UEANIUM AND GEOLOGY JOLY. 379 



As regards the claims which such figures have upon our considera- 

 tion, my assumptions as to thickness and radio-activity of the spe- 

 cially rich surface layer are doubtless capable of considerable amend- 

 ment. It will be found, however, that the assumed factors may be 

 supposed to vary considerably, and yet the final results prove such as, 

 I believe, can not be ignored. Indeed those who are in the way of 

 making such calculations, and who enter into the question, will find 

 that my assumptions are not specially favorable, but are, in fact, 

 made on quite independent grounds. Again, a certain class of effects 

 has been entirely left out of account, effects which will go toward 

 enhancing, and in some cases greatly enhancing, the radio-thermal 

 activity. I refer to the thickening of the crust arising from tangen- 

 tial pressure, and, at a later stage, the piling up and overthrusting 

 of mountain-building materials. In such cases the temperature of 

 the deeper parts of the thickened mass must still further rise under* 

 the influence of the contained radium. These effects only take place, 

 indeed, after yielding has commenced, but they add to the element of 

 instability which the presence of the accumulated radio-active de- 

 posits occasions, and doubtless increase thermal metamorphic actions 

 in the deeper sediments and result in the refusion of rocks in the 

 upper part of the crust.* 



The effect of accumulated sediment is thus necessarily a reduction 

 in the thickness of that part of the upper crust which is capable of 

 resisting a compressive stress. Over the area of sedunentation, and 

 more especially along the deepest line of synclinal depression, the 

 crust of the globe for a period assumes the properties belonging to an 

 earlier age, yielding up some of the rigidity which was the slow in- 

 heritance of secular cooling. Along this area of weakness — from its 

 mode of formation generally much elongated in form — the stressed 

 crust for many hundreds, perhaps thousands, of miles finds relief, and 

 flexure takes place in the only possible direction — that is, on the whole 

 upward. In this way the prolonged anticline bearing upward on its 

 crest the whole mass of deposits is formed, and so are borne the moun- 

 tain ranges in all their diversity of form and structure. 



We have in these effects an intervention of radium in the dynamics 

 of the earth's crust, which must have influenced the entire history of 

 our globe, and which, I believe, affords a key to the instability of the 

 crust; for after the events of mountain building are accomplished, 

 stability is not attained, but in presence of the forces of denudation 

 the whole sequence of events has to conmience over again. Every 



"Prof. C. Schmidt (Basel) has recently given reasons for the view that the 

 Mesozoic schists of the Simplon at the period of their folding were probably 

 from 15,000 to 20,000 meters beneath the surface (Ec. Geol. Helvetia;, Vol. IX, 

 No. 4, p. 590). As another instance consider the compression of the Laramide 

 range (Dawson, Bull. Oeol. Soc. Amer., Vol. XII, p. 87). 



