Terrestrial Surface Materials. 703 



It must be remembered that the calcareous rocks are 

 relatively o£ little importance in estimating the average 

 radioactivity of the sediments. Estimates of their relative 

 abundance concur in representing them as constituting but 

 a few per cent, (less than 10) of the total sedimentary mass. 

 We infer from this that 1*5 may be accepted as the average 

 radium content of the sedimentary rocks. Results on the 

 amount of thorium in the sediments show that they contain 

 about 1*16 x 10~ 5 gram per gram*. From these data we get 

 for the rate of development of radioactive heat in these 

 rocks : — 



For the radium 9 x 10 ~ u cal. per sec. per gram. 

 „ thorium 7*6 x 10~ 14 „ ,, „ 



giving a total of 16*6 x 10 _u cal. The corresponding value 

 for the igneous rocks is, as we have seen, 25 x 10" u cal. 



It is only in the geosynclines, wherein the great body of 

 the sediments has collected throughout vast periods of geo- 

 logical time, that the geological importance of the radioactive 

 heat of the sediments becomes conspicuous. The sedimentary 

 deposits may amount to more than 10 kilometres in thickness. 

 These great masses are laid down upon the normal igneous 

 crust , and the intensification of the deep-seated temperatures 

 thereby brought about would appear adequate to account for 

 the leading facts of mountain elevation. The matter has 

 been dealt with elsewhere |. 



The results obtained by the fusion and decomposition of 

 rocks in the electric furnace assign to the sediments 1*5 and 

 to the igneous rocks 2*5 as mean radium content. Of the 



existence of a substantial difference between these two classes of 

 materials there appears to be no doubt. 



The denudative loss of radium (which, of course, stands for 

 a measure of the parent substance, uranium) to the ocean is 

 greater than appears at first sight from the foregoing figures; 

 for there is a reduction of mass attending the processes of 

 denudation amounting to about 33 percent, of the original rock. 

 Hence in the sediments we have but 67 per cent, of the primary 

 rock, and this containing but 60 per cent, of the original radium 

 content. The total loss has been about 60 per cent, of the 

 radium of the primary rock ; or for every tonne of igneous 

 rock converted to sediment the loss of radium has been 

 15 x 10" 7 gram. This is carried into the ocean and remains 

 in its waters and its underlying deposits. 



* Joly, Phil. Mag-. July & Aug. 1910. 



t Joly, Address Section C, Brit. Assoc. 1008. 



