642 THE JOURNAL OF GEOLOGY. 



have differed materially in their results. Mr. W J McGee esti- 

 mated that the mean age of the earth is 15,000 million years, 

 and that 7,000 million had elapsed since the beginning of Paleo- 

 zoic time. 1 In a subsequent note he modifies this conclusion and 

 gives as a mean estimate 6,000 million years, of which 2,400 mil- 

 lion have elapsed since the beginning of the Paleozoic. This is 

 based on a minimum estimate of the age of the earth of 1 0,000, - 

 000 years and a maximum estimate of five million million (5,000,- 

 000,000,000) years. 2 Professor Warren Upham concludes that 

 Quartenary time comprises about 100,000 years. He applies 

 Professor Dana's time-ratio, and finds on this basis that the 

 time needed for the earth's stratified rocks and the unfold- 

 ing of its plant and animal life must be about 100 millions of 

 years. 3 



From the foregoing estimates of geologic time the only con- 

 clusion that can be drawn is that the earth is very old, and that 

 man's occupation of it is but a day's span as compared with the 

 eons that have elapsed since the first consolidation of the rocks 

 with which the geologist is acquainted. 



When I began the preparation of this paper it was my inten- 

 tion to carefully analyze the sedimentary rocks of the entire geo- 

 logical series as exposed upon the North American continent. I 

 soon found, however, that the time at my disposal would make 

 this impracticable, and I decided to take up the history of the 

 deposits that accumulated in Paleozoic time on the western side 

 ■of our continent, in an area that for convenience I shall call the 

 Cordilleran sea. This was chosen as ( 1 ) I was personally 

 acquainted with many of its typical sections; (2) there was a 

 broad and almost uninterrupted sedimentation during Paleozoic 

 time; and (3) there is a prospect for obtaining more satisfactory 

 data as a basis of calculation, since calcareous deposits are in 

 excess of those of mechanical origin. 



We will now consider certain points in relation to the growth 



1 American Anthropologist, Vol. £, 1892, p. 340. 



2 Science, Vol. 21, 1893, P- 3°9- 



3 Am. Jour. Sci., Vol. 45, 1893, PP- 217-218. 



