LA VAS IN THE GREA T BASIN REGION 645 



Reasoning on the basis of the deductions specified, we may 

 speculate briefly concerning the cause of the two revolutions, 

 the reappearance of the intermediate magma, and the exhaustion 

 of the old, highly differentiated magmas. In explanation of 

 this, the hypothesis may be advanced that magma basins or lava 

 reservoirs may be almost entirely exhausted by the expulsion of 

 lavas to the surface, and that this emptying may permit refilling 

 by new material from lower regions. 1 It is very possible that 

 the processes of differentiation can only go on under certain 

 circumstances, such as are probably afforded by the compara- 

 tively quiet magma basins, and that in the lower regions there 

 may be so much mixing that segregation is impossible. 2 There- 

 fore, when the magma basin is exhausted and receives a new sup- 

 ply from below, it is of material similar to that which filled the 

 basin before differentiation altered it. It is probable that in 

 this way the history of many petrographic provinces, when 

 closely studied far back into geologic time, will be found to be 

 not a simple, single process, but a succession of several or many 

 differentiation cycles, some of which will probably be found to 

 .be complete, and some interrupted by this or that accident. It 

 is probable that the existence of recurrent lavas will be found 

 true at many points. 3 In Alaska the writer has found that the 



Valley, and here was considered to be extrusive. The writer's grounds for consider- 

 ing that the Walker River Range granite is equivalent to the basal rhyolite cannot be 

 given in this paper, but will appear subsequently. If they prove sound, then the 

 older monzonite very likely represents a pre-rhyolitic monzonitic effusive rock, or at 

 least a less siliceous pre-rhyolite magma. 



1 Since writing the above the writer has chanced, upon the following sentence of 

 Iddings (" Origin of Igneous Rocks :" Bull. Phil. Soc, Washington,. Vol. XII, 1892-4, 

 p. 179) : " It is also possible to find a recurrence of different varieties at one center of 

 eruption, which may be accounted for by supposing successive supplies of magma 

 from some depth, which differentiate into similar varieties before their final eruption," 

 He also finds that the same idea had been previously expressed by Sir Archibald Gei- 

 kie (Quar. Jour. Geol. Soc, London, Vol. XLVIII, 1892, p. 178), as follows : " And as 

 the successive protrusions took place within the same circumscribed region, it is evi- 

 dent that in some way or other, during the long interval between the two periods, the 

 internal magma was renewed as regards its constitution, so that when eruptions again 

 occurred they once more began with basic and ended with acid materials." 



2 Iddings ( op. cit., p. 196) considers that the general or undifferentiated magma 

 remains undifferentiated on account of being solid, that is, being in a state of poten- 

 tial liquidity. 



3 Geikie (op. cit.) in his study of the history of volcanic activity in Great Britain 



