Qo6 The Age of Pleochroic Haloes. 



(d) The rate of formation of the experimental staining is 

 very fast compared with the rate of formation of the halo. 

 It does not seem likely that any significance is to be ascribed 

 to this fact. For the distribution of the alpha rays projected 

 into the mica in the experiment is actually a very sparse one. 

 Molecularly speaking they are finally separated by very 

 great distances ; so that the molecular groups, in which the 

 entering rays expend their energy, remain generally isolated 

 and undisturbed by succeeding rays. The interval of time 

 between succeeding rays would not, therefore, appear to 

 matter, and the resulting staining would be the same for each 

 ray, 



(e) Exposure to light does not appear to affect, one way 

 or the other, the staining produced in this mica by the alpha 

 ray. An experiment in which spots D and E were half 

 covered by lead foil and then exposed in bright sunlight, 

 concentrated by a lens, for several hours, revealed no effect 

 whatever. Strong heating obliterates haloes. In the experi- 

 ment strong heating was avoided. 



(/) Thorium is absent from these nuclei. No thorium 

 haloes have been found in this mica. 



Comparing the value indicated for the age of the Devonian 

 by the foregoing results with estimates arising from the 

 measurement of accumulated radioactive debris, it will be 

 seen that the halo indicates a somewhat excessive age. The 

 helium ratio gave Rutherford * 241 millions of years for a 

 specimen of fergusonite, probably from ancient rocks. 

 Strutt, by the same method, arrived at 150 millions of years 

 as the age of the Carboniferous Limestone and 710 millions 

 as the age of the Archaean f. Boltwood's results with the 

 lead ratio ranged from 246 to 1640 millions of vears ; the 

 higher being referable, probably, to Archaean rocks "f. 

 Becker, dealing with Pre-Cambrian minerals of the Llano 

 Group, found, by the lead ratio, from 1671 to 11,470 millions 

 of years; regarding his figures as a " reductio ad absurdum "§. 

 Boltwood criticised these results, questioning the reliability 

 of the material dealt with. Becker urged in reply that there 

 was no evidence of any effect on the lead ratio arising from 

 incipient alteration of the material. A. Holmes || investigated 

 the age of a Devonian syenite by the lead ratio, and found 

 this to be 370 millions of years ; he rejects, however, results 

 showing a greater age, on the ground that original lead was 

 probably present in sufficient quantity to falsify the result. 



* Phil. Mag., Oct. 1906, p. 368. 



t Proc. E. S., 1908-1910. % Am. J. of Sc, 1907. 



§ Bull. Geol. Hoc. Am., 1908. 



|| Proc. It. S., June 1911. 



