680 



TRANSACTIONS OF SECTION C. 



virtue of its thermal expansion must inevitably bring the entire mass to the surface. 

 This reasoning would, at any rate, apply to material situated at a considerable dis- 

 tance inwards, and may possibly be connected with vulcanicity and other crustal 

 disturbances observed at the surface.^ The other view, that the addition of 

 uranium to the earth was mainly an event subsequent to its formation in bulk, 

 so that radio-active substances were added from without and, possibly, from a 

 solar or cosmic source, has not the same a priori probability in its favour.'* 



I have in this part of my Address briefly to place before you an account of my 

 experiments on the amounts of radium distributed in surface materials. Here, 

 indeed, direct knowledge is attainable ; but this knowledge takes us but c. very 

 few miles inwards towards the centre of the earth. 



'J'hc Igneous Rocks. — The basalt of the Deccan, to which I have referred, 

 known to cover some 200,000 square miles to a depth of from 4,000 to 6,000 feet, 

 or more, appears to be radio-active throughout. A fine series of tunnel and 

 surface specimens sent to me by the Director of the Indian Geological Survey has 

 enabled me to examine the radio-activity at various points. It is remarkable that 

 the mean result does not depart much from that afforded by a long series of 

 experiments on North of Ireland basalt and on the basalt of Greenland. 



Again, the granites and syenites — and those of Mourne, Aberdeen, Leiuster, 

 Plauen, Finsteraarhoru have been examined — while variable, yet approximate to 

 the same mean result. 



In the Simplon and St. Gothard tunnels igneous rocks have been penetrated 

 at considerable depth beneath the surface. The greatest true depth is attained, 

 I think, in the central St. Gothard massif. It is remarkable, and may be signifi- 

 cant, that in these rocks I have reached the lowest radio-activities I have met — 

 down to almost one billionth of a gram of radium per gram ; although the generiil 

 mean of the St. Gothard igneous rocks, owing to the high radio-activity of the 

 Finsteraar granite at the north end of the tunnel, is not exceptionally low. Radio- 

 active minerals seem common in the Simplon rocks, involving considerable variations 

 in successive experiments. Some of the highest results are omitted in the mean 

 given below, but as it is difficult to know what to allow for purely sporadic radium 

 the mean is not verj' certain. In the case of a specially high result I asked I'ro- 

 fessor Emil Werner to determine the uranium : my result was confirmed. My 

 list of mean results on igneous rocks up to the present is the following ; — 



Basalts (14) 

 Granites (6) . . 

 Syenites (1) 



The general mean is 6'1. 



From the igneous rocks have originated the sediments after a toll of dissolve! 

 substances has been paid to the ocean. It does not of course foUgw necessarily 

 that the percentage of radium, or more correctly of uranium, in the sedimentary 

 rocks should be less than in the igneous. The residual materials might keep the 

 original percentage of the parent rock, or even improve upon it. There are 

 reasons for believing, however, that there would be a diminution. 



Those sedimentary rocks which have been derived from materials formerly in 

 solution offer a different problem. In their case there is little or none of the 

 original materials carried into the secondary rock, and the radio-activity will 

 depend mainly upon how far uranium is precipitated or abstracted with the rock- 

 making substances. In other words, upon how far the waters of the ocean will 

 restore to the rocks what it has borrowed from them. 



This brings me to consider the condition of the ocean as preparatory to quoting 

 experiments on the sediments. 



' See Appendix A. - Nature, Ixxv. p. 294. 



' This number is to be multiplied by lO-^-, and represents billionths of a gram 

 of radium per gram of material investigated. Throughout the rest of my Address 

 this understanding holds, unless where a different meaning is specified. The 

 numbers in parentheses signify the nurnbey of different specirnens investigated. 



