Age of the Earth. 375 



following as summarizing the data of bis earlier papers : — 



Spbserosiderite from Rhine Provinces. Oligocene . . 8*4 xlO 6 years. 



Haematite, Co. Antrim. Eocene . . . . ; 31 „ „ 



„ Forest of Dean. Carbonif. Limestone. . 150 „ }J 

 Sphene, Renfrew Co., Ontario. Archaean 710 „ ; , 



These are advanced as minimum values, the loss of helium 

 being impossible to estimate. 



Boltwood first investigated the age by the accumulation of 

 lead (Am. Journ. Sc. vol. xxiii. 1907). Very high figures 

 were obtained; ranging from 246 to 1320 millions of years. 

 Becker criticizes these results (Bull. Geol. Soc. Am. vol. xix. 

 p. 113, 1908), 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 1671 to 11,470 millions of 

 years. Boltwoud questions the suitability of the minerals 

 on the score of incipient or advanced alteration. Becker 

 in reply urges that there is no evidence 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 im- 

 purity 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 (Proc. B. S. June 1911). 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, segerine, nepheline, felspar, &c. The ratio of lead to 

 uranium ranges from 0*04:1 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 presence 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 0043 to O'OoO. 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 original lead did not enter into the constitution of the 

 others. The agreement among the ratios renders this 

 improbable, however. 



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