MEASURING GEOLOGIC TIME— LANE 253 



one line of the lead spectrum can be broken by sufficient magnification.^^ 

 Aston ^' obtained the proportion of isotope, by using a mass spectro- 

 graph in which the atoms shot off from a target pass between magnetic 

 poles which deflect the heavier atoms from their course less than the 

 lighter ones. He registers the atoms on the photographic plate 

 which they strike. In Dempster's ^^ improved apparatus the atoms 

 are exposed to a continuous field which swings them around in a 

 great arc and gets them farther apart. But estimations of the 

 strength of lines are limited in accuracy and are lil^ely to be interfered 

 with by lines for other compounds. A. O. C. Nier's present appar- 

 atus for the cumulative record of the effect of these atoms promises 

 greater accuracy and uses smaller quantities. 



A difficulty already mentioned is in getting the quantity required 

 to make an analysis, a separation of isotopes, and an age determina- 

 tion. Still the quantity needed has steadily diminshed until it is 

 surprising how complete an analysis F. Hecht and L. Kroupa, in 

 Vienna, can make with only 50 milligrams of uraninite, about the 

 weight of a postage stamp. ^^ Even with their methods, minerals 

 such as zircon and allanite that have only a little uranium or thorium, 

 often only 2 or 3 percent, offer difficulties, and if the lead in them 

 is only a hundredth part of that, which is the case for Tertiary or 

 younger minerals, it can readily be seen how refined and accurate 

 methods of analysis must be. Moreover, recent work on successive 

 zones of the same crystal have shown a change in composition due 

 to a transfer of material. In comparatively fresh material, that 

 appears to be satisfactory, the uranium seems to be the more soluble 

 as a uranous compound and at least films of secondary uranic yellow 

 compounds are found in radioactive deposits generally. Yet these 

 very studies tend to show that such changes are not often over 10 

 percent, and that the ages are in a general way correct. 



ROCK AGES 



As has been said, in rocks there is so much ordinary lead that 

 until the separation of small quantities of isotopes of lead and the 

 identification of ordinary lead, say by the 204 isotope, can be safely 

 accomplished with small quantities, no reliable age determinations 

 can be obtained. But helium has not the same difficulty, if we take 

 compact rocks containing no air and which have been molten. Melt- 

 ing drives out most of the helium and other gases as well. Helium 

 being a gas, is likely to escape, and until a few years ago it was gen- 

 s' Rose, J. L., and Stranathan, R. K., Geologic time and atomic composition of radiogenic lead. Phys. 

 Rev., vol. 60, pp. 792-796, 1936. 

 3« Aston, F. W., Mass spectra and isotopes. Longmans, 1933. 



" Hecht, F., and Kroupa, L., Die Bedeutung der quantitative Mikroanalyse radioaktive Mineralien 

 fiir die geologische Zeitmessung. Zeitschr. Chem., vol. 108, pp. 82-103, 1936. 



