A HISTORY OF SCIENCE 



mouth of a great river furnishes a clew to the rate of 

 denudation of the area drained by that river. Thus 

 the studies of Messrs. Humphreys and Abbot, made for 

 a different purpose, show that the average level of the 

 territory drained by the Mississippi is being reduced by 

 about one foot in six thousand years. The sediment is, 

 of course, being piled up out in the Gulf at a proportion- 

 ate rate. If, then, this be assumed to be an average 

 rate of denudation and deposit in the past, and if the 

 total thickness of sedimentary deposits of past ages 

 were known, a simple calculation would show the age 

 of the earth's crust since the first continents were form- 

 ed. But unfortunately these "ifs" stand mountain- 

 high here, all the essential factors being indeterminate. 

 Nevertheless, the geologists contended that they could 

 easily make out a case proving that the constructive 

 and destructive work still in evidence, to say nothing 

 of anterior revolutions, could not have been accom- 

 plished in less than from twenty-five to fifty millions of 

 years. 



This computation would have carried little weight 

 with the physicists had it not chanced that another 

 computation of their own was soon made which had 

 even more startling results. This computation, made 

 by Lord Kelvin, was based on the rate of loss of heat 

 by the earth. It thus resembled the previous solar 

 estimate in method. But the result was very dif- 

 ferent, for the new estimate seemed to prove that 

 a period of from one hundred to two hundred millions 

 of years has elapsed since the final crust of the earth 

 formed. 



With this all controversy ceased, for the most grasp- 



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