198 ANNUAL REPOET SMITHSONIAN INSTITUTION, 19 3 2 



years as a possible upper limit, but this result is more interesting 

 than conclusive. 



Kovarik treats also in brief the possibility of age determination 

 based on the helium that is formed by radioactive decay and discusses 

 the features known as pleochroic halos, or radiohalos, as Hirschi 

 argues they should more appropriately be called. Both these mat- 

 ters, however, are considered in ampler detail by Holmes in Part IV.^ 



In Part IV ^ Holmes deals exhaustively with the application of 

 radioactivity to the measurement of geologic time. Practically all 

 the available information — geologic, mineralogic, and chemical — has 

 been assembled and critically evaluated. It will be seen that this 

 information has already attained an astonishing bulk. It is clear 

 that the field bristles with problems. Much of the work already 

 done is only of suggestive value, owing to the lack of correlation be- 

 tw^een the geologic, analytical, and atomic-weight investigations. 



Probably the phenomena of radioactivity that come oftenest to the 

 attention of the geologist are the pleochroic halos. They are here 

 discussed in detail and the problems that they evoke are fully dis- 

 cussed. Although the halos are known to be effects of alpha particles 

 ejected from radioactive inclosures in certain minerals, their actual 

 mode of growth is not fully understood. Joly believes that they 

 are of centripetal growth, but Schilling, working on the superbly 

 developed halos in the fluorite of Wolsendorf, has demonstrated be- 

 yond much doubt that they are of centrifugal growth. Some halos 

 have been formed by the cumulative effect of alpha particles ejected 

 at the rate of but one a year. In pleochroic halos we have a means 

 of detecting radioactivity ten million times more sensitive than elec- 

 trical methods. Halos can not be used to determine the age of 

 minerals, although Rutherford and Joly tried to do this for the 

 biotite in a Devonian granite. But they had to guess the value of 

 one factor in their calculations and, as has well been said, one might 

 therefore as well guess the final answer. The fact of great import to 

 the theory of age determination based on radioactivity that emerges 

 from the study of the halos is that the rate of disintegration of 

 " uranium " and " thorium " was the same in pre-Cambrian time as 

 it is now. This reassuring conclusion on the constancy of the rate 

 of disintegration of uranium during geologic time is particularly 

 the result of the work of Kerr-Lawson, who developed an improved 

 technique in his investigation of the halos in biotite from a pre- 

 Cambrian pegmatite in Ontario. 



The results of Holmes's world-wide survey of the data on age 

 determination by atomic disintegration are summarized in Table 

 LXXXII.^ The lead ratios are listed in numerical order, and the 



' Of the bulletin referred to in footnote 1 on the first page of this article. 



