Age of Pleochroic Haloes. 651 



The nuclei are in most cases possessed o£ all the optical 

 appearances of zircon ; they are transparent, colourless, and 

 of high refractive index. An attempt to determine the 

 index by use o£ refracting liquids failed. In the cases tried 

 the nuclei were probably buried in the mica. There is no 

 doubt that the index is considerably higher than that of the 

 mica. Some may be orthite ; some, possibly, brookite. 

 Many of the nuclei in certain parts of the granite show no 

 definite crystalline shape, but are granular in appearance. 

 In other places they are all in the form of well- developed 

 dimetric crystals placed with their long axis parallel with 

 the cleavage of the mica. These definite crystals are beyond 

 doubt zircons. They seem somewhat less radioactive than 

 the irregular nuclei. Again, some nuclei are plate-like 

 crystals, rectangular in shape and developed in the plane of 

 cleavage. It is not practicable to ascertain the volume of 

 these as their thickness cannot be measured. Probably their 

 mass is often comparable with that of the smallest nuclei. 



It would be very desirable to ascertain directly the radium 

 content of these nuclei. An attempt was made by reducing 

 a considerable quantity of the mica to a very fine powder, and 

 then effecting separations according to density by use of 

 heavy liquids, to isolate the zircons. But nothing of a 

 minuteness comparable with the nuclei was extracted. It is 

 probable that their removal from the mica is difficult. It is 

 not safe to use solvents, for there is evidence that zircons 

 may carry their radioactive constituents as a surface accre- 

 tion and always as a purely foreign admixture. We could 

 not then be sure that a solvent which acted on the mica did 

 not also remove the radioactive constituents of the zircon, 

 thus falsifying the result in the very direction which is most 

 to be avoided. 



It would appear that, for the present, we can only estimate 

 the uranium content by analogy with known results obtained 

 on large zircons. Strutt * has given results on 14 zircons of 

 very different geological ages. The quantity of U 3 8 in 

 grains per gram of zircon varies from 1*4 X 10~ 4 to 75'3 x 10~ 4 . 

 The highest result is very exceptional, the next highest being 

 38xl0~ 4 , and this is from the exceptionally radioactive 

 lavas of Vesuvius. After this 13\3xl0~ 4 is the highest. 

 The mean of all is 14xl0~ 4 . In other experiments Strutt 

 found 865 x 10~ 12 gram of radium per gram in zircons | • 

 This involves less than 40 x 10~ 4 gram per gram of U 3 8 . 



* Proc. R. S., A, Jxxxiii. p. 298. 

 t Proc. R. S., A, lxxviii. p. 150. 



