NOVEMBEE 20, 1908] 



SCIENCE 



701 



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, Leinster, 

 Plauen, Finsteraarhorn have been ex- 

 amined—while variable, yet approximate to 

 the same mean result. 



In the Simplon and St. Gothard tunnels 

 igneous rocks have been penetrated at con- 

 siderable depth beneath the surface. The 

 greatest true depth is attained, I think, in 

 the central St. Gothard massif. It is re- 

 markable, and may be significant, 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 general 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 excep- 

 tionally low. Radio-active minerals seem 

 common in the Simplon rocks, involving 

 considerable variations in successive experi- 

 ments. Some of the highest results are 

 omitted on the mean given below, but as it 

 is difficult to know what to allow for purely 

 sporadic radium the mean is not very cer- 

 tain. In the case of a specially high result 

 I asked Professor Emil Werner to deter- 

 mine the uranium: my result was con- 

 firmed. My list of mean results on igneous 

 rocks up to the present is the following : 



Basalts (14) 5.0" 



Granites (6) 4.1 



Syenites ( 1 ) 6-8 



Lewisian gneiss (3) 5.7 



"This number is to be multiplied by 10"", and 

 represents billionths of a gram of radium per 

 gram of material investigated. Throughout the 



Simplon (32) 7.6 



St. Gothard (32) 5.1 



The general mean is 6.1. 



From the igneous rocks have originated 

 the sediments after a toll of dissolved sub- 

 stances has been paid to the ocean. It does 

 not of course follow 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 im- 

 prove upon it. There are reasons for be- 

 lieving, however, that there would be a 

 diminution. 



Those sedimentary rocks which have been 

 derived from materials formerly in solu- 

 tion 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 sub- 

 stances. 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 ex- 

 periments on the sediments. 



The Ocean and its Sediments. — The 

 waters of the ocean, covering five sevenths 

 of the earth's surface to a mean depth of 

 3.8 kilometers, represent the most abundant 

 surface material open to our investigation. 

 As the mean of a very large number of 

 experiments upon twenty-two different 

 samples of sea-water from various widely 

 separated parts of the ocean, I obtain a 

 mean of 0.016 X 10"^^ gram per cubic centi- 

 meter. There is considerable variability. 

 Taking the mass of the ocean as 1.458 X 

 10-18 tonnes, there must be about 20 X 10"^ 

 rest of my address this understanding holds, 

 unless where' a different meaning is specified. 

 The numbers in parentheses signify the number 

 of diflferent specimens investigated. 



