PALEOGEOGRAPHIC MAPS 



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the Atlantic, beneath which was deposited the widespread OHgocene 

 limestone, characterized by the faunas of a warm oceanic current. 

 This fauna spread north along the southeastern coast of the United 

 States. 



I am indebted to Dr. Wm. H. Dall and Dr. Ralph Arnold for 

 discussion of the distribution ot marine faunas and their relation to 

 inferred currents. 



In outline, North America during the Miocene resembled the 

 continent during the Eocene. The surface was, however, less 

 mountainous. The sites of the Sierra Nevada and of the Coast 

 Range of British Columbia were plains or low hilly lands. The 

 Rocky Mountains of the United States were comparatively low. In 

 British Columbia, and thence southward through Washington, 

 Oregon, and Nevada occurred outflows of lava, which covered 

 many thousand square miles, but which in general were not from 

 volcanoes. Though probably subordinate in volume of lava 

 erupted, volcanoes were numerous and they gave off quantities of 

 volcanic ash, which formed deposits in lakes, particularly in western 

 Montana and British Columbia. 



The elevation of the Rocky Mountains of western Montana and 

 British Columbia by overthrust, and subsequently the development 

 of longitudinal valleys and separate ranges by vertical displacements, 

 probably began in the Miocene period and may have culminated dur- 

 ing Pliocene or early Quaternary time. 



In the West Indian region the close of the OHgocene period was 

 marked by a notable disturbance, which raised a folded mountain 

 chain from Puerto Rico to Cuba and probably continuously to 

 Yucatan. It may also have closed the Isthmus of Tehuantepec and 

 possibly have temporarily connected Honduras with South America. 

 Another possible line of connection is around the eastern end of the 

 Caribbean through the Windward Islands. If, however, such a land 

 link united North and South America it was but temporary. 



The effect of the Cuban elevation, or of some other geographic 

 change not yet suggested, was to shut off from the northern Gulf and 

 southern Atlantic coasts the warm currents which had sustained a 

 rich southern fauna and to admit the cool northern waters with their 

 appropriate life. A very pronounced faunal change, without any 

 marked stratigraphic break in the sediments, was the result. 



