UEANIUM AND GEOLOGY — jOLf, B81 



oceanic beds; for if, following Professor Spencer," we ascribe their 

 deposition to Eocene times, a less definite time interval is indicated; 

 but the rate could hardly have been less than 3 millimeters in a cen- 

 tury. The site of the deposit was probably favorable to rapid 

 growth. 



We have already found a maximum limit to the average thickness 

 of true oceanic sediments, and such as would obtain over the ocean 

 floor if the rate of collection was everywhere the same and had so con- 

 tinued during the past. If there is one thing certain, however, it is 

 that the rates of accumulation vary enormously. The 1,200 or 1,500 

 feet of chalk in the British Cretaceous, collected in one relatively 

 brief period of submergence, w^ould alone establish this. Huxley 

 inferred that the chalk collected at the rate of 1 inch in a year. 

 Sollas showed that the rate was probably 1 inch in forty years. 

 Sir John Murray has advanced evidence that in parts of the Atlantic 

 the cables become covered with Globigerina ooze at the rate of about 

 10 inches in a century. Finally, then, we must take it that the fair 

 allowance of one-seventh of a mile may be withheld in some areas 

 and many times exceeded in others. 



Now, it is remarkable that all the conditions for rapid deposition 

 seem to prevail over those volcanic areas of the Pacific from which 

 ascend to the surface the coral islands — abundant pelagic life and 

 comparatively shallow depths. Indeed, I may remind you that the 

 very favorable nature of the conditions enter into the well-known 

 theory of coral-island formation put forward by Murra3^ 



The islands arise from depths of between 1,000 and 2,000 fathoms. 

 These areas are covered with Globigerina ooze having a radio-activity 

 of about 7 or 8. The deeper-lying deposits around — red clay and 

 radiolarian ooze — show radio-activities up to and over 50. From 

 these no volcanic islands spring. 



These facts, however, so far from being opposed to the view that 

 the radio-activity and crustal disturbance are connected, are in its 

 favor. For while those rich areas testify to the supply of radio- 

 active materials, the slow rate of growth prevailing deprives those 

 deposits of that characteristic depth which, if I may put it so, is of 

 more consequence than a high radio-activity. For the rise in tem- 

 perature at the base of a deposit, as already pointed out, is propor- 

 tional to the square of the thickness. In reality the dilution of the 

 supplies of uranium which reach the calcareous oozes flooring the 

 disturbed areas is a necessary condition for any effective radio- 

 thermal actions. 



It might appear futile to consider the matter any closer where so 

 little is known. But in order to give an idea of the quantities in- 



" Quart. Journ. Geol. Sue, Vol. LVIII, p. 354 et seq. 



