704 Radioactivity of Terrestrial Surface Materials. 



I have shown elsewhere that it is possible to use the 

 amount of sodium in the ocean as a measure of the total 

 quantity of primary rock which has been denuded in the 

 course of geological history*; and later 1 calculated by its 

 means the total mass of the sub-oceanic sediment sf. These 

 estimates are independent of the duration of geological time. 

 They, in fact, do not involve the rate of denudation, but only 

 the integral of its products. The security they possess may 

 be judged from the fact that the only assumption involved 

 in their derivation is that the sodium of the ocean is a product 

 of solvent denudation which has been contributed to the 

 ocean and which, practically, in its entirety now remains to 

 our observation. The rest of the argument is based on our 

 knowledge of the sodium content of average igneous and 

 sedimentary rocks. 



The mass of igneous rock, parent to the sediments, proves to 

 be 84*3 X 10 16 tonnes. The radium brought into the ocean has, 

 therefore, been 84*3 X 10 16 x 15 x 10~ 7 or 1264 x 10 9 grams. 

 This amount must be contained in its waters and in the sub- 

 oceanic sediments. We may first make a deduction for the 

 amount now in solution. Estimates of oceanic radium vary. 

 If we take 4 x 10 -12 gram per kilogram or 4 x 10~ 9 per tonne, 

 we have, multiplying by the mass of the ocean, i. e. by 

 1*32 x 10 18 tonnes, the quantity 5*3 x 10 9 grams as the total 

 amount in solution. The deduction is trifling and leaves 

 1259 x 10 9 grains assignable to the sub-oceanic sediments. 



The mass of these in the aggregate is 19*5 x 10 16 tonnes, 

 and hence we must ascribe to them an average radium 

 content of 6'4x 10~ 12 gram per gram. 



The following measurements of the radium content of 

 oceanic deposits have been made by the fusion method {: — 



Four samples of Globigerina ooze from depths 

 of 1990 to 2493 fthms 33 



Three of the above four samples 3*1 



A sample of blue mud, depth 1240 fthms 1*5 



"ooze" „ 720 „ 1*7 



Two samples of Radiolarian ooze from depths 

 of 2600 and 2750 fthms 13'1 



A sample of red clay, depth 2350 fthms ll'O 



From these figures we see that the quantity of radium 

 assignable to the oceanic deposits is in fair agreement with 



* Joly, Trans. K. D. S. vol. vii. 2nd ser., 1899. 

 t ' Radioactivity and Geology/ pp. 57-58. 



| They are considerably lower than the results which I formerly 

 obtained by solution, but must be considered more reliable. 



