Products of the Radio-active Elements. 263 



of the low proportion of lead and helium, and the practical 

 absence of thorium in ix, is quite in accord with the geological 

 indications that this material is of an age greatly inferior 

 to that of the primary uraninites. In xxn the very notable 

 amount of lead shown by the analysis would seem to offer no 

 serious obstacle to the theory, since this material occurs inti- 

 mately associated with the sulphide of lead and other similar 

 minerals, and the massive and amorphous form in which it is 

 found would indicate that the conditions under which it was 

 originally deposited were not favorable to the separation of a 

 pure uranium compound. The statement of Hillebrancl* that 

 nitrogen (helium) and the rare earths were practically absent 

 in specimens of secondary uraninite from Pribram, Joachims- 

 thal and Johanngeorgenstadt, which he examined, is also of 

 interest in this connection. The experience of Debiernef in 

 separating actinium from a secondary uraninite of this charac- 

 ter, is, however, indicative of the existence of small amounts 

 of thorium in these minerals. 



Other Radio-active Minerals. 



In the table which follows (Table II) will be found some 

 data compiled from various sources on the composition of a 

 number of primary and secondary radio-active minerals. 



As bearing on the topic under discussion it is interesting to 

 note the following : — 



1. The greatest proportion of helium with respect to the 

 uranium and lead present has been observed in those primary 

 minerals which have the lowest emanating power and the 

 highest specific gravity, i. e., in the most compact and least 

 porous minerals. Examples are furnished by thorianite, fergu- 

 sonite, samarskite and monazite. (Of the varieties of thorite, 

 much greater proportions of helium have been observed in the 

 variety known as orangite, which has also the greatest density.) 



2. Greater proportions of lead and helium with respect to 

 uranium are found in those primary minerals which occur in 

 the oldest geological formations. This point is well illustrated 

 by thorianite, which is found in Ceylon in a geological forma- 

 tion which is probably of the Archean period. 



3. The primary minerals containing the greatest proportion 

 of thorium are in general the most hydrated. 



In considering the secondary radio-active minerals certain 

 probable conditions must be recognized. Where these minerals 

 are formed by the alteration of primary minerals in place, 

 namely, where the primary mineral is acted on by underground 



* Bulletin U. S. Survey, No. 78, p. 72. 

 % Compt. rend., cxxx, 906 (1900). 



