646 /. E. SPURR 



probably Silurian basalt or basaltic diabase of the Rampart 

 series is identical in composition with the olivine-basalts of the 

 Pliocene. 1 Yet between these two periods a great variety of vol- 

 canic rocks, including other basalts, appeared in Alaska. 



In studying the succession of lavas it must be borne in mind 

 that the processes of differentiation are quite independent of 

 the causes which produce the expulsion of lavas. Therefore,, 

 while the differentiation in a magma basin may go on so that all 

 intermediate types between the initial intermediate one and the 

 final extremes are represented, yet the causes producing eruption 

 will probably occur only at different points in this process, so- 

 that the record will be only partial and perhaps not to be inter- 

 preted except by comparison with other localities. For this 

 reason, the observed succession must be studied with regard to 

 its general aspects rather than to its details. In interpreting the 

 succession, also, the possibility, or even probability, of many 

 intermediate forms being brought about by accidental mixing 

 during the general processes of differentiation, must be borne in 

 mind. This error may be eliminated if at each period of vul- 

 canism we select the extreme important types, omitting the 

 associated intermediate ones as possibly formed by mingling. 

 The variations introduced by mingling and those brought about 

 by irregular volcanic eruptions are such that it is difficult to 

 apply to them any law. It has been assumed, for example, that in 

 general basalts overlie rhyolites where these occur close together. 

 This is held by Messrs. Hague and Iddings 2 as well as by previ- 

 ous observers. But Marvine 3 observed, in the Colorado River 

 region, basalt lying upon rhyolite, and the present writer observed 

 the same relation in the Pleistocene lavas of Meadow Valley 

 canyon, which is a part of the drainage of the Colorado. 



J. E. Spurr. 



from the earliest pre-Cambrian times to the Tertiary, has found that similar rocks 

 recur at many different points in the succession. He finds also similar series, so that 

 he is led to divide the whole succession into natural groups or periods of volcanic 

 activity. See also Iddings, op. cit., pp. 145, 179, 196. 



1 Geology of the Yukon Gold District. Eighteenth Ann. Rept., U. S. Geol. Surv., 

 Part III, Economic Geology, p. 241. 



2 Mon. U. S. Geol. Surv., Vol. XX, p. 86. 



3U. S. Geol. Surv., W. 100th Men, Part III, p. 205. 



