32 PKKSIDKNTIAL ADDUKSS SECTION B. 



the minerals in which uranium and thorium occur, radio-active 

 changes have been going on continuously for vast periods of 

 time. Hence, we should expect to tind in these minerals the 

 ultimate products of the atomic changes which have taken place, 

 and the older the geological formation to which the mineral 

 belongs, the greater ought to be the quantity of the products. 

 It was found that these minerals contained the gas helium, and 

 it is known that helium is one of the products of radio-active 

 sul)Stances. and is being evolved by them at an uniform rate. 

 If, then, calculations could be made of the amount of the stored- 

 up helium, and of the elements giving rise to it, from these data 

 the age of the minerals containing the radio-elements might be 

 estimated. 



There is always the prol)ability, however, that the helium 

 may escape from the containing mineral, which would naturally 

 invalidate any estimate of this kind. 



A variation of the above method has been applied to the 

 pleochroic halos surrounding minute ,i;rains of radio-active 

 minerals, occurring in the mica of certain granites, the halo 

 having been caused by the action of the helium produced. 



Lead is also supposed to be a i)roduct of the disintegration 

 of uranium and thorium, and various estimates have been made 

 on this basis as to the geological age of minerals of different 

 geological periods, containing uranium and associated with lead. 



The ages assigned to geological periods by Strutt and others 

 by these methods are considerablv greater than those arrived 

 at by other methods such as that from measuring the thickness 

 of the sediments and their rate of deposition, and amount to 

 several himdreds of millions of years for some of the earlier 

 geological systems. 



The whole subject, however, is exceedingly complicated, 

 and the present state of our knowledge of the exact nature and 

 results of radio-active changes is hardly sufficient to justify 

 thorough reliance on these methods as yet for estimating the 

 age of the earth ; though as cliemical and geological investigations 

 progress side by side, the radio-active method may become an 

 imjjortant line of research. 



The whole studv of radio-activitv in its bearing on geology 

 has, I am sure, a great future l)efore it, and in opening up to 

 our knowledge such vast and unexpected stores of energy within 

 the earth's crust, it gives a new significance to many problems, 

 such as volcanic action, magmatic movements, and differentiation, 

 and to the whole historv and evolution of the earth, and of the 

 structure and condition of its interior and of its crust. 



