382 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1908. 



volved I may state that, if my calculations are correct, a rate of dep- 

 osition comparable with that of the chalk prevailing for ten million 

 years would, on assumj^tions similar to those already explained when 

 discussing the subject of mountain building, occasion a rise of the 

 deeper isogeotherms by from 20 to 30 per cent of their probable 

 normal depth. 



In making these deductions as to the influence of radium in sedi- 

 mentary deposits I have so far left out of consideration the question 

 of the time which must elapse in order that the final temperature- 

 rise in the sediments must be attained. The question we have to 

 answer is: Will the rate of rise of temperature due to radium keep 

 pace with the rate of deposition, or must a certain period elapse after 

 the sedimentation is completed to any particular depth before the 

 basal temperature jDroper to the depth is attained ? 



The answer appears to be, on an approximate method of solution, 

 that for rates of deposition such as we believe to prevail in terrigenous 

 dejDOsits — even so great as 1 foot in a century, and up to depths of 

 accumulation of 10 kilometers and even more — the heating waits on 

 the sedimentation. Or, in other w^orcls, there is thermal equilibrium 

 at every stage of growth of the deposit ; and the basal temperature 

 due to radio-active heating may at any instant be computed by the 

 conductivity equation. For accumulations of still greater magnitude 

 the final and maximum temj^erature appears to lag somewhat behind 

 the rate of deposition. 



From this we may infer that the great events of geological history 

 have primarily waited upon the rates of denudation and sedimenta- 

 tion. The sites of the terrigenous deposits and the marginal oceanic 

 precipitates have many times been convulsed during geological time 

 because the rates of accumulation thereon have been rapid. The com- 

 l^arative tranquillity of the ocean floor far removed from the land 

 may be referred to the absence of the inciting cause of disturbance. 

 If, however, favorable conditions prevail for such a period that the 

 local accunudations attain the sufficient depth, here, too, the stability 

 must break down and the j^ermanency be interrupted. 



Upheaval of the ocean floor, owing to the laws of deep-sea sedi- 

 mentation, should be attended with effects accelerative of deposition — 

 a fact which may not be without influence. But although ultimately 

 sharing the instability of the continental margins, the cycle of change 

 is tuned to a slower periodicity. From the operation of these causes, 

 possibly, have come and gone those continents, which many believe 

 to have once replaced the wastes of the oceans, and which with all 

 their wealth of life and scenic beauty have disappeared so completely 

 that they scarce have left a wreck behind. But those forgotten 

 worlds may be again restored. The rolled-up crust of the earth is 

 still rich in energy borrowed from earlier times, and the slow but 



