12 Clarence King — Age of the Earth. 



the maximum, then the minimum rate thence declining into 

 the region of inadmissible rates. 



The probable conditions of the true gradient are as to initial 

 excess and age such as fall below the diabase line into solidity 

 and emerge at the surface with a rate which has not declined 

 below the mean (B A) rate of 64 ft. to 1° F. From the point 

 of view of solidity no gradient of initial excess above 2,000° C. 

 is admissible : that of 2,560° C, even after 100x10" years cool- 

 ing still shows a deep shell of fusion (sixty-four miles from top 

 to bottom), and since it emerges on the minimum rate it has 

 already fallen below the admissible tangent. 



Gradient d, 1,950° C. and 15xl0 6 years, just cooled to the 

 maximum surface rate has still an inadmissible fluid shell, but 

 if refrigeration had been continued for 7x1 6 to 9xl0 6 years 

 more the line would have fallen below the solidity line and 

 its surface rate would not have passed the mean value. Hence 

 a 1,950° 24xl0 6 year earth is possible and marks about the 

 superior limit admissible for initial excess. 



From the point of view of age no greater time of cooling is 

 allowable than enough to bring the gradient for any initial 

 excess to the mean surface rate. Thus the condition for 

 excess and age exclude a line of over 2,000° C. and 24xl0 6 

 years. Conductivity remaining of the value used, any higher 

 excess involves fluidity, and any greater age an inadmissible 

 surface rate. 



To the extent, therefore, that solidity is a valid criterion 

 and so far as the melting temperature of diabase may be 

 supposed to apply to the depth examined, there is no escape 

 from an earth of the low age and temperature given except by 

 impugning the rate of surface augmentation and the value 

 of rock conductivity here employed. 



Whoever has examined the B. A. committee's reports and 

 summaries on underground temperatures must have realized 

 the obstacles to the evaluation of a true mean rate. The 

 range of observations is wide, from high rates due to residual 

 vulcanism to low ones produced by neighboring bodies of cold 

 water, such as are described by Wheeler from mines near 

 Lake Superior.* It is not, however, likely that by rejecting 

 anomalies and assigning probable weight to further observa- 

 tions the present value will be moved to an important extent. 



We have seen that all probable distributions of earth-tem- 

 perature involve in the initial stage a great solid nucleus, 

 practically the whole body of the earth, with a shallow surface 

 shell of fusion. In the case of the 1,741° C. 20xl0 6 year 

 earth there was an initial melted shell of 53 miles. Obvi- 



* This Journal, vol. xxxii, 1886. 



