RADIOACTIVITY AND THE EARTh's AGE 133 



Again, in spite of Mr Koenigsbergers valuable classification of 

 gradients, we need more information on the normal gradient, as lie 

 points out himself — indeed, I understand that he is making observations 

 on the conductivity of rocks and the normal gradient. 



Mr A. C. Lane ^^ has discussed the gradient in Michigan and considers 

 1° Fahrenheit in 100 feet an average value ia trap and limestone; but 

 the neighborhood of large bodies of water modifies the applicability o:£ 

 this result. 



On the whole, however, considering the good agreement between my 

 results for a cooling earth and the age as computed by other means, it 

 seems reasonable to conclude that a tenth represents the order of magni- 

 tude of the fraction of the gradient due to such causes as radioactivity, 

 and that it very probably lies between an eighth and a sixteenth. 



Geophysicists should not forget, however, that radioactivity is only one 

 of the causes which may influence the gradient. Even relatively fresh 

 rocks are rarely quite undecomposed, and eventually this must be taken 

 into account. The disintegration of a gram of radium liberates millions 

 of times as much heat as the peroxidation of a gram of ferrous silicate; 

 but these silicates are also millions of times as abundant as is uranium. 



Mention has been made above of Mr Eutherf ord's extremely interesting 

 idea of determining the age of uranium minerals from the amount of 

 helium, or preferably of the lead, which they contain, and geologists 

 would assuredly rejoice in the discovery of a valid method of this kind. 

 One condition of its acceptance would clearly be that it should give 

 periods of the same order of magnitude as is indicated by purely geo- 

 logical data. 



]^ow from the helium found in an analysis of fergusonite by Messrs 

 Ramsay and Travers, Mr Rutherford ^^ computes an age of at least 500 

 million years, and from an uranium mineral at Glastonbury, Connecticut, 

 analyzed by Mr Hillebrand, a similar antiquity. The Glastonbury granite 

 gneiss is equivalent to the Wilbraham gneiss of Mr Emerson,^* who pro- 

 nounces it unequivocally early Cambrian. Messrs Rice and Gregory "^ 

 feel some uncertainty as to its age, but do not suggest a new position for 

 it. For the present purpose, it is sufficient to regard it as at the bottom 

 of the Cambrian. Mr Walcott's estimate of the lapse of time since the 

 begiimuig of the Cambrian is nearly 28 million years, or about an 

 eighteenth part of that suggested by Mr Rutherford. 



^ Annual Report of the Michigan Geological Survey, 1901, p. 244. 



" Radioactive transformations, p. 189. 



^^U. S. Geological Survey, Monograph 29, 1898. 



™ Connecticut Geological and Natural History Survey, Bulletin 6, 1906, p. 116. 



