GEOLOGIC TIME. 673 



throughout, I have increased the various factors above those 

 usually accepted : thus, for mechanical sedimentation, 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 

 fifteenfold (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 drainage 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 maybe 

 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. Jas. 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 Targe 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 Paleo- 

 zoic, Mesozoic and Cenozoic periods is: 12, 3, and 1, respect- 

 ively 1 . Prof. Henry S. Williams applies the term geochro?iology> 

 1 Manual of Geology, 1875, P- 5^6. 



