September io, igo8] 



NA TURE 



465 



torrent, assists to spread out once more on the floor of 

 the ocean the heat-producing , substance. With this 

 rhythmic succession of events appear bound up those posi- 

 tive or negative movements of the strand which cover and 

 uncover the continents, and have swayed the entire course 

 of evohition of terrestrial life. 



Oceanic Deposits. — The displacements of the crust which 

 we have been considering are now Itnown to be by no 

 means confined to the oceanic margins. The evidence 

 seems conclusive that long-continued movements have been 

 in progress over certain areas of the sea floor, attended 

 with the formation of those numerous volcanic cones upon 

 which the coral island finds foundation. Here there are 

 plainlv revealed signs of instability and yielding of the 

 crust (although, perhaps, of minor intensity) such as are 

 associated with the greater movements which terminate 

 in mountain building. I think it will be found, when 

 the facts are considered, that we have here phenomena 

 continuous with those already dealt with, and although 

 the conditional element of a sufficient sedimentary accumu- 

 lation must remain speculative, the evidence we possess 

 is in favour of its existence. 



One of the most interesting outstanding problems of 

 deep-sea pljysiography is that of the rates of accumula- 

 tion of the several sorts of deposit. In the case of the 

 more rapidly collecting sediments there seems no serious 

 reason why the matter should not be dealt with observa- 

 tionallv. t hope it may be accomplished in our time. 

 For my present purpose I should lilce to know what 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 geologv 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 moan ol 

 these estimates Sfives a rate of collection of 5 millimetres 

 in a century. This sounds a very slow rate of growth, 

 hut 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 Prof. 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 millimetres in a century. The site of the 

 deposit was probablv 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 collection \\"as 

 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 1200 or 

 1500 feet of chalk in the British Cretaceous, collected in 

 one relatively brief period of submergence, would alone 

 establisli this. Huxley inferred that the chalk collected at 

 the rate of i inch in a year. Sollas showed that the rate 

 was more probably i Inch in forty years. .Sir John Murray 

 has advanced evidence that in parts of tlie 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 mav remind you that the verv favourable nature 

 of the conditions enter into the well-known theory of 

 coral Island formation put forward by Murray. 



The islands arise from depths of between 1000 and 2000 



' O. T.G.S . xlviii., p. 2Tn. 



- IhW., Iviii., p. 35 J et seq. 



NO. 2028, vol . 78] 



fathoms. These areas are covered with Globigerina ooze 

 liaving a radio-activity of about 7 or 8. The deeper-lying 

 deposits around — red clay and radiolarian ooze — show 

 radio-activities up to and more than 50. From these no 

 volcanic islands spring. 



ihese facts, however, so far from being opposed to the 

 view that the radio-activity and crustal disturbance are 

 connected, are in its favour. 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 

 temperature 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 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 involved I may state that, if my calcula- 

 tions 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 dis- 

 cussing the subject of mountain building, occasion a rise 

 of the deeper isogeotherms 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 con- 

 sideration the question of the time which must elapse in 

 order that the final temperature-rise in the sediments must 

 be attained. The question we have to answer is : Will 

 the rate of rise of temperature due to radium keep pace 

 with the rate of deposition, or must a certain period elapse 

 after the sedimentation is completed to any particular 

 depth, before the basal temperature proper to the depth 

 is attained? 



The answer appears to be, on an approximate method 

 of solution, that for rates of deposition such as we believe 

 to prevail in terrigenous deposits — even so great as i foot 

 in a century, and up to depths of accumulation of 10 kilo- 

 metres and even more — the heating waits on the sed'- 

 mentatlon. Or, in other words, there is thermal equil- 

 ibrium at every stage of growth of the deposit ; and the 

 basal temperature due to radio-active heating may at any 

 instant be computed by the conductivity equation. For 

 accumulations of still greater magnitude the final and 

 maximum temperature appears to lag somewhat behind 

 the rate of deposition. 



From this we may infer that the great events of geo- 

 logical historv have primarily waited upon the rates of 

 denudation and sedimentation. The sites of the terri- 

 genous deposits and the marginal oceanic precipitates have 

 rnanv times been convulsed during geological time^ because 

 the rates of accumulation thereon have been rapid. The 

 comparative tranquillity of the ocean floor far removed from 

 the land may be referred to the absence of the inciting 

 cause of disturbance. If. however, favourable conditions 

 prevail for such a period that the local accumulations attain 

 the sufficient depth, here, too, the stability must break 

 down and the permanency be interrupted. 



Upheaval of the ocean' floor, owing to the laws of deep- 

 sea sedimentation, should be attended with effects 

 accelerative of deposition — a fact which may not be with- 

 out influence. But although ultimately sharing the in- 

 stabilltv of the continental margins, the cycle of change 

 is tuned to a slower periodicity From the operation of 

 these causes, possibly, have come and gone those continents 

 which manv believe' to have once replaced the wastes of 

 the oceans, and which with all their wealth of life and 

 scenic beautv have disappeared so completely that they 

 scarce have left a wreck behind. But those forgotten 

 worlds mav be again restored. The rolled-up crust of the 

 earth is still rich In energy borrowed from earlier times, 

 and the slow but mighty influences of denudation and 

 deposition are for ever at work. And so. perchance, in 

 some remote age the vanished GondwAna Land, the lost 

 Atlantis, mav once again arise, the seeds of resurrection 

 even now being- sown upon their graves from the endless 

 harvests of pelagic life. 



