284 li. A. Duly — Mechanics of Igneous Intrusion. 



The depth of the pressure-solid basement supposed to form the 

 foundation of such magmas, the density stratification of 

 the liquid magmas themselves, the distribution of diluting 

 " mineral izers," the efficiency of buttresses left in the act of 

 stoping, the possibility of progressive lateral cooling in stock 

 or " batholith," the strength and thickness of the chamber 

 vault, are among those elements. 



The difficulty of reconciling the idea of stoping as a process 

 extensively operative in intrusion, with the apparent necessity 

 of postulating flotation for the crust overlying still liquid 

 batholiths, is partly met by recognizing the influence of com- 

 pression in the magmas under their heavy crustal loads. The 

 full significance of such compression cannot, for lack of exist- 

 ing experimental data, be told, but, from the known analogous 

 compressibility of glass, it is probable that the quantitative 

 value of the compression in great depths is considerable. As 

 we have seen, this conclusion is in no sense inimical to the 

 possibility of stoping, since rifted blocks are under the same 

 pressure as the magmas enclosing them. 



Further, it has been seen from a discussion of Table IV, 

 that many solid crystalline schists and sediments (staple rocks 

 in the earth's crust) might be expected to float on the heavier 

 gabbros when the latter are highly fluid. Reasons are given 

 below for the conclusion that earth-magmas are, in general, 

 arranged according to the scheme of density stratification, and 

 that, beneath the less dense, acid magmas, as well as underly- 

 ing the solid crust, there is a world-circling layer of potentially 

 molten gabbro magma. If this view be 'accepted, it is possible 

 to credit some roof-support for a large batholith to the immer- 

 sion in gabbro of the bases of the walls and of large interven- 

 ing pillars of solid roof-rock invaded by the batholith. The 

 required strength of the whole roof becomes, on that supposi- 

 tion, of comparatively low degree, equivalent, namely, to that 

 of the relatively small vaults between the main walls and pil- 

 lars ; such vaults as belong to the associated stocks so com- 

 monly believed by geologists to represent the upper portions 

 of huge, regional batholiths. Provided that the roofs of the 

 stocks remain intact, their chambers, filled with magma less 

 dense than the overlying crust, will represent so many floats 

 for the roof of the whole batholith. 



Needless to say, the body of desired facts ascertained in 

 field observations is insufficient to permit of an adequate 

 test of these hypothetical postulates. The problem is, there- 

 fore, at present, largely indeterminate. It might, perhaps, 

 be adequately treated by actual analytical study of type 

 occurrences where the phenomena of nature indicate either 

 that wholesale foundering has not taken place, or, on the other 



