254 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



I have now traversed that part of our subject of which I feel com- 

 petent to speak. The upshot is that radio-active methods of research 

 indicate a moderate multiple of 1,000,000,000 years as the duration 

 of the earth's crust as suitable for the habitation of living beings, 

 and that no other considerations from the side of pure physics or 

 astronomy afford any definite presumption against this estimate. 



The arguments from geology and biology I must leave to our col- 

 leagues from other sections. May I venture to say that I for one 

 consider the topics with which they will deal as not less interesting 

 and important than those which it has been my privilege to try to lay 



before you. 



By Prof. W. J. Soixas, F. R. S. 



Huxley once sagely remarked that the zoologist must take his time 

 from the geological clock. The geologist is thus charged with a 

 great responsibility which he would willingly share with the physicist 

 and astronomer. One of the earliest attempts to determine the age 

 of the earth by purely geological means was made by the late Dr. 

 Samuel Haughton, who based his calculations on the rate of depo- 

 sition of sediment supposed to be evenly distributed over the whole 

 floor of the ocean. This led to the conclusion that the time which 

 must have elapsed since the first appearance of the dry land is of 

 the same order of magnitude as that now presented for our considera- 

 tion by Lord Eayleigh. 



Soon, however, it was discovered, as a result of exploration by 

 the Challenger, that deposition is limited to a comparatively narrow 

 belt bordering the continents — a limitation due to several causes, 

 chief among them the fact that sediment sinks much more rapidly in 

 salt water than fresh. On taking account of this factor Haughton's 

 period was reduced to about 100,000,000 years. At the same time a 

 new method was devised by Professor Joly which depends on the rate 

 at which sodium is supplied to the sea, and this led to a similar re- 

 sult. 



Antecedent to these attempts, another method, based on the rate 

 at which the earth is losing heat, had been employed by Lord Kelvin, 

 and this gave at first an estimate concordant with the preceding — i. e., 

 100,000,000 years. Later, however, this allowance was reduced to 

 forty, or preferably to twenty, millions, and by the uncompromising 

 Professor Tait to ten millions. 



These estimates proved very embarrassing to the geologists, who 

 found it impossible to compress the events of the earth's history into 

 so restricted an interval without unduly "hurrying up the phe- 

 nomena." Lord Kelvin, however, was inflexible and impressively as- 

 serted that he could conceive of no escape from his conclusions. 



