ORIGIN OF URANIUM AND THORIUM 127 



syenitic rocks. The pitchblende of ore veins is not crystallized and is in 

 all probability derived from the pegmatites by a secondary process. No 

 andesites, basalts, peridotites, etcetera, have been found to carry uranium 

 in weighable quantities, and there is much evidence that when freshly 

 erupted they are not considerably radioactive.^^ Old ejecta, especially 

 when porous, seem quite as active as average superficial rocks. That 

 mediosilicic and subsilicic rocks at or near the surface of the earth and 

 overlying or adjoining granitic rocks should acquire radioactivity from 

 emanations and solutions is entirely intelligible. 



Since uranium must have been stable when the minerals of which it is 

 a chief component crystallized, the genesis of uranium must be possible 

 under the conditions w^hich accompany the formation of pegmatites. 

 Now of these we know something. At a pressure of one atmosphere, 

 silica crystallizes as quartz only below 800° centrigrade, as was shown 

 by Messrs Day and Shepherd,^"' and Mr Miigge ^^ has recently demon- 

 strated by a new and brilliant method that the dihexahedral quartz of 

 pegmatites could form under ordinary pressure only above 570° centi- 

 grade. Facts presented by Mr Brogger also show that granitic and 

 syenitic rocks have formed in many cases at depths of only a few hun- 

 dred meters from the surface,^^ and in any case the granitic massives, 

 with their effusive modifications, being the least dense of rocks, are rela- 

 tively superficial. These data would indicate that at temperatures of 

 less than 1000° combined with pressures of a hundred or more kilograms 

 per square centimeter, uranium might form under proper chemical con- 

 ditions. It may therefore yet prove possible to combine lead and helium 

 in bombs, even though neither high temperatures nor great pressures 

 alone suffice to arrest the disintegration of radioactive substances. The 

 attempt should be made.*^ 



Eadioactivity and the Earth's Age 



Much the most important aspect of radioactivity is its bearing on the 

 age of the earth. It is very plausibly held that the observed increment 

 in underground temperatures must be due in part, and may be due in 



^ Nasini and Levi : Accad. Lincei. Rendiconti, vol. 15, 1906, p. 391, and O. Soarpa, 

 idem, vol. 16, 1907, p. 44. 



=0 American Journal of Science, vol. 22, 1906, p. 276. 



21 Neues Jahrbuch fiir Mineralogie, 1907. 



22 Die Mineralien der Syenitpegmatitgaenge, 1890, p. 224. 



^ Since this section of this paper was completed and communicated to colleagues, an 

 important paper on the evolution and devolution of the elements, by Messrs A. C. and 

 A. E. Jessup, has appeared in the Philosophical Magazine for January, 1908. In it 

 the periodic law has been employed in somewhat the same way as here. I print my 

 reflections without change, as an independent contribution to the subject. 



