32 JR. A. Daly — Mechanics of Igneous Intrusion. 



where alone there seems, in this third period, to have occurred 

 the injection of molten magma in masses of batholithic size — 

 in no known case accompanied by wholesale foundering. 



Again, granting the hypothesis that a visible post-Archean 

 batholith is the acidified, upper portion of a basaltic body 

 originally injected to a level less than about ten or fifteen 

 kilometers from the earth's surface (perhaps tile level of no 

 strain), it is not difficult to see that extensive foundering may 

 be impossible. Only after some differentiation or acidification 

 of the primary magma would any part of it become less dense 

 than the average roof-rock. Xenoliths of the heavier gneisses 

 and schists would, however, sink. When dissolved in the pri- 

 mary magma their material — added to that dissolved along the 

 main contact-surfaces — would lower the density and inaugu- 

 rate the stage of general stoping. Only when the resulting 

 syntectic magma has been formed in large amount is there 

 any danger of roof-foundering. But it is evident that, in the 

 process of dissolving the engulfed blocks, the magma is losing 

 heat. In every post-Archean batholith the magma, because of 

 exhaustion of the heat-supply, seems to have been arrested in 

 its upward course at average distances of one or more kilo- 

 meters from the earth's surface. The syntectic magma, less 

 dense than the roof-rock, is thus necessarily of limited depth. 

 That depth represents the thickness of the couche which 

 endangers the stability of the roof. If, now, we imagine the 

 buckling of the roof with the complete immersion and sinking 

 of certain parts of it, the foundering must be limited by the 

 width of the injected body (seldom over fifty kilometers) and 

 by the thickness of the acid couche (ten kilometers or less). 

 Extensive floods of rhyolite and allied rocks may have issued 

 at the surface in consequence of partial foundering (faulting), 

 but great crustal catastrophes involving large areas would not 

 be expected. 



Finally, it should be noted that post-Archean granitic intru- 

 sions have regularly followed periods of prolonged orogenic 

 crushing, during which accumulated tangential stresses are 

 effectually relieved. As the magmas work their way up into 

 the folded terranes there is relatively little chance for the 

 buckling of the roof. Until it is buckled and immersed in the 

 magma it cannot sink. Now the heat of the magma, though 

 it shatters the roof-rock at the immediate contact of solid and 

 fluid, must tend to expand the roof, tighten it, prevent normal 

 faulting and so strengthen the roof. The cover of the batho- 

 lith is thereby kept in an exceptionally rigid condition. Its 

 strength is, initially, that of a domed shell spanning diameters 

 not very many times the thickness of the shell. The strength 

 is increased, as with the groined roofs and arches of Gothic 



