458 



NA TURE 



[September io, 190'? 



The 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 4000 to 6000 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 re- 

 markable 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 

 examined — while variable, yet appro.ximate 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 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 exceptionally low. 

 Radio-active minerals seem common in the Simplon rocks, 

 involving considerable variations in successive experi- 

 ments. 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 very 

 certain. In the case of a specially high result I asked 

 Prof. 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 (i) 



•ni 



S-o- 

 41 

 6 8 



Lewisian Gneiss (3). 57 



Simplon (32) 7'6 



St. Gothard (32) ,. 5-1 



The general mean is 6'i. 



From the igneous rocks have originated the sediments 

 after a toll of dissolved substances 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 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 w-aters 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. 



The Ocean and its Sedimetifs. — The waters of the ocean, 

 covering five-sevenths of the earth's surface to a mean 

 depth of 3.8 kilometres, represent the most abundant sur- 

 face 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 0016x10-'- gram 

 per cubic centimetre. There is considerable variability. 

 Taking the mass of the ocean as 1-458x10" tonnes, there 

 must be about 20x10° grams (20,000 tons) of radium in 

 its waters. 



The experiments which I h.ave been able to make on 

 deep-sea deposits, thanks mainly to the kind cooperation 

 of Sir John Murray, apply to ten different materials of 

 typical character. 



The results are so consistent as to lead me to believe 

 that although so few in number they cannot be far wrong 

 in their general teaching. 



1 This number is to be multiplied by 10-12, 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 number of different 

 specimens investigated. 



NO. 2028, VOL. 78] 



The means are : — 



Extension : 



Radium Milhonsof 



square miles 



Globigerina Ooze 7'2 49*5 



Radiolarian ,, 307 2*5 



Red Clay 33-3 51-5 



Diatom Oozes have not yet been examined. 



It is apparent from these results that the more slowly 

 collecting sediments are those of highest radio-activity, as 

 if the organic materials raining downwards from the 

 surface of the ocean carried everywhere to the depths 

 uranium and radium abstracted from the waters, but in 

 those regions where the conditions were inimical to the 

 preservation of the associated calcareous tests there was 

 the less dilution of the radio-active substances accumu- 

 lating beneath. The next table shows that radio-activity 

 and the percentage of calcareous matter in these deposits 

 stand in an inverse relation : — 



Calc.um 

 carbonate 



Radium 



Globigerina Ooze, Challenger 33S 



„ .. ,. 290 



Red Clay „ 5 



,, 276 



Radiolarian Ooze „ 272 



274 



per cent. 



92 24 ... 67 1 



64'34 ••• 7'4 



1200 ... I5'4 



2S-28 ... 52-6 



I0'I9 ... 22-8 



3S9 ■• SO'J 



The percentages of calcium carbonate are from the Report 

 of the Challenger Expedition. The Red Clay in the table, 

 which reads as an apparent exception, is probably a case 

 of recent change in the character of the deposit, for the 

 evidence of manganese nodules and sharks' teeth brought 

 up with this clay is conclusive as to the slow rate of its 

 collection. Readers of Sir John Murray's and Prof. 

 Renard's report will remember inany cases where recent 

 change in the character of a deposit is to be inferred. 



A point of much importance in connection with our views 

 on oceanic radio-activity is that of the presence in the 

 waters and in the deposits of the parent radio-active sub- 

 stance, uranium. The evidence that the full equivalent 

 amount of uranium is present is, I believe, conclusive. 



In the first place, to so vast a reservoir as the ocean 

 the rivers cannot be supposed to supply the radium 

 sufficiently fast to make good the decay. In a very few 

 thousand years, in the absence of uranium, the rivers must 

 necessarily renew almost the entire amount of radium pre- 

 sent. I have made examination of the water of one great 

 river only — the Nile. The quantity of radium detected 

 was 00042x10-'^ per cubic centimetre. That is less than 

 the oceanic amount. In short, it is evident that the 

 uranium must accumulate year by year in the oceanic 

 reservoir, like other substances brought in by the rivers, 

 and that the present state of the waters is the result of 

 such actions prolonged over geological time. 



While this reasoning is conclusive as regards the waters 

 of the ocean, it does not assure us that the sediments 

 accumulating in their depths are throughout as radio-active 

 as their surface parts would indicate. There might be 

 a precipitation of radium unattended by uranium, in which 

 case their deeper parts would not be radio-active. 



Against this possibility there is the evidence of such 

 true deep-sea deposits as were formed in past times and 

 to-day still preserve their radio-activity. For instance, the 

 chalk, which, considering that it was undoubtedly a very 

 rapidly formed deposit, exhibits a radio-activity quite com- 

 parable with that of the Globigerina Oozes, deposits which 

 it most nearly resembles. In this deposit, clearly, the 

 uranium must have collected along with the calcareous 

 materials. We can with security argue that the similar 

 oozes collected to-day must likewise contain uranium. In 

 the case of the Red Clays we have the direct determina- 

 tion of the uranium which Prof. Emil Werner was so good 

 as to make at my request. Considering the difficulties 

 attending its separation, the result must be taken as sup- 

 porting the view that here, too, the radium is renewed 

 from the uranium. Regarding the efforts of other 

 observers to detect uranium in such deposits, it is note- 

 worthy that without the guidance of the radium, enabling 

 specially rich materials to be selected for analysis, the 



