380 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



fresh accession of snow to the firn, every passing cloud contributing 

 its small addition to the 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 positive or negative move- 

 ments of the strand which cover and uncover the continents, and have 

 swayed the entire course of evolution of terrestrial life. 



Oceanic deposits. 



The displacements of the crust which we have been considering are 

 now known 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 for- 

 mation of those numerous volcanic cones upon which the coral island 

 finds foundation. Here there are plainly 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 con- 

 sidered, that we have here phenomena continuous with those already 

 dealt with, and although the conditional element of a sufficient sedi- 

 mentary accumulation must remain speculative, the evidence we pos- 

 sess 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 case of the more rapidly collecting sediments 

 there seems to be no serious reason why the matter should not be dealt 

 with observationally. I hope it maj^ 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 noncalcareous 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 geolog}^ 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 dur- 

 ing 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 esti- 

 mates gives a rate of collection 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 assigned to the deposition of these 



« Quart. Journ. Geol. Soc, Vol. XLA^III, p. 210. 



