THE ABYSS OF CENTURIES. 289 



matter to have been infinitely dispersed, and assumes that all 

 the heat ever existing in the contracting mass arose from the 

 process of contraction. Of course, if the sun's matter in 

 other words, the matter of our primitive nebula did not ex- 

 tend an infinite distance, the time for contraction would be 

 shortened. He says a temperature permitting the existence 

 of water on the earth would have been reached ten million / 

 years ago. 



Professor Newcomb calculates further, that the sun in its 

 process of cooling will reach a darkened or planetary state in / 

 twelve million years. So the whole life-time of the sun from 

 infinitely expanded fire-mist to final darkness, spans only 

 thirty million years. 



Sir William Thomson has calculated the time required for 

 the earth to cool from incipient incrustation to its present 

 state. He thinks it would not exceed eighty million years. 

 Rev. O. Fisher, in making the same calculation deduces thirty- 

 three million years. These estimates do not cover precisely 

 the same stage of cooling as either of Newcomb's, but the 

 numbers appear to be relatively higher. 



Another way of getting at the length of any particular 

 period of the world for instance, the time elapsed since the close 

 of the Tertiary is first to compute the relative duration of 

 the different ages on the basis of thickness of formations, and 

 then divide some assumed total age in the same proportion as 

 the relative ages. To make this plainer: A formation two 

 hundred feet thick has probably consumed more time than a 

 formation one hundred feet thick. We do not know whether 

 the rate of accumulation was the same for the two formations, 

 but we must assume it the same, if the materials are the same ; 

 if however, one is of limestone and the other of fragmentai 

 materials (sand, pebbles, clay) we may assume the rate of 

 accumulation five times as great for the latter as for the lime- 

 stone. Thus the formation one hundred feet thick, of lime- 

 stone would be equivalent in time to 500 feet of sandstone. 

 Now, we have carefully studied the thickness of all the for- 

 mations, making due allowance for all the limestones. We 



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