366 
'/'A- A mi rii-n a Geologist. 
December, 1893 
out I have increased the various factors above those usually accepted; 
— thus for mechanical sedimentation, the erosion of one foot in 200 
years is used. If the usually accepted average of one foot in 3,000 
years is taken the time period must be increased fifteen fold (21,000,000 
years), or the area of denudation from 1,600,000 square miles to 
24,000,000, — or three times the present area of the North American 
continent. 
In the estimate for the amount of chemical denudation the largest 
average is taken — 70 tons of calcium per square mile per annum — and 
the assumption made that all calcium derived from the adjoining drain- 
age area was deposited within the Cordilleran sea. Again, the total 
supply provided per annum to ocean waters of Paleozoic time is taken 
as 3.78 times greater than the amount annually contributed to ocean 
waters to-day; of this, four times as much is assumed to have been 
taken out per annum per square mile as was taken by the remaining 
area in which calcium was being deposited. 
The area of the Cordilleran sea is given as 400,000 square miles, but 
it was probably 600,000, if not much more. It may be claimed that the 
area tributary to the Cordilleran sea was greater than I have estimated. 
The evidence, such as it is, is against such a view. As a whole I think 
the estimate of 17,500,000 years for the duration of Paleozoic time in 
the Cordilleran area is below the minimum rather than above it. 
If the estimated rate of the deposition of coral limestones — five feet 
in 1,000 years— given by Prof. James D. Dana is correct, the 19,000 feet 
of Paleozoic limestone in central Nevada would have required 3,800,000 
years to have accumulated under the most favorable [local conditions 
surrounding a coral reef. With the exception of large deposits of 
corals in Devonian rocks no appearance of a coral reef is recorded in 
the Cordilleran area. 
Time-Ratios of Geologic Periods. 
The time ratio adopted by Prof. James D. Dana for the Paleozoic, 
Mesozoic and Cenozoic periods is: 12; 3; and 1, respectively.* Prof. 
Henry S. Williams applies the term qeochronoJogy, giving the standard 
time-unit used the name geochrone. The geochrone used by him in ob- 
taining a standard scale of geochronology is the period represented by 
the Eocene. His timescale gives 15 for the Paleozoic; 3 for the Mes- 
ozoic; and 1 for the Cenozoic, including the Quaternary and the Re- 
cent. + 
The Rev. Samuel Haughton obtained the following time-ratios from 
the maximum thickness of strata as they occur in Europe: 
S<\le of Geological Time. 
Period. 
From Theory of 
CooliDg Globe. 
From Maximnm 
Thickness of Strata. 
Azoic 
33.0 per cent. 
41.0 
2ti.ll 
4 M.:\ per cent. 
42.5 
23 2 
Total 
100.0 
100.0 
♦Manual of Geology, 1875, p. 586. 
TJournal of Geology, Chicago, vol. 1, 1893, pp. 394-295. 
