578 HISTORICAL GEOLOGY 



velopment of marls and limestone of this age in Central America and 

 the West Indies shows that submergence was widespread in these 

 regions. An island was raised in northern Florida early in the epoch 

 which, by further arching of the sea bottom, became joined to the 

 mainland in the Miocene. 



Western Interior. — On the Great Plains region continental de- 

 posits of this epoch occur at various points from British Columbia 

 to Mexico, and outcrop from two to three hundred miles east of the 

 Rocky Mountains. They seldom rest upon the Eocene, but on the 

 worn surfaces of the Upper Cretaceous, showing that while deposi- 

 tion was taking place in the mountain basins of the Eocene, the re- 

 gion of the Great Plains was an open, rolling country, traversed by 

 streams which were degrading its surface. " A picture of the plains 

 region in Oligocene times is that of broad, gentle, eastward slopes from 

 the Rocky Mountains, plane or gently undulating and not moun- 

 tainous, bearing broad streams with varying channels, sometimes 

 spreading into shallow lakes, but never into vast fresh-water sheets. 

 Savannahs were interspersed with grass-covered pampas traversed 

 by broad, meandering rivers. This land was dry in dry seasons, but 

 was flooded in very high-water periods. The materials were partly 

 erosion products of the Rocky Mountains and Black Hills, such as 

 true sandstones and conglomerates, but they include also fine layers 

 of volcanic dust, wind-borne from distant craters in the mountains, 

 far out on the plains of Nebraska and Kansas." (Osborn.) 



Pacific Coast. — On the Pacific coast the Oligocene was an epoch of 

 elevation and erosion, during which the land was not high except in 

 a few places, as is indicated by the fine character of most of the sedi- 

 ments. The areas of deposition on what is now land were compara- 

 tively small. 



Oligocene of Other Continents. — In general, the distribution of 

 land and water was different in the Oligocene from what it was in the 

 preceding epoch. One important transgression of the sea covered 

 Germany and Belgium and at the time of greatest extension joined 

 the North Sea with the Mediterranean and Aral seas. In France 

 and Russia large areas were beneath the water. In the Paris basin 

 the presence of salt and gypsum furnishes a clue to the climate during 

 a portion of the epoch. In various parts of Europe (Germany, Switz- 

 erland, southern France, and Bavaria) extensive swamps were present 

 in which were accumulated the lignite deposits that are now workable 

 to some extent. 



