THE earth's radiation 201 



.Agf. in kilom. 



68 X W If 2.58 



100 " 4.74 



200 " 7.72 



300 " 9.03 



600 " 10.75 



1,000 " 11.68 



1,314 " 12.09 



The second column gives the thickness of the active shell, the activity 

 being supposed constant, and the third column states the proportion of 

 the emitted heat which is due to radioactivity. 



Mr. Hayford^s level of isostatic compensation lies at a depth of 120,900 

 meters, or, not to be meticulous, at 121 kilometers.^® Such compensation 

 at an eutectic level seems natural and comprehensible; but under that 

 interpretation the age of the earth, with the data now available, is only 

 68 X 10^ years, and it does not seem possible that any corrections in the 

 values of the constants should increase the a.ge thus determined to more 

 than 100 X 10^ years.''^ On the other hand, if the eutectic level is at 



"^ Supplementary investigation of the figure of the earth and isostasy. Coast and 

 Geod. Surv., 1910. 



^^ To avoid a possible misinterpretation, it may be expedient to refer to a difference 

 of opinion which has arisen between Mr. .loly and myself. In discussing the age of the 

 earth from the accumulation of sodium in the ocean, as first suggested in modern times 

 by Mr. .Toly, I made allowance for the continual diminution of the exposed area of 

 Archean and igneous rocks from which the sodium is ultimately derived. The computed 

 age came out about 70 X 10" years. (Smithsonian Misc. Coll., vol. 56, 1910, No. 6.) 

 In Science I'rogross for .luly, 1914, p. 4.5, Mr. Joly expresses his dissent from my view, 

 maintaining the uniformity of the sodium supply and referring to his paper of 1890 in 

 the Trans. R. Dublin Soc. for reasons. In that paper he stated that the possible 

 diminution in the area of feldspathic rocks "should undoubtedly lead us to widen the 

 margin we allow for error in our estimate of geological time ;" and he drew attention to 

 the fact that such analyses of ancient slates and modern silts as were at his comtnand 

 tended to show a decrease with increasing age of the content in alkalies. He neverthe- 

 less adhered to the hypothesis of uniformity seemingly because old sediments freshly 

 eroded give up a fresh portion of their sodium content. This, of course, is true. Sodium 

 extraction is a slow, perhaps asymptotic process. But ultimately the alkali all comes 

 from the Archean and igneous rocks, and if the source of supply wore cut off it would 

 gradually cease or approach zero. Suppose that the original feldspathic rocks had been 

 in some way protected from decomposition or denudation once for all at the beginning of 

 the Cambrian. Would the reworking of pre-Cambrian sediments still be yielding as much 

 sodium as is now derived from continental areas about one-fourth of the area of which 

 is occupied by ancient feldspathic rocks? Surely not. The supply could be kept up only 

 if decomposition were undiminished by a superjacent detrital layer. But every member 

 of this Society knows that at many points In the northern part of the continent a layer 

 of compact tnrf, only .3 or 4 inches thick, has been sufficient to preserve intact glacial 

 scratches and polish, while exposed areas of the same rocks in the same region have lost 

 both. In the south a coating of saprollte a score or two of feet in thickness seems like- 

 wise a complete protection against weathering. In short, I adhere to my opinion that 

 as the amount of sodium In the ocean increases the available continental supply of 

 Bodltim decreases. 



