1022] on The Age of the Earth 499 



with the question at issue, the external radius of the Eocene halo- 

 ring — no allowance being made for the nuclear radius — is 0*0135 mm. 

 The same observers obtained for the Devonian halo 0*0146 mm. — 

 without allowance for the nucleus. The nuclear correction, as I have 

 said, would have increased the discrepancy, but the correction is a 

 difficult one. There is no reason to believe that more than 1 per 

 cent, of this difference can be ascribed to the chemical composition 

 or density of the micas, both of which have been investigated. 



Still more recently I have found these primary ring-haloes in the 

 micas of Arendal and Yitterby, which are said to be of Archaean age, 

 and which are certainly extremely ancient. These haloes appear to 

 possess a radial dimension of 0*0160 mm., or a little less. Here, 

 again, the nature of the mica does not appear to be responsible. 

 According to these measurements it would appear that the radius of 

 the Eocene halo-ring must be increased by about 7 per cent, to 

 attain the size of the Devonian halo-ring, and that this is, in radial 

 dimension, about 10 per cent, smaller than the Archaean. It would 

 seem as if we might determine a geological chronology on the 

 dimensions of these halo-rings ! 



The foregoing results, if confirmed, would give strong support to 

 the view that some factor, variable over geological time, had affected 

 the ranges and periods of certain elements concerned in building up 

 the uranium halo. However, too much stress must not be placed on 

 these measurements till they are confirmed by haloes in yet other 

 micas. Pending further investigations, I return to the fact that the 

 uranium halo of Devonian age does not conform to the ionisation 

 curve of the uranium family as determined on present-day measure- 

 ments. Serious discrepancy seems confined to the shorter ranges, 

 more especially with that primary range which is most influential in 

 determining the rate of production of uranium lead. 



We do not appear to be in a position to deny the possibility that 

 uranium 1 may have slowed down in its rate of decay over geological 

 time. Such laboratory observations as can be extended to the case 

 of short-lived elements would not, probably, shed any light on the 

 matter. It is a possibility long a^o suggested by Rutherford. But 

 if this is the explanation we must admit that in the case of thorium 

 any corresponding effect must have been much smaller. On the 

 whole the former influence of one or more isotopes of uranium — 

 which possibly may almost have disappeared— seems the more pro- 

 bable explanation. Hypothetical isotopes of uranium have been 

 invoked by highly competent authorities to meet the difficulties 

 affecting the ionisation accounts of the uranium family of elements. 

 Boltwood suggests as " not impossible " that what we now call 

 uranium consists of three radio-elements : a parent element and two 

 isotopic products all emitting a-rays.* In 1917 A. Piccard put 



* Phil. Mag., 6 S. vol. xl. p. 50, 1920. 



