AGE OF THE EARTH—JOLY. 287 
Years. 
Spherosiderite from Rhine Provinces—Oligocene............ 8.4108 
Hematite, County Antrim—Eocene . : ee oer ee 
Hematite, Forest of ean ee nrane: aacuione: NE 150 X10 
Sphene, Renfrew County, Ontario—Archean............... 710 108 
These are advanced as minimum values, the loss of helium being 
impossible to estimate. 
Boltwood first investigated the age by the accumulation of lead.t 
Very high figures were obtained, ranging from 246 to 1,320 millions 
of years. Becker criticizes these results,? pointing out that certain 
radioactive minerals of well-determined age (Llano Group, not far 
below the Cambrian) afford on the same principles ages which are 
quite incredible, ranging from 1,671 to 11,470 millions of years. Bolt- 
wood questions the suitability of the minerals on the score of incipient 
or advanced alteration. Becker in reply urges that there is no evi- 
dence to show that alteration can affect the ratios. Becker considers, 
further, that Brégger’s views, as cited above, show that lead may fe 
occluded as an impurity in such minerals, and that the amount of this 
impurity will vary from crystal to crystal, in accord with the results 
of the observations. 
The subject of the lead ratio has been lately taken up by A. Holmes? 
Holmes selects minerals from the intrusive nephelene syenite of the 
Christiania district, supposed by Brégger to be of Middle or Lower 
Devonian age; most probably the latter. Seventeen minerals are 
investigated, among which are thorite, biotite, zircon, egerine, nephe- 
line, feldspar, etc. "The ratio of lead to uranium ranges from 0.041 to 
0.500. There is found to be an increase in the value of the ratio with 
diminution in the amount of uranium; a result suggesting the pres- 
ence of original lead. Holmes, accordingly, rejects about half the 
results (those which give the higher ratios) and finds a mean among 
eight results which range from 0.043 to 0.050. The mean of these 
gives for post-Lower Devonian time 370 million years. It must be 
admitted that this result is not entirely satisfactory; it contains an 
element of arbitrary choice, and although it is possibly true that the 
minerals with least uranium contain too much original lead to be 
reliable, we are by no means sure that even larger amounts of origi- 
nal lead did not enter into the constitution of the others. The agree- 
ment among the ratios renders this improbable, however. 
Holmes enters into the question of the geological positions of the 
results cited by Boltwood, and concludes that they may be tabulated 
as follows. His own mean result is included in the table. 
1 Amer. Journ. Sci., vol. 23, 1907. 
£ Bull. Geol. Soc. Amer., vol. 19, p. 118, 1908. 
Proc. Roy. Soc., June, 1911. 
