l'SKStnE^'TIAL ADDRESS. 68^ 



■floor. If tlie rate of collection had been iiuiforin iu every part of thd oceau 

 throughout geological time, a depth of about one-seventh of a mile (240 metres) 

 of deposit would cover the ocean bed. 



\Vhile, I believe, we can place considerable reliance on this approximation, we 

 are less sure when we attempt an estimate of its mean radio-activity. If we assume 

 lor it an average radio-activity similar to that of Globigerina Ooze, we tind that 

 the quantity of radium involved must be considerably over a million tons. Apart 

 from the value which such estimates possess as presentiog us with a perspective 

 view of the great phenomena we are dealing witb, 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 f those of detrital character 

 is indicated in the toUowing sandstones, slates, and shales : — ■ 



Shales, sandstones, grits (10) i-4 



Slates (Cambrian, Devonian) 47 



Mud from Amazon .......... 3'iJ 



Some of the above are from deep borings in Cai-boniferous 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 therefore owe their uranium wholly or in part to oceanic source : — 



Marsupites chalk 4-2 



Green sandstone .......... 49 



Green sand (dredged) ......... 4o 



Limestones and dolomites [Trenton, Carboniferous, Zechstein, 



Lias, Solenhofen (7)] 41 



Keuper gypsum .......... 69 



Coral rock, Funafuti bore (4)- ....... 1'7 



Trias- Jura sediments, Simplon : 17 rocks of various characters . 6-9 



Mesozoic sediments, St. Gothard : 19 rocks of various characters . 4-2 



The general mean on sixty-two rocks is 47. 



Making some allowance for uncertainties in dealing with the Simplon rocks, 

 I think the experiments may be taken as pointing to the result : — ■ 



Igneous rocks from 5 to 6. 



Sedimentary rocks from 4 to 5. 



If our estimate of oceanic radium be applied to the account of the sedimentary 

 rocks in a manner which will be understood from what 1 have already endeavoured 

 to convey, there will be found to exist a fair degree of harmony between the great 

 quantities wtiich we have found to be in the sediments of the ocean and the 

 impoverishment of the sediments which the experiments appear to indicate. 



In all these results fresh and uuwdathered material has been used. The 

 sand of the Arabian desert gave me but 0'4. Similarly low results have been 

 found by others for soils and such materials. These are not to be included when 

 we seek the radio-activity of the rocks. 



As regards generally my experiments on the radium-content of the rocks, 

 I cannot say with couhdence that there is anything to indicate a detinite falling 

 off iu radio-activity iu the more deeply seated materials i have dealt with. The 

 central St. Gothard and certain parts of the Deccan have given results in favour 

 of such a decrease. On the other baud, as will bd seen later, the granite at the 

 north end of the St. Gothard and the primitive gueiss of the Simplou show no 

 diminution. According to the view I Dave put forward above as to the origin 

 of the surface richness iu radium it is I tuink to be expected that, while the 

 richest materials would probably rise most nearly to the surface, there might be 

 considerable variability in the radio-activity of the deeper parts of the upper crust. 



' For these rocks, and for much other valuable material, I have to thank Mri 

 • D. Tate, of the Scottish Geological Survey. 



■■' I'or these I have to thank the Trustees of the British Museum and Jlr. A4 

 Smith Woodward, F.R.S. 



