REPORT OF THE CHIEF ASTRONOMER 751 



SESSIONAL PAPER No. 25a 



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 substratum. It is first of all assumed 

 that the substratum, though a true basalt for many miles 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 temperature of four hundred degrees involves an expansion of only about 

 one per cent in volume. An underlying couch e 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 conception also involves the possibility that the observed 

 temperature gradient continues without important change, deep into the sub- 

 stratum. It is here also assumed that the gradient, 3° C. for 100 metres of 

 descent, applies to the crust and to the tipper 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 

 gradient is assumed throughout. 



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

 immediately below the solid crust is not superheated but is at the melting point 

 of basalt at that depth. The accepted temperature gradient gives, at the depth 

 of 38 kilometres, a temperature of 1140° C. Vogt has calculated that the 

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

 atmospheric pressure is all molten at about 1140° C, we may conclude that 

 the bottom of the crust, in accordance with the assumptions, averages about 40 

 kilometres 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 metres 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. Part of this superheat is lost by con- 



