694 TRANSACTIONS OF SECTION C. 



For my present purpose I should like to know wliat may or may not be assumed 

 in discussing the accumulation of radio-active sediments on the ocean floor. 



As regards the rate of collection of the non-calcareous deposits, the nearest 

 approach to an estimate is, I think, to be obtained from the exposed oceanic deposits 

 of Barbados. In the well-known paper of Jukes Brown and Harrison* on the 

 geology of that island, it is shown that the siliceous radiolarian earths and red 

 clays aggregate to a thickness of about 300 feet. These materials are true 

 oceanic deposits, devoid of terrigenous substances. They collected very probably 

 during Pliocene and, perhaps, part of Pleistocene times. Now, there is evidence 

 to lead us to date the beginning of the Pliocene as anything from one million to 

 three million years ago. The mean of these estimates gives a rate of collection 

 of 5 millimetres in a century. This sounds a very slow rate of growth, but it is 

 too fast to be assumed for such deposits generally. More recent observations 

 might, indeed, lead us to lengthen the period assigned to the deposition of these 

 oceanic beds ; for if, following Professor Spencer,- we ascribe their deposition to 

 I'jocene times, a less definite time-interval is indicated ; but the rate could hardly 

 have been less than three millimetres in a century. The site of the deposit was 

 probably favourable 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 col- 

 lection was everywhere the same and had so continued 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 rela- 

 tively brief period of submergence, would alone establish this. Huxley inferred 

 that the chalk collected at the rate of one inch in a year. SoUas showed that the 

 rate was more probably one inch in forty years. Sir John Murray has advanced 

 evidence that in parts of the Atlantic the cables become covered with Globigeriua 

 ooze at the rate of about 10 inches in a century. Finally', tlieu, we must take it 

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

 and man)' times exceeded in others. 



Now it is remarkable that all the conditions for rapid deposition seem to pre- 

 vail over those volcanic areas of the I'acific from which ascend to the surface the 

 coral islands— abundant pelagic life and comparatively shallow depths. Indeed, 

 I may remind you that the very favourable natui-e of the conditions enter into the 

 well-known theory of coral island formation put forward by Murray. 



The islands arise from depths of between 1,000 and :i,000 fathoms. These 

 nreas are covered with Globigerina ooze having a radio-activity of about 7 or 8. 

 U'he deeper-lying deposits around — red clay and radiolarian ooze — show radio- 

 activities up to and over 60. 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 connecteJ, are in its favour. For while those 

 rich areas testify to the supply of radio-acive 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-activit}'. For the rise in tem- 

 perature at the base of a deposit, as already pointed out, is proportional 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 efl^ective 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 involved I may state that, 

 if my calculations are correct, a rate of deposit comparable with that of the chalk 

 prevailing for ten million years would, on assumptions similar to those already 

 explained when discussing the subject of mountain building, occasion a rise of the 

 deeper isogeothnrms by from 20 to 30 per cent, of their probable normal depth. 



In making these deductions as to the influence of radium in sedimentary 

 deposits, I have so far left out of consideration the question of the time which mus . 



» Q.J.G.S., xlviii. p. 210. « Jbid., Iviii. p. 354 et seq. 



