Terrestrial Surface Materials. 701 



rocks entering into the fusion results are taken, the general 

 mean is 2*60. Any one of these means is considerably in 

 -excess of what is deducible from the results obtained by 

 solution. 



When the presence of thorium is taken into ncconnt we 

 have the data for a complete evaluation of the number of 

 calories developed per unit of time in average surface rock. 

 The general average for the thorium content of igneous 

 rocks appears to be about 2'60 x 10~ 5 gram per gram*. 

 ]f we take 2 x 10~ 5 for purposes of calculation, we find 

 13'28 x 10~ 14 calorie as the heat developed per second per 

 gram. Taking the radium as 2xl0~ v2 , we have to add 

 12-00 xlO" 14 calorie. The total is 25 X 10~ u gram-degree 

 of heat per second. A material of this degree of radioactivity 

 can only be supposed to extend downwards a relatively small 

 distance. Assuming the surface gradient to be 1° in 35 

 metres, the conductivity to be 0*004, and the specific gravity 

 of the rock to be 2*7, we find that a downward extension of 

 17 kilometres (10*6 miles) suffices to account for all the heat 

 reaching the surface. 



This conclusion lands us in the difficulty of accounting for 

 the high temperatures which are believed to extensively 

 prevail deep in the crust. At the base of the 17 kilometre 

 layer the temperature due to radioactivity would be no more 

 than 246° C. We are from this driven to the conclusion 

 that rocks of this degree of radioactivity do rot, in point of 

 fact, extend downwards till all the radioactive materials 

 present are exhausted. The low radioactivity of certain of 

 the basalts probably affords an indication of the vertical dis- 

 tribution of rocks in the earth's crust. It seems a probable 

 conclusion that such basic and feebly radioactive materials 

 make up the deeper parts of the surface crust, and by carrying 

 the heat-producing elements to a sufficient depth, account 

 for such temperatures prevailing beneath as many geological 

 phenomena seem to require. Thus it will be found on cal- 

 culation that if basalts with a radium content of 0*5 x 10 " 13 

 and a thorium content of 1*0 X 10~ 5 carried most of. the 

 radioactive materials, temperatures of between 500° C. and 

 G00° ('. would, exist at depths of some 40 kilometres (25 

 miles) from the surface f. It must be remembered, however, 

 that the thermal state in the depths of the crust is largely 



* Joly, Cong. Interned, de JRadiologie et tfEhdricite, 1911, p. 37G. 



t In a recent determination Mr. Louis Smyth finds the thorium- content 

 of the typical composite basalt to be 0*47 X 10- 5 gram per gram. This 

 result would involve a greater extension downwards and higher basal 

 temperatures than those cited above. 



