22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



experimental garden and the sandstone of Craigleith quarry, all at Edin- 

 burgh. Different weights were given to these observations, biit how is not 

 explained. Now, in considering the diffusivity of the earth it does not 

 seem to me that the ragged pellicle of detrital matter on its surface need 

 be considered. Over large areas it is absent and in most places the sedi- 

 mentary rocks are saturated with water, so that their own intrinsic 

 diffusivity is a minor feature of the flow of heat. The great bulk of the 

 rocks in the shell affected by cooling are massive and at least comparable 

 with the trap of Calton Hill, which is chiefly composed of Carboniferous 

 basalt and andesite. The conductivity of this rock ^v;as observed by 

 Forbes and Thomson (Kelvin) for no less than eighteen 5^ears; the 

 thermal capacity was determined by Eegnault, so that the value of the 

 diffusivity, 0.00786, is undoubtedly very accurate. It does not stand 

 alone. A committee of the British Association,' Herschel and Lebonr, 

 reported for whin and traps k = .0067, and for serpentine from .0059-i to 

 .0073, while Ayrton and Perry got for porphyritic trachyte .0103. I do 

 not think a better choice can be made than the Calton Hill trap, and its 

 diffusivity with the meter and year as units is the value which will be 

 assumed here, i. e.. k = 24.8037. 



That K varies with temperature and with pressure is probable. That in 

 iron it decreases with increasing temperature is known and analogy would 

 point to the conclusion that it should increase with pressure. Possibly 

 diffusivity is simply related to density and for the same or similar rocks 

 tends in the earth to a nearly constant value. At present it seems un- 

 avoidable in this problem to regard it as constant. 



The outer portion of the earth is composed of various rocks which are 

 believed to be arranged roughly in the order of density. If so the perido- 

 tites underlie the basaltic rocks, while the andesites and rhyolites overlie 

 them. These latter are less fusible than diabase. How deep the level lies 

 which would answer to the upper surface of the basaltic rocks cannot be 

 told with certainty. The best that can be done is to assume that Laplace's 

 law of density is valid for a few score miles from the snrface and to con- 

 sider roughly the effects of heat and pressure. In this way I have reached 

 the conclusion that at about 40 miles, or 0.01 times the radius, where the 

 density should be 2.86, the temperature perhaps 1300° C. and the pressure 

 17,400 atmospheres, basaltic rocks may begin to appear in place. A pres- 

 sure of 13 or 14 atmospheres per degree centigrade is probably of the 

 order of magnitude needful to preserve constancy of volume in a heated 

 solid, while at atmospheric pressures the densities of basaltic rocks are 

 from 2.85 to 3.10, with minor exceptions. The outer crust to a depth of 

 40 miles is thus probably less fusible than basalt. 



iBrit. Assoc. Ad. Sci., 1881. 



