NO. 6 AGE OF THE EAKTII — BECKER 27 



The least satisfactory of the assumptions made in the foregoing dis- 

 cussion of refrigeration is that the depth of the diabase couche is 40 miles 

 or O.Olr. This was estimated from Laplace's law, but that is a mere 

 approximation or at best a mean. In order to test the consequences of a 

 variation in the thickness of the refractory shell (nr) I have also com- 

 puted some cases for nr = 0.0075r^ or say 30 miles. Of these I will give 

 only that appropriate to an initial surface temperature of V = 1300°. 

 The results are : 



T 70. 



c 0.00403 



1/c 248. 



G°C 46.3 



G°P 84.3 



Thus the age falls out notably greater and the gradient lower, leaving 

 double the margin for chemical heat estimated in the case of the 60- 

 million-year earth. While these results are less probable than those for 

 the younger earth, they do not seem impossible and, in my opinion, 70 

 million years may be taken as a maximum value of the age of the earth 

 as determined from refrigeration. 



With such constants as are now available it seems to me that the age of 

 the globe considered as a cooling body must be between 70 million and 55 

 million years, limits not differing greatly from those found by other means 

 in the earlier part of this publication. 



Conclusion. 



In the stratigraphical method of determining the age of the ocean the 

 weak point is the uncertainty of the duration of pre-Cambrian time. The 

 best determination of the date of the base of the Cambrian seems to be 

 that by Mr. Walcott, who places it at 27,640,000 years ago. The order of 

 magnitude of the pre-Cambrian period is probably the same, so that 

 stratigraphy indicates an age of the ocean of, say, between 50 and 65 mil- 

 lion years. This is in accord with Mr. Sollas's most recent results, for he 

 regards 80 million as a maximum without being able to give a definite 

 account of nearly so long a period. 



Considering sodium accumulation as an asymptotic process, as it un- 

 questionably is, the weak point is the possibility that the primitive ocean 

 was salt, or that there have been continents in the oceanic basin. These 

 possibilities do not affect an estimate of the maximum age, 74 million 

 years, but preclude a definite minimum. Assuming that neither of these 

 possibilities was realized, the minimum would be about 46 million years. 



Eefrigeration, so dealt with as to exclude tidal instability, and com- 

 puted on the basis of Mr. Hayford's level of isostatic compensation, with- 



