THE STORY OF NINETEENTH-CENTURY SCIENCE 



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 formed. 

 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 com- 

 putation 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 different, for'the new 

 estimate seemed to prove that since the final crust of 

 the earth formed a period of from one hundred to two 

 hundred millions of years has efapsed. 



With this all controversy ceased, for the most grasp- 

 ing geologist or biologist would content himself with a 

 fraction of that time. What is more to the point, how- 

 ever, is the fact, which these varying estimates have 

 made patent, that computations of the age of the earth 

 based on any data at hand are little better than rough 

 guesses. Long before the definite estimates were under- 

 taken, geologists had proved that the earth is very, very 



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