848 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



density of the matter through which it is passing. The ranges in air 

 liave heen carefully measured, and the corresponding values for biotite 

 can easily be calculated. The largest sphere from the uranium family 

 should have a diameter of 1/30 mm., and that from the thorium family 

 a diameter of 1/25 mm. Now the maximum diameters of halos have 

 been carefully measured, and surely enough there are two types of halos 

 with exactly the diameters theoretically to be expected. Clinching the 

 argument by practical demonstration, Eutherford has made artificial 

 lialos in glass and in flakes of biotite. 



The color of a halo depends on two factors: (a) the radioactivity of 

 the central inclusion, and (h) the age of the mineral in which it occurs. 

 As the helium atoms accumulate in their spherical shells the color gradu- 

 ally deepens. It is interesting and significant to notice that halos are 

 found only in fairly old rocks. Granites of Tertiary age are practically 

 free from perceptible halos, whereas in biotite granites of Permian and 

 Devonian age they are frequently well developed. 



Eecently Joly and Eutherford have attempted to estimate the age of the 

 biotite halos in the Leinster Granite of County Carlow (Lower Devonian) 

 by means of their color. Artificial halos were made in the same biotite 

 under controlled conditions which made it possible to measure accurately 

 both the radioactivity and the time for which it acted. Artificial halos 

 are made with a high degree of radioactivity acting for a short time. 

 Natural halos are made with a low degree of radioactivity acting for a 

 long time. Suppose that a natural halo is found with exactly the same 

 depth of color as an artificial one. If, now, the radioactivity of the central 

 inclusion can be estimated, then the time for which it must have acted 

 can easily be calculated. The period of time so found would, of course, 

 be a measure of the age of the halo and of the mineral in which it had 

 developed. Once the natural halo has been matched in color with an 

 artificial halo, the only problem that remains is the estimation of the 

 radioactivity of the zircon at the center of the natural halo. 



The volume of the inclusion can be determined by working with a 

 high-power microscope. Unfortunately it is impossible to separate the 

 zircons and measure their uranium contents directly. The difficulty is 

 surmounted by an appeal to probable limits. No zircons are known to 

 contain more than 10 per cent of uranium. If this figure is employed 

 throughout for all the halos examined, some of them, the paler ones, will 

 give ages that are obviously too low, while others, the darker ones, will 

 give ages that approach the correct figure, and may perhaps even exceed 

 it. The values actually arrived at by Joly and Eutherford varied l)etween 

 50 and 470 million years, the larger figure being certainly nearer the 



