142 G. F. BECKER KELATIOXS OF RADIOACTIVITY TO COSMOGONY 



shell; that no gradient higher than 1° Fahrenheit in 57 feet can be 

 accounted for by uniform radioactivit)^ and that average radioactivity is 

 probably a maximum where water interferes with the escape of radium 

 emanation. It also appears inevitable to ascribe a great part of the tem- 

 perature gradient of the earth to its original heat. The whole subject 

 would be much clearer if the origin of granite and the depth to which 

 it extends were kaown. 



That the granites are relatively superficial seems to be proved by their 

 low density, the relatively small silica content of most modern effusive 

 rocks, which must come to the surface through the granite, and finally 

 the absence of tridymite, together with the presence of much water in 

 the granular rocks. 



If, as Kelvin believed and as it seems most rational to believe, the 

 rocks of the earth's lithoid shell consolidated at their melting tempera- 

 tures, no granitic rocks can have formed until all the others had congealed. 

 It does not seem possible consistently to imagine a globe in the process of 

 cooling in which a layer of freshly consolidated basalt immediately under- 

 lies a sea of granitic magma; for the basalt surface would have a tem- 

 perature of 1170° plus an amount corresponding to the pressure at that 

 particular depth, while since the inversion point of trid}anite at atmos- 

 pheric pressure is some 800°, the lower surface of granitic sea would 

 have a temperature of, say, 800° plus an amount corresponding to the 

 pressure. A temperature gap or discontinuity would exist between the 

 two rocks. Under such conditions, if they could exist, the lower part of 

 the granite would rapidly pass over into rhyolitie or trachytic magmas 

 and the temperature curve would be inflected. On the other hand, I 

 find no difficulty in imagining the earth consolidating to a surface of 

 rhyolitie or trachytic composition similar perhaps to that wliicli the moon 

 now presents to our vision, but at a temperature of, say, 1300°. The 

 surface would be white hot and the atmosphere would contain, inter alia, 

 almost all of the water of the globe at a temperature nearly a thousand 

 degrees above its critical temperature. Surely when the sun solidifies 

 there will be such an epoch in its liistory, and the small density of Jupiter 

 points to the existence of such conditions there. As soon as the surface 

 of the earth solidified, its temperature must have sunk with great rapidity 

 to the critical point of water (365°), and at the enormous atmospheric 

 pressure of several himdred kilograms per square centimeter the intensely 

 heated aqueous vapors must have acted on the rh3'-olitic shell with extra- 

 ordinary energy. They were not only forced in imder pressure, but 

 sucked in by the shrinkage of the cooling lava. Here, then, were ideal 



