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THE JOURNAL OF GEOLOGY.

have differed materially in their results. Mr. W J McGee estimated that the mean age of the earth is 15,000 million years, and that 7,000 million had elapsed since the beginning of Paleozoic time.[1] In a subsequent note he modifies this conclusion and gives as a mean estimate 6,000 million years, of which 2,400 million have elapsed since the beginning of the Paleozoic. This is based on a minimum estimate of the age of the earth of 10,000,000 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 unfolding of its plant and animal life must be about 100 millions of years.[3]

From the foregoing estimates of geologic time the only conclusion 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 intention to carefully analyze the sedimentary rocks of the entire geological 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. 5, 1892, p. 340.
  2. ↑ Science, Vol. 21, 1893, p. 309.
  3. ↑ Am. Jour. Sci., Vol. 45, 1893, pp. 217-218.
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