288 ANNUAL REPORT SMITHSONIAN INSTI UTION, 1911. 
- : Millions of 
Geological period. Pb/U. years. 
WALD ONEROUS sae veree ee ye cee ey ora es See oe ea echo en RUD ami apse RO ae 0. 041 340 
MD GVOTIAN erate se ee ree Se nee ae ee Le ee Ee eee ee Se ee eee ee ence eee 045 370 
IP re-Carboniierouss. cas et cen Saco te ie nee net ae ae Race EERE: ee eaters - 050 410 
SUTIN OLLOLG OMA CI ATI aatee rat este vp eo eae ee ele . 053 430 
Pre-Cambrian: 
W) 
sry alae atte Mena pled cael t-te iar igs 
| «155 1,270 
Tnited? States? . pe secre we ere En Seal Sheer Sal Ae may ppt ey pee 130 
175 1, 435 
WeylOMs ee cee eee Rete = MRR OC oct eee = ERAS SOE Rone RE RAE EERE 2M aR 2335] 20 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 Paleozoic 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 morelocally. The thicknesses of the severalstrata Ihave 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- 
