( 819 ) 
It is seen, that, if we disregard the sample from the carboniferous 
formation, the numbers do not differ much, at least less than was found 
for igneous rocks. We do not observe either any relation between 
radium content and geological age: the figures in the table are distri- 
buted arbitrarily. The same holds for the four specimens of marbles ; 
the two first are geologically more recent than the last, but a cor- 
responding difference in the quantity of radium present is not to be 
found. The general mean of the above numbers is 1,4 and agrees 
fairly well with the mean, which can be calculated from different (in 
all ten) values, given for limestone by Srrurr *), Eve *), COLERIDGE 
Farr and Frorance’), ScuLuNpr and Moore‘) and which amounts to 1,3. 
On the contrary a much higher mean of 3,3 would follow from 
Joty’s*) data, which relate to twelve rocks of this kind. We cannot 
yet settle the question, whether this difference must be ascribed to 
chance or is caused by the small deviations between the methods 
of determination. In this connexion, though, I must draw attention 
to the fact that Jory, measuring a limestone from Vaihingen obtained 
the value 3,0, while in my table the number 2,1 is found; this would 
tend to make influence of the method of working more probable. 
In conclusion I would like to make a remark on the often 
expressed opinion that sedimentary rocks contain generally less 
radium than igneous ones do. As a matter of fact, the mean of all 
sedimentary rocks measured is less than that of the igneous, but, 
if we divide the last into groups, it becomes evident, that this 
difference is only caused by the high radium content of the granites, 
whilst the fact, that an especially great number of samples of this 
rock have been measured, tends to make the general mean higher. 
If we calculate e.g. the mean of the ten results, obtained by Farr 
and Frorarcr and by FrLetcuer’) for trachyte, we obtain 1,0, a lower 
value than that given above for limestone. Such a result will probably 
be also obtained for other igneous rocks; I hope to return to this 
subject later on, after having measured further samples of these rocks. 
Inorg. Chem. Laboratory University of Amsterdam. 
1) Proc. Roy. Soc, A 78, 150 (1906). 
2) Phil. Mag. [6] 14, 231 (1907). 
3) Phil. Mag. [6] 18, 812 (1909). 
4) U. S. Geological Survey, Bulletin 395 (1909). 
5) Radioactivity and geology, London 1909, p. 60. 
6) Phil. Mag. [6] 20, 36 (1910). 
