364 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



or abstracted deposits at the bottom of the ocean. We are to dis- 

 tribute this quantity over its floor. If the rate of collection had 

 been uniform in every part of the ocean throughout geological time 

 a depth of about one-seventh of a mile (240 meters) of deposit would 

 cover the ocean bed. 



While I believe we can place considerable reliance on this approxi- 

 mation, we are less sure wdien we attempt an estimate of its mean 

 radio-activity. If we assume for it an average radio-activity similar 

 to that of Globigerina ooze, we find that the quantity of radium 

 involved must be considerably over a million tons. Apart from the 

 value which such estimates possess as presenting us with a perspective 

 view of the great phenomena we are dealing with, it will now be seen 

 that it supports the finding of the experiments on sedimentary rocks 

 and leads us to anticipate a real difference in the radio-activity of the 

 two classes of material. 



The sedimentary rocks. 



The radium content of those of detrital character is indicated in 

 the following sandstones, slates, and shales: 



Shales, sandstones, grits (10) 4.4 



Slates (Cambrian, Devonian) 4.7 



Mud from Amazon 3.2 



Some of the above are from deep borings in Carboniferous rocks 

 (the Balfour and Burnlip bores), "^ and from their nature, where not 

 actually of fresh-water origin, can owe little to oceanic radio-activity. 

 Many of the following belong to the class of precipitates, and there- 

 fore owe their uranium wholly or in part to oceanic source : 



Marsnpites clialk 4. 2 



Green sandstone 4. 9 



Green sand (dredged) 4.5 



Limestones and dolomites [Trenton, Carboniferous, Zeehstein, 



Lias, Solenhofen (7)] 4.1 



Keuper gypsum G. 9 



Coral roclc, Funafuti bore (4)^ 1.7 



Trias-Jura sediments, Simplon : Seventeen rocks of various 



cliaracters 6.9 



Mesozoic sediments, St. Gotbard : Nineteen i-ocks of various 



characters 4.2 



The general mean on 02 rocks is 4.7. 



Making some allowance for inicertainties in dealing with the 

 Simplon rocks, I think the experiment may be taken as pointing 

 to the result : 



" For these rocks, and for much other valuable material, I have to tlianic; Mr. 

 D. Tate, of the Scottish Geological Survey. 



^ For these I have to thank the trustees of the British Museum aud Mr. A. S. 

 Woodward, F. R. S. 



