THE TERTIARY PERIOD 435 



of the continent was sometimes submerged, and was thus 

 the site of deposition ; and that at other times it was out of 

 water, and was liable to erosion. The changes of level, 

 whether of the land or of the sea, were not great in any in- 

 stance. One result of the slight warping to which eastern 

 United States was subjected during the Tertiary period was 

 the formation of an island within the present confines of 

 Florida, and later the addition of this island to the mainland 

 in the form of a peninsula. No deposits older than the Ter- 

 tiary limestones are exposed in that state. 



At several places in Texas, Louisiana, and California wells 

 drilled down into the Tertiary sediments have yielded petroleum. 

 This oil, and the natural gas which usually accompanies it, is prob- 

 ably produced by the slow decomposition of animal and vegetable 

 matter which was mixed with the sediments at the time they were 

 deposited. Certain sandy beds become saturated with the gas and 

 liquid and when one of these is pierced by the drill a flowing well 

 may result. In some cases the gas pressure is so great that the oil 

 is blown out in a jet. These "gushers" often wreck the buildings 

 and derricks over the wells, and much oil is wasted before the 

 fountain can be controlled. 



Only a part of the oil produced in the United States comes from 

 Tertiary beds. That of Ohio and Indiana is in the Paleozoic rocks, 

 and the Kansas oil is only a little younger. Doubtless the condi- 

 tions for the formation of gas and oil have been present somewhere 

 in all the geologic periods. 



Local sedimentation on the Pacific coast. In the Ter- 

 tiary period, the Pacific coast was apparently somewhat 

 abrupt and rugged, although perhaps less so than it is to-day. 

 Erosion was the chief activity along the western slope. 

 Here and there, however, deposits of Tertiary age have been 

 found, those in the coast ranges of California being largely of 

 marine origin, while farther north, near Puget Sound and 

 in Alaska, early Tertiary beds containing coal seams are 

 known. The latter were evidently laid down in swampy 

 lowlands near the coast, but not submerged by the sea. 



Alluvial deposits in the Great Plains. Throughout most 

 of the Tertiary period the Great Plains were much nearer 



