LENGTH OF GEOLOGIC TIME 1119 



while ]\IcGee has suggested a possible age of 7,ooo,(XX),ooo years 

 for our earth. Since we have not as yet ascertained the actual 

 thickness of the stratified rocks of the earth, and since we know so 

 little about the rate of erosion, we must conclude that all such esti- 

 mates are premature and almost valueless, and that even the esti- 

 mates of the proportional length of duration of the various divi- 

 sions are extremely hypothetical. Geological time was long, very 

 long, as measured in terms of human chronology — long enough to 

 permit the development of the multif^irious forms of life upon the 

 earth. Only a part of this time is recorded in the known rocks of 

 the earth's crust — for there are probably many lost intervals, the 

 duration of which we cannot even estimate. 



BIBLIOGRAPHY XXXI. 



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CLARKE, J. M. 1903. The Naples Fauna in Western New York. New 

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DANA, JAMES D. 1895. Manual of Geology. Fourth Edition. 

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