﻿Thorium Content of the Earth's Crust. 489 



o£ gelatinous silica may lead to better boiling of the solution. 

 The amount of HC1 used, too, seems to have little effect on 

 the result, as the solutions made up with 50 c.c. and 100 c c. 

 of acid respectively gave nearly identical results. Thus 

 from this series of trials it would appear that the exact 

 method of treating the rock has not much effect on the final 

 result, which is satisfactory. 



It is rather difficult to arrive at a mean result for the 

 thorium content from the values obtained. We can either 

 simply take the mean of all the results obtained, or we can 

 attach to each result a weight proportional to the number of 

 rocks in the powder from which the result was obtained. 

 The means obtained by both methods are given. A slightly 

 higher result is obtained by the second method. This is due 

 to the fact that there is a larger number of acid rocks in the 

 powders than either intermediate or basic rocks. Of course 

 by neither method can we hope to obtain the real mean 

 value of the thorium content of the earth's crust. To do 

 this, we should know its average composition, i. e. the per- 

 centage of acid, intermediate, and basic types in it. The 

 amount of sedimentary rocks could probably be neglected 

 without making much difference in the result. It is generally 

 thought that the composition of the lithosphere approximates 

 to that of a diorite or andesite, i. e. it is intermediate in 

 chemical character. On this supposition the average thorium 

 content would be about 1*6 x 10~ 5 . This value is of course 

 obtained only from surface materials, and we are not justified 

 in assuming that the thorium content is not different at some 

 distance below the surface. The mean for the acid rocks 

 alone is 2*08 x 10" 5 and for the basic rocks 0*56 x 10~ 5 . 

 These means are obtained by the second method. By the 

 same method we obtain a general mean for the experiments 

 of 1*50 X 10~ 5 . In these results the miscellaneous rocks are 

 neglected. The values are obtained from 86 acid, 48 inter- 

 mediate, and 56 basic rocks. If we take the simple arith- 

 metical means we obtain slightly different values, i. e. acid 

 2*13, intermediate 1'50, and basic 0*51. The corresponding- 

 general mean is 1*38 X 10"°. 



In conclusion I wish to express my sincere thanks to 

 Dr. Joly for his most kind aid and advice during the progress 

 of the experiments. 



Iveagh Geological Laboratory, 

 December 1914. 



