November 27, 1908] 



SCIENCE 



741 



phenomena continuous with those already 

 dealt with, and although the conditional 

 element of a sufficient sedimentary accu- 

 mulation must remain speculative, the evi- 

 dence we possess is in favor of its existence. 



One of the most interesting outstanding 

 problems of deep-sea physiography is that 

 of the rates of accumulation of the several 

 sorts of deposit. In the ease of the more 

 rapidly collecting sediments there seems no 

 serious reason why the matter should not 

 be dealt with observationally. I hope it 

 may be accomplished in our time. For my 

 present purpose I should like 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 ap- 

 proach 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, per- 

 haps, 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 col- 

 lection of 5 millimeters 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 as- 

 signed to the deposition of these oceanic 

 beds; for if, following Professor Speneer,^^ 

 we ascribe their deposition to Eocene times, 

 a less definite time-interval is indicated; 



™e. J. G. S., XLVIII., p. 210. 

 ^-'Ibid., LVIII., pp. 354 ff. 



but the rate could hardly have been less 

 than 3 millimeters in a century. 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 

 everyTvhere the same and had so continued 

 during the past. If there is one thing cer- 

 tain, however, it is that the rates of accu- 

 mulation vary enormously. The 1,200 or 

 1,500 feet of chalk in the British Creta- 

 ceous, collected in one relatively brief 

 period of submergence, would alone estab- 

 lish this. Huxley inferred that the chalk 

 collected at the rate of one inch in a year. 

 SoUas showed that the rate was more prob- 

 ably one inch in forty years. Sir John 

 Murray has advanced evidence that in parts 

 of the Atlantic the cables become covered 

 with Glohigerina ooze at the rate of about 

 ten 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 aU the condi- 

 tions 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 com- 

 paratively 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 for- 

 ward by Murray. 



The islands arise from depths of between 

 1,000 and 2,000 fathoms. These areas are 

 covered with Glohigerina ooze having a 

 radio-activity of about 7 or 8. The deeper- 

 lying deposits around— red clay and radio- 

 larian ooze— show radio-activities up to and 

 over 50. From these no volcanic islands 

 spring. 



These facts, however, so far from being 



