734 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



mineralisateurs.' Moreover, every batholith occurring in the Forty-ninth 

 Parallel section is clearly exotic with respect to the surrounding formations. 

 This exotic relation applies not only to satellitic offshoots but also to the main 

 bodies exposed. In this respect, as in many others not here mentioned, the 

 combined influence of mineralizing agents and of the basal warming of geo- 

 synclinals cannot account for the granitic masses of the Boundary belt. 



Hypothesis op Magmatic Stoping. 



Summary. — The general statement of this hypothesis may be conveniently 

 quoted from the third of the writer's papers on the ' Mechanics of Igneous 

 Intrusion.'* 



1. Each acid, batholithic magma has reached its present position in the 

 earth's crust largely through the successive engulfment of suites of blocks broken 

 out of the roof and walls of the batholith. 



2. The blocks (xenoliths) are completely immersed in the magma, partly 

 through the confluence of apophyses which have been injected on joints and 

 other planes of weakness in the country- rock ; more often the blocks represent 

 the effect of shattering, due to the obviously unequal heating of the solid rock at 

 magmatic contacts. 



3. The sunken blocks must be dissolved in the depths of the original fluid, 

 magmatic body, with the formation of a synteetic, secondary .magma. 



4. The visible rock of each granite batholith or stock has resulted from the 

 differentiation of a synteetic magma. 



In applying the hypothesis to the explanation of actual field occurrences 

 other general considerations seem necessary. Stoping and abyssal assimilation 

 en the batholithic scale are begun by the primary basaltic magma. This magma 

 carries the heat required for the double action.f The source of the magma is to 

 be found in the general basaltic substratum beneath the earth's solid crust. 



These subsidiary elements of the problem here to be discussed have been 

 described in the first intrusion paper and, more fully, in the later communication 

 on ' Abyssal Igneous Injection.':}: No one of these additional conceptions is 

 essential to the idea of stoping per se. All of them may prove incorrect without 

 invalidating the stoping hypothesis in its main feature. Combining them and 

 the idea of stoping, the writer has constructed a general working hypothesis for 

 the origin of the igneous rocks. It seems, therefore, expedient in the present 

 chapter to discuss the problem in its larger aspect. 



Believing that assimilation by magmatic action of some kind is responsible 

 for practically all the chambers occupied by those intrusive3 with which he i3 

 more or less intimately acquainted, the writer has sought for field evidence as to 



* American Journal of Science, Vol. 26, 1908, p. 19. 



t Again it may be noted that the question whether the substratum is actually or 

 only potentially fluid is not vital in this connection. The observed rigidity of the 

 planet may be due, not to its being a true solid, but to the direct influence of gravity, 

 which binds the earth-shells so effectively that bodily tides are almost wholly prevented. 

 In any case rigidity and solidity are not synonymous terms. 



X Amer. Jour. Science, Vol. 22, 1906, p. 195. 



