B. A. Daly — Mechanics of Igneous Intrusion. 33 



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, not have been 

 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 development 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 gen- 

 eral questions not immediately affecting the stoping hypoth- 

 esis. 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 temperature may be 

 obtained by deductively considering the. temperature gradient 

 or, secondly, by assuming that the initial temperature of the 

 abyssally injected basalt is not far from that of the hottest 

 basaltic lava known in volcanoes. 



The first method is only applicable on certain assumptions 

 as to the thermal and material constitution of the basaltic sub- 

 stratum. It is first of all assumed that the substratum, though 

 a true basalt for many kilometers of depth, is faintly stratified 

 according to density differences. The chemical contrast 

 between successive shells of the substratum may be extremely 

 slight and yet sufficient to prevent convection-currents, even 

 though the bottom shell of the substratum is several hundreds 

 of degrees hotter than the uppermost shell. A rise in temper- 

 ature of four hundred degrees involves an expansion of only 

 about one per cent in volume. An underlying couche of 

 basalt at 1600° C. would, therefore, if its specific gravity at 

 1200° C. were 2*93, not convectively displace an overlying 

 couche of magma at 1200° C. and with a specific gravity of 

 2*90. Such faint density stratification, if assumed, goes far to 

 explain the general stability of the earth's crust and so far is 

 in accord with the facts of post-Archean geology. This con- 

 ception also involves the possibility that the observed temper- 

 ature gradient continues without important change, deep into 



Am. Jour. Sci.— Fourth Series, Vol. XXVI, No. 151.— July, 1908. 

 3 



