638 /. E. SPURR 



LAW OF SUCCESSION OF LAVAS 



The most natural deduction from all these harmonious 

 observations is that the Great Basin, southward into the Mojave 

 Desert, together with a portion at least of the Sierra- Nevada, 

 constitutes a petrographic province ; that is to say, it is under- 

 lain by a single body of molten magma, which has supplied, at 

 different periods, lavas of similar composition to all the different 

 parts of the overlying surface. The limits of this subcrustal 

 basin, however, are not yet defined in any direction. 



In studying the eruption of different lavas from this magma 

 basin at different periods, it is instructive to inquire whether or 

 not the succession follows any definite laws. Mr. Iddings 1 inter- 

 preted the usnal law of succession in volcanic rocks as this : that 

 a series begins with a rock of average composition, and passes 

 through less siliceous and more siliceous ones to rocks extremely 

 high in silica and others extremely low in silica — that is, the 

 series commences with a mean and ends with extremes. This 

 interpretation of Iddings was based on his work in the Yellow- 

 stone Park and vicinity, at Eureka, Washoe, and elsewhere. 

 From studies of the eruptive rocks in the vicinity of Christiania 

 Professor Brogger also thought to have determined a definite law 

 of succession, by which the lavas progress from the most basic to 

 the most acid varieties. Sir Archibald Geikie, 2 from a study of 

 igneous rocks in Great Britain, has come to the same general 

 conclusion as to the succession. 



The section of Tertiary volcanics exposed at Eureka and 

 Washoe, as given by Mr. Hague, begins with what is designated 

 by the present writer No. 2 in the succession, and does not reach 

 back to the basal biotite rhyolite. The general succession for 

 the Great Basin, leaving out this basal' rhyolite, appears, then, to 

 be as follows : 



2. Siliceous intermediate. 



3. Acid (and basic). 



4. Basic intermediate. 



5. Basic (and acid). 



'The origin of igneous rocks, Bull. Phil. Soc. Washington, Vol. XII, p. 145. 

 2 Quar. Jour. Geol. Soc. Lond., 1892, Vol. XLVIII, p. 177. 



