3-i 7?. .A. Daly — Mechanics of Igneous Intrusion. 



the substratum. It is here also assumed that the gradient, 

 3° C. for 100 meters of descent, applies to the crust and to 

 the upper part of the substratum at least. It must be noted, 

 however, that the gradient may very considerably steepen in 

 the depths, because of the fact that the thermal conductivity 

 and diffusivity of rock both decrease in large ratio with increase 

 of temperature. The amount of steepening of the gradient is 

 unknown, but our ignorance on this point is unessential to the 

 principle of the following argument, in which the normal gra- 

 dient is assumed throughout. 



Thirdly, it is assumed that, under normal conditions, the 

 substratum shell immediately below the solid crust is not super- 

 heated but is at the melting-point of basalt at that depth. The 

 accepted temperature gradient gives, at the depth of 38 kilo- 

 meters, a temperature of 1140° C. Yogt has calculated that 

 the pressure at this level raises the melting-point about 50° C. 

 Since basalt at atmospheric pressure is just melted at about 

 1190° C, we may conclude that the bottom of the crust, in 

 accordance with the assumptions, averages 38 kilometers below 

 the present surface. If the earth is cooling down, the crust 

 was evidently somewhat thinner during Tertiary and pre- 

 Tertiary batholithic intrusion. 



If, now, a broad geosynclinal prism of sediments, 10,000 

 meters thick in the middle, is laid down on the site of 

 a future mountain-range, the isogeotherms must rise. The 

 uppermost layer of the substratum, where most deeply 

 buried, will thus tend to assume a temperature of nearly 300° 

 C. above normal. If the sedimentary prism be folded and 

 overthrust as in the usual large-scale orogenic disturbance, the 

 substratum below the mountain-range may be still more effect- 

 ively blanketed, with a further rise of the isogeotherms. 

 Quickened erosion may, however, largely offset this thickening 

 by the mountain-building process, and it would be unsafe to 

 postulate a total rise of temperature of more than 300° C. in 

 the substratum of the area. Fart of this superheat is lost by 

 conduction into the crust, the lower basic part of which may 

 be thus melted. An unknown but possibly considerable frac- 

 tion of the total superheat may remain in the original substra- 

 tum, and this amount of superheat would characterize the 

 basalt when rapidly injected into the crust. 



In the partial release of pressure in the act of injection we 

 have another, but probably less important, source of super-* 

 heat — averaging some fraction of the 50° C. by which the 

 melting-point is raised at the bottom of the 38-kilometer crust. 

 A third source of superheat is found in the conversion into 

 heat of the mechanical energy necessary for injecting a viscous, 

 melt into an opening cavity. 



