AGE OF THE EARTH — JOLY. 287 



Years. 



Sphserosiderite from Rhine Provinces — Oligocene 8.4 X 10 6 



Haematite, County Antrim — Eocene 31 X10 6 



Haematite, Forest of Dean — Carboniferous Limestone 150 X10 6 



Sphene, Renfrew County, Ontario — Archaean 710 X10 6 



These are advanced as minimum values, the loss of helium being 

 impossible to estimate. 



Boltwood first investigated the age by the accumulation of lead. 1 

 Very high figures were obtained, ranging from 246 to 1,320 millions 

 of years. Becker criticizes these results, 2 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 1 1,470 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 Brogger's views, as cited above, show that lead may be 

 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. 3 

 Holmes selects minerals from the intrusive nephelene syenite of the 

 Christiania district, supposed by Brogger to be of Middle or Lower 

 Devonian age; most probably the latter. Seventeen minerals are 

 investigated, among which are thorite, biotite, zircon, asgerine, 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, 1007. 

 3 Bull. Geol. Soc. Amcr., vol. 19, p. 113. 1908. 

 Proo. Roy. Soc, June, 1911. 



