304 



HISTORICAL GEOLOGY 



of lava (Fig. 185). . . . The average thickness of flows seems to 

 be about seventy-five feet. 



"The plateau was long in building. Between the layers are 

 found in places old soil beds and forest grounds and the sediments 

 of lakes. ... So ancient are the latest floods in the Columbia 

 Basin that they have weathered to a residual yellow clay from 



thirty to sixty feet in depth 

 and marvelously rich in the 

 mineral substances on which 

 plants feed. In the Snake 

 River Valley the latest lavas 

 are much younger. Their sur- 

 faces are so fresh and un- 

 decayed that here the effusive 

 eruptions may have continued 

 to within the period of hu- 

 man history." 1 



Volcanic activity must 

 have been very pronounced 

 along the Rockies during the 

 Tertiary, as shown by exten- 

 sive and often very thick de- 

 posits of tuff or volcanic ash 

 (e.g. San Juan and Florissant 

 formations) . Many volcanoes 

 also broke forth on the Col- 

 orado Plateau of Utah, New 

 Mexico, and Arizona, in the 

 latter state especially there 

 being cones exhibiting all 

 stages of denudation from 

 very recent cinder cones to others where only the merest rem- 

 nants or " volcanic necks" are left. 



In the northern Coast Range mountains of the United States 

 there was considerable volcanic activity in the Eocene and much 

 throughout the Range in the Miocene. 



Very pronounced vulcanism occurred in the Cascade Moun- 

 tain region during the Eocene and to the middle Miocene. During 

 Pliocene time there was great volcanic activity with outpourings 



1 W. H. Norton: Elements of Geology, pp. 400-401. 



Fig. 186 

 Mount Lassen in northern California 

 in eruption August 22, 1914. Smoke 

 and volcanic ash rose to a height of 

 10,000 feet. (From a photograph by 

 Restinson, Red Bluff, Cal.) 



