THE MIOCENE PERIOD 813 



throughout the west, and igneous activity affected nearly or quite 

 every state west of the Rocky Mountains, and the eruptions were 

 from fissures as well as volcanoes. Among the conspicuous centers 

 of activity " the basin of the Columbia l and the Yellowstone 

 National Park 2 may be mentioned. Locally, forests were buried 

 by the volcanic ejecta, and in favorable situations their trunks were 

 petrified (Fig. 548) . Great areas of the sedimentary beds of the 

 period are concealed by the lavas, but the extrusions were by no 

 means confined to the areas where sedimentation was in progress. 

 The lavas of at least a considerable part of 200,000 or 300,000 



Fig. 547. Court-House and Jail Rocks. Buttes of the Arikaree (Miocene) 

 formation of western Nebraska. (Darton, U. S. Geol. Surv.) 



square miles of lava-covered country in the western part of 

 the United States issued during the Miocene period, or during the 

 time of crustal deformation which brought it to a close. 



Volcanoes were active in the Antillean region of Central 

 America and the West Indies, and the Andean system of South 

 America, as well as in North America. 



Close of the Miocene. Slow warpings of the surface seem to have 

 been in progress throughout the Cordilleran region during the 

 Miocene period, accompanied by faulting and vulcanism, and 



1 Landes, Wash. Geol. Surv., Vol. II, and Smith, G. O., Ellensburg folio, 

 |U. S. Geol. Surv. 



2 Western folios, U. S. Geol. Surv., notably the Yellowstone National Park 

 folio. Most of the folios showing Neocene formations show volcanic rocks 

 of Neocene age. 



