocene times ; 

 a cool current flowed southward along the coast and entered 

 the Gulf of Mexico through the strait between the Florida 

 island and the mainland, bringing the northern animals with 

 it and driving out the tropical forms. This complete faunal 

 change, which might fairly be called a revolution, was the most 

 sudden and striking in the Tertiary history of the continent. 



On the Pacific coast also there was a depression, which 

 caused a renewed transgression of the sea. The Coast Range 

 formed a chain of reefs and islands in the Miocene sea, which 



