288 



ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1911. 



Geological period. 



Carboniferous 



Devonian 



Pre-Carboniferous 



Silurian or Ordovician 

 Pre-Cambrian: 



Sweden 



United States 



Ceylon 



Pb/U. 



0.041 

 .045 

 .050 

 .053 



.125 

 .155 

 .160 

 .175 

 .20 



Millions of 

 years. 



340 



370 

 410 

 430 



1,025 

 1,270 

 1,310 

 1,435 

 1,640 



The Swedish minerals are from pegmatites of an age younger than 

 Jatulian. The results obtained from them show, among 17 specimens 

 examined, two well-marked groups, having the ratios tabulated. 

 There is nothing in the rocks to indicate any difference in age. Of the 

 United States minerals, those having the lesser ratio are from granites 

 intruded into the Llano Group (Texas) of metamorphosed sediments. 

 Their age is, therefore, younger than the sediments, which are early 

 Algonkian. Those with the higher ratio are from Burnet County, 

 Tex., and Douglas County, Colo. The geological evidence is similar to 

 that of Llano County. 



The evidence for the pre-Cambrian age of the Ceylon thorianite 

 is the resemblance of the rocks to the fundamental complex of India. 

 The tabulated values are the means of several results cited by Bolt- 

 wood, some of which are in closer mutual agreement than others. 



These results greatly transcend Strutt's in the antiquity they assign 

 to Palaeozoic and pre-Cambrian time. This fact can be explained by 

 the escape of helium. The possibility of occluded lead entering se- 

 riously into such determinations will, doubtless, form the subject of 

 future research. Meanwhile it seems improbable that the higher 

 average ratios of the oldest minerals can find explanation in this 

 manner. I have already dwelt sufficiently, in view of our very 

 deficient knowledge, on these points. 



The discordance between the radioactive indications of time and 

 those derived from the stratigraphical column appears clearly when 

 we plot one against the other (fig. 1). The assumption made in plot- 

 ting the sedimentary thicknesses is that these, inter se, are roughly 

 comparable as regards the rate of accumulation. As I have already 

 pointed out, this seems probable save in the case of the earlier pre- 

 Cambrian sediments, which we might expect would have been accu- 

 mulated more locally. The thicknesses of the several strata I have laid 

 out according to the data collected by Sollas. The radioactive times 

 are plotted above the points on the base line to which their geological 

 positions assign them. We have from the lead ratios two early- 



