750 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



when the resulting syntectic magma has been formed in large amount is there 

 any danger of xoof -foundering. But it is evident that, in the process of dissolv- 

 ing the engulfed blocks, the magma is losing heat. In the normal post-Cambrian 

 batholith the magma, because of exhaustion of the heat supply, seems to have 

 been arrested in its upward course at average distances of a few thousand feet 

 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 thirty miles) and by the thickness of the acid couche (perhaps eight miles 

 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 intrusions 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 architecture, by the presence of roof -pendants ; and by thermal expansion, 

 the whole is strongly knit together. Immersion and foundering of roof-sections 

 may, therefore, have been seldom possible in the case of post-Archean batholith 

 or stock. 



In spite of the highly theoretical nature of some of the foregoing argument, 

 it appears to the writer to carry weight enough to warrant our regarding the 

 difficulty in question as not destructive of the stoping hypothesis. The problem 

 needs further study in connection with this and all other conceptions of granitic 

 intrusion. 



Supply of the Necessary Heat; Magmatic Superheat and its Causes. — 

 Whether the observed average temperature gradient within the earth's crust is to 

 be explained as due to original heat (inherited from an early epoch in the devel- 

 opment of the earth either from a gaseous or planetesimal nebula), or whether 

 the gradient is due to the evolution of heat with the break-up of radium and 

 other radio-active substances, are general questions not immediately affecting 

 the stoping hypothesis. We need go no further back in the thermal problem 

 than to secure an estimate of the minimum temperature of the primary magma 

 when abyssally injected and thus prepared for stoping and assimilation. This 

 estimate is evidently not easy to make. A rough idea of the probable tempera- 

 ture may be obtained by deductively considering the temperature gradient or, 



