278 CJ^EOLOGY OF THE EUREKA DISTRICT. 



Natural succession of volcanic roclcs. 



OrtU-r. Subdivision. 



1. Pro]iylite «. Hornblendepropylite. 



h. Quartz-propylite. 



c. Aiigite-propylite. 

 'J. Audesite a. Hoi'iibleude-andesite. 



h. Quartz-audesite (Dacite). 



c. Aiigite-andesite. 



3. Trachyte «. Uorubleude-plagiodase-tracbyte. 



h. Sauidine-tracbyte (quartziferous). 

 c. Augite-tracbyte. 



4. Neolite a. Rbyolite. 



b. Basalt. 



This 231'esents a much more complex system and could hardly be 

 accepted u^jon the simple conditions of a uniform and widespread liquid 

 mass, as held by Waltershausen and modified by Richthofen, in requiring 

 frequent elevation and depression of the loci of eruption in accord ^\ith the 

 changes in the composition of the lava thrown out at the surface. Mr. 

 King is fully aware of the many ])hysical obstacles encountered, and 

 explains the many oscillations and abrupt alternatio-ns in the volcanic 

 products which his system calls for by a carefully considered hypothesis of 

 his own, quite at variance with the views advanced by his predecessors. 

 In place of a broad belt or magma of liquid lava encircling- the earth 

 beneath the sedimentary cnist, he holds to the opinion of local reservoirs of 

 molten matter within the superficial crust, each of his orders being the 

 product derived from one of these reservoirs, or, as he calls them, 

 "extremely localized and only temporarily existing pools of fusion." 

 He says: 



Under my hypothesis, l)y \vhii-b fusion is the temporary result of erosion, each 

 one of Ricbthofen's orders, with its acidic and pyroxenic members, would be considered 

 as the product of a single ephemeral lake. A period of erosion under this concei)tiou 

 would result in the formation of a lake. The cessation of ei'osiou, either from climatic 

 causes or from the degradation of centers of erosion, would place a limit to the 

 expansi(m in dei)th of fusion; in other woi-ds, would define the time limits and the 

 vertical expansion of the lake.' 



' Op. clt., p. 716. 



