UEANIUM AND GEOLOGY JOLY. 377 



in a trough 40,000 feet deep, filling the trough to the brim. It thus appears 

 that epochs of mountain making have occurred only after long intervals of 

 quiet in the history of a continent. 



The generally observed fact that the deposition of sediments in 

 some manner involves their ultimate upheaval has at various times 

 led to explanations being offered. I think I am safe in saying that 

 although the primary factor, the compressive stress in a crust which 

 has ceased to fit the shrinking world within it, has probably been 

 correctly inferred, no satisfactory explanation of the connection be- 

 tween sedimentation and upheaval has been advanced. The mere 

 shifting upw^ard of the isogeotherms into the deposits, advanced as 

 a source of local loss of rigidity by Babbage and Herschel, need not 

 involve any such loss so long as the original distance of the isogeo- 

 therms from the surface is preserved. 



We see in everj^ case that only after great thicknesses of sediments 

 have accumulated is the upheaval brought about. This is a feature 

 which must enter as an essential condition into Avhatever explanations 

 we propose to offer. 



Following up the idea that the sought-for instability is referable 

 to radio-thermal actions, we will now endeavor to form some approxi- 

 mate estimate of the rise of temperature which will be brought about 

 at the base of such great sedimentary accumulations as have gone 

 toward mountain building, due to the radium distributed throughout 

 the materials. 



The temperature at the base of a feebly radio-active layer, such as 

 an accumulation of sediments, is defined in part by radio-active en- 

 ergy, in part by its position relative to the normal isogeotherms, 

 whether these latter are in turn due to or influenced by radio-thermal 

 supplies or not. It is convenient and, I think, allowable to consider 

 these two effects separately and deal with them as if they Avere inde- 

 pendent, the resultant state being obtained by their summation. 



In dealing Avith the rise of temperature at the base of a radio-active 

 layer we arrive at an expression which involves the square of the 

 depth. This is a very important feature in the investigation, and 

 leads to the result that for a gi^^en amount of racliimi diffuse dis- 

 tribution through a great depth of deposit gives rise to a higher basal 

 temperature than a more concentrated distribution in a shallower 

 layer. 



But this will not give us the whole effect of such a deposit. An- 

 other and an important factor has to be taken into account. We 

 haA^e seen that the immediate surface rocks are of such richness in 

 radium as to preclude the idea that a similar richness can extend many 

 miles inward. 



Now, it is upon this surface layer that the sediments are piled, and 

 as they grow in thickness this original layer is depressed deeper and 

 deeper, yielding under the load, until at length it is buried to the full 



