MEASUKEME^s'TS BASED ON RADIOACJTIVITY 845 



to quote freely from a recent article by Arthur Holmes,^^ who by his re- 

 search has contributed much to this subject. The following three topics 

 are quoted entire, as they give in brief space the essentials of the methods 

 and the original article will be seen by but few American geologists. The 

 importance of tlie whole subject from a geological standpoint is such that 

 this presentation should be given in an American geological publication. 



ACCUMULATION OF HELIUM ^^ 



The proof is complete that helium is a stable product of the decay of 

 both uranium and thorium. It is always present in radioactive minerals. 

 In 1903 Eamsay and Soddy demonstrated its genesis directly from radium 

 emanation. In 1910 Strutt went further and measured its rate of for- 

 mation in pitchblende and thorianite. His results show that one gram 

 of uranium^*^ generates helium at the rate of 1 cc. in 10 million years, 

 and that one gram of thorium®^ generates helium at the rate 1 cc. in 30 

 million years. These figures have been verified quite independently by 

 directly counting the a-particles — helium atoms — emitted by various 

 members of the radioactive families. Strutt found that the thorianite 

 which he used in his experiments originally contained 280,000,000 times 

 more helium than the amount which the same mineral could generate in 

 a year. The inference is clear. That large volume of helium must have 

 taken 280,000,000 years to accumulate. 



Before going a step further and asserting that this is also the age of 

 the mineral, two questions must be asked and answered : 



{a) Was there any helium present in the mineral at the time of its 

 crystallization, or has it been generated since ? 



{!)) Can we be sure that no helium has escaped from the mineral 

 during the period which has elapsed since its crystallization ? -, 



In the first case it is only necessary to notice that ordinary rocks and 

 minerals contain only the slightest traces of helium, and that what little 

 there is can be fully explained by the small quantities of the radio- 

 elements which are always present. If any strongly radioactive mineral 

 in an igneous rock did contain a little helium as an original impurity, its 

 amount would soon become negligible in proportion to the large quantities 

 suljsequently generated. 



The second question is not so easily disposed of. It has been experi- 

 mentally demonstrated that as soon as a mineral is exposed to the air it 



^ Radioactivity and the measurement of geological time. Proc. of the Geologists' 

 Asso., vol. xxvi, part 5, 1915, pp. 289-309. The following three topics are quoted from 

 pp. 294-300, 303-309, with some omissions. 



^ By Arthur Holmes. 



"0 When in equilibrium with all its daughter elements. 



