MEASURING GEOLOGIC TIME— LANE 251 



much of it there is by the quantity of Pb (204), the results will be 

 uncertain until there is better agreement as to what isotopes and how 

 much of them uranium contains, and how fast they change. In this 

 field G. P. Baxter and A. O. Nier are actively at work. 



Potassium is radioactive through givmg off a beta particle — an 

 electron. As recently as 1933, when Aston wrote Isotopes, although it 

 was recognized that the radioactivity was due to an isotope heavier 

 than the common K (29), it was supposed to be caused by K (41) 

 changing to Ca (41). It has since become apparent that the radio- 

 activity was due to a K (40), not before recognized. This makes no 

 longer apphcable some ingenious suggestions of A. Holmes, depending 

 on the recognition of the supposedly relatively rare isotope of calcium 

 Ca (41), The calcium isotope Ca (40) actually is very common. The 

 most that can be said now is that the ratio of calcium to potassium will 

 give a ma>dmum age to rock and mineral and to the earth, as A. Keith 

 Brewer has recently pointed out.^° Of course, in all minerals that 

 contain potassium there must be calcium, and if there is very little 

 calciiun, the maximum may be near the true age. 



But to use potassium or rubidium changes, accurate analysis of very 

 small quantities will be needed. The methods of quantitative spec- 

 troscopy must be developed. We may look for similar developments 

 with other elements. 



PLEOCHROIC HALOS 



We are entitled to infer that the rate of disintegration of atoms has 

 not markedly changed during geologic times from the fact that the 

 value of pleochroic halos around minute crystals containing a radio- 

 active element are about the same in the older rocks as in the younger 

 ones, or as in recent artificial preparations. The halos to which I 

 refer have been noticed since the days of Rosenbusch and have been 

 recently studied by D. E. Kerr-Lawson ^^ and G. H. Henderson ;^2 

 they are illustrated by plate 2, after Henderson's latest paper. They 

 depend upon the fact that the heUum buUets flying off when an atom 

 explodes change the color of the surrounding mineral, which acts as 

 host for the minute specks of zircon or allanite or whatever radio- 

 active mineral may be their source. In particular they ajffect the 

 iron of the mica, so that, whUe they are hardly \'isible when the mica 

 is dark (that is, the light is vibrating parallel to the cleavage), they 

 are much plainer when the mica is Ught (the hght which passes 

 through it is vibrating perpendicular to the cleavage). In other words, 

 the effect is much the same as that produced by these rays on a 

 photographic plate. 



'0 Radioactivity of potassium and geologic time, Science, vol. 86, No. 2226, p. 198, Aug. 27, 1937. 



" Kerr-Lawson, D. E., Pleochroic halos in biotite from near Murray Bay, P. Q., Univ. Toronto Studies, 

 Geol. Ser., No. 24, Contr. to Canadian Mineralogy, 1927. 



'2 Henderson, G. H., A quantitative study of pleochroic halos. — 1, Proc. Roy. Soc, ser. A, vol. 145, pp. 

 663-581, 591-698, 1934; vol. 158, pp. 199-211, 1937. 



