NO. 6 AGE OF THE EARTH— BECKER 17 



opinion that the age indicated is between 70 and 50 million, probably 

 closer to the upper limit than to the lower one. 



The extreme values of Mr. Joly's ratio, N /my, which I am led to adopt, 

 do not differ greatly from those preferred by Mr. Sollas, viz., 80 million 

 and 150 million, while the ages computed from the logarithmic expression 

 fall in very well with Mr. Sollas's results from stratigraphic reasoning. 

 Again, if Mr. Walcott's estimate is amended by making the age of the 

 earth twice as great as the lapse of time since the beginning of the Cam- 

 brian, the resulting 55 million j^ears accord with the result here reached. 

 Stratigraphy and sodium denudation as here discussed thus give results 

 which are at any rate of the same order of magnitude. 



AGE FROM REFRIGERATION 



Historical Note 



Kelvin's famous and epoch-making paper on the secular cooling of the 

 earth was published in 1862.^ His problem was to find the time which 

 would elapse before a globe completely solid from center to surface and 

 having throughout a certain uniform initial temperature would cool so 

 far as to reduce the surface gradient of temperature to any given value. 

 He assumed an initial temperature of 3900° C, a diffusivity of 0.01178 

 in c. g. s. units and a final surface gradient of 1° C. in 27.76 m. or 1° F. 

 in 50.6 feet. These data discussed by one of Fourier's theorems give for 

 tlie age of the earth 98 x lO*' years. Kelvin, however, expressly directed 

 attention to the fact that the effect of temperature in modifying diffusivi- 

 ties is almost unknown, and that the original distribution of temperature 

 is uncertain. He also referred to the great differences in the surface 

 gradient of temperature, which varies with the locality, as he stated, from 

 1° F. in 15 feet to 1° F. in 110 feet. He, therefore, allowed very wide 

 limits in his estimate and place.l the age between 20 million and 400 

 million years. 



In 1893 Clarence King made a very important contribution to the 

 subject' by introducing the criterion of tidal stability. Mr. Barus deter- 

 mined for him the melting point of diabase in terms of depth. If in any 

 hypothetical earth consisting solely of diabase the temperature in any 

 couche were to exceed the melting point of diabase, then tidal instability 

 would set in, the crust would break down and chaos would reign for the 

 time being. In a real earth the same result would follow provided the 

 couche were in a region where diabase or equally fusible rocks are to 

 be expected. Excluding such cases. King found that the age of the earth 

 could not exceed 24 million years when Kelvin's values for diffusivity and 



1 Trans. R. S. Edinburgh, reprinted in Thomson and Tait, " Natural Philosophy," Pt. II, p. 468. 



2 Am. Jour. Sci., vol. 45, 1893, p. 1. 



