THE BIRTH-TIME OF THE WORLD 49 



measurements to a wonderful degree of refinement, found the 

 following ages for mineral substances originating in different 

 geological ages : 



Oligocene 8'4 millions of years. 



Eocene 31 „ „ „ 



Lower Carboniferous . . .150 „ „ „ 



Archaean . . . . . .710 „ „ ,, 



Periods of time much less than, and very inconsistent with, 

 these were also found. The lower results are, however, easily 

 explained if we assume that the helium — which is a gas under 

 prevailing conditions — escapes in many cases slowly from the 

 mineral. 



Another product of radioactive origin is lead. The sugges- 

 tion that this substance might be made available to determine 

 the age of the earth also originated with Rutherford. We are 

 at least assured that this element cannot escape by gaseous 

 diffusion from the minerals. Boltwood's results on the amounts 

 of lead contained in minerals of various ages, taken in conjunc- 

 tion with the amount of uranium or parent substance present, 

 afforded ages rising to 1,640 millions of years for Archaean and 

 1,200 millions for Algonkian time. Becker, applying the same 

 method, obtained results rising to quite incredible periods : from 

 1,671 to 11,470 millions of years. Becker maintained that 

 original lead rendered the determinations indefinite. The more 

 recent results of Mr. A. Holmes support the conclusion that 

 " original " lead may be present and may completely falsify 

 results derived from minerals of low radioactivity in which the 

 derived lead would be small in amount. By rejecting such 

 results as appeared to be of this character, he arrives at 

 370 millions of years as the age of the Devonian. 



I must now describe a very recent method of estimating the 

 age of the earth. There are, in certain rock-forming minerals, 

 colour-changes set up by radioactive effects. The minute and 

 curious marks so produced are known as haloes ; for they 

 surround, in ring-like forms, minute particles of included sub- 

 stances which contain radioactive elements. It is now well 

 known how these haloes are formed. The particle in the centre 

 of the halo contains uranium or thorium, and, necessarily, along 

 with the parent substance, the various elements derived from it. 

 In the process of transformation giving rise to these several 

 derived substances, atoms of helium, projected with great 

 4 



