MEASURING GEOLOGIC TIME— LANE 249 



tainties produced by Ithe "background effect" of unwanted particles 

 are still great. 



The beta ray is an electron. Its loss produces inconsiderable change 

 in the atomic weight but does change the place of the remaining element 

 in the periodic table, in other words changes it to another element of 

 practically the same atomic weight — a so-called isobar. For instance, 

 Hahn and Mattauch have recently discovered (and their results 

 are being confirmed in the chemical laboratory of the U. S. Geological 

 Survey) that the rubidium of which the atomic weight is 87 (about 

 25-28 percent of rubidium) changes to strontium of which the atomic 

 weight is also 87. Of this there is in strontium generally only 7.5 

 percent." 



Here we are brought face to face with two of the principal difficulties 

 in the measurement of the age of minerals and rocks by atomic dis- 

 integration. How do we know that the elements produced by disinte- 

 gration were not already there? If this is answered by saying that they 

 have a pecuHar atomic weight, then the problem arises, how to find 

 that weight, especially when one has but very small quantities. 



The third ray given off is the gamma ray, which is only an X-ray. 

 So far it has not been considered in geologic age determination. Yet 

 its work in penetrating the wrapping and darkening photographic 

 plates led to the discovery of atomic disintegration. But as such rays 

 are known to affect the formation and disintegration of elements they 

 may have to be taken into account sometimes, though preliminary 

 investigation would suggest that they are a very minor factor. Plate 

 1, figure 2, is a print of a polished piece of radium ore from Great Bear 

 Lake, Canada, made by its own light. 



The methods of getting ages of rocks and minerals have been mainly: 

 For the rocks, the amount of helium (alpha-ray product) which has 

 accumulated from the explosion of the uranium and thorium. The 

 lead is very small in comparison even with the few grams per ton of 

 ordinary lead present, and until recently it has been impossible even 

 to attempt to get the proportion of isotopes in the rock lead, as it 

 would involve getting about a gram of lead from a rock which con- 

 tained less than 22 grams in a ton. Of this only a small portion would 

 be the lead derived from the explosion of the uranium and thorium 

 atoms, perhaps 10 percent. 



For instance, Dr. Urry has computed for the "whinsill," a well- 

 known sheet of intrusive basalt that occurs in the British Carbonif- 

 erous, that the lead derived from the uranium and the thorium would 

 be about 0.049 gram per ton, whereas the actual amount of lead 

 present was 3 to 5 grams per ton. M. F. Conner found similar results 

 in concentrates which he analyzed. 



In molten rocks the hehum gas was mainly driven off at the time 



" Die Naturwissenschaften, vol. 25, no. 12, p. 189, 1937. 



