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1894.] Geology and Paleontology. 793 
General Notes. 
GEOLOGY AND PALEONTOLOGY. 
Geologic Time indicated by the Sedimentary Rocks of 
North America.—Various geologists have speculated as to the age of 
the earth, basing their estimates on both geologic and paleontologic data. 
The latest contribution to the subject is from Dr. Charles Walcott. His 
unit is the aye of the Paleozoic rocks of the Cordilleran area in western 
North America. A careful consideration of all the factors of denuda- 
tion and deposition leads him to consider that it would have required 
17,500,000 years for the deposition of the calcium and the mechanical 
sediments of Paleozoic time. He concludes his paper as follows: 
“ Taking as a basis 17,500,000 years for loce we inen the time 
ratios 12, 5 and 2 for Paleozoic, M g Plisto- 
cene) respectively, the Mesozoic is given a time duration of 7, 240, 000 
years, the Cenozoic of 2,900,000 years, and the entire series of fossiliferous 
sedimentary rocks of 27,650,000 years. To this there is to be added 
the entire period in which all of the sediments were deposited between 
the basal crystalline archean complex and the base of the Paleozoic. 
Notwithstanding the immense accumulation of mechanical sediments 
in this Algonkian time, with their great unconformities and the great 
differentiation of life at the beginning of Paleozoic time, I am not 
willing, with our present information, to assign a greater period than 
that of the Paleozoic—or 17,500,000 years. Even this seems excessive. 
Adding to it the time period of the fossiliferous sedimentary rocks, the 
result is 45,150,000 years for post-Archean time. Of the duration of 
Archean or pre-Algonkian time, I have no estimate based on a study 
of Archean strata to offer. If we assume Houghton’s estimate of 33 
per cent. for the Azoic period and 67 per cent. for the sedimentary 
iin Archean time would be represented by the period of 22,250,000 
yea 
" T estimating for the Archean, Houghton included a large series 
of strata that are now placed in the Algonkian of the Proterozoic of 
the U. S. Geol. Survey; and I think that his estimate is more than 
one-half too large; if so, ten million years would be a fair estimate, or 
rather conjecture, for Archean time. 
