GEOLOGIC TIME. 331 



for deposit i^er annum in the Cordilleran sea. At this rate it would have 

 required 10,300,000 years to have deposited the 15,007,244,800,000,000 

 {2 thousand trillion] tons (900,000,000 mile-feet) of calcium in the Cor- 

 dilleran sea; adding- to this the 1,200,000 years estimated for the depo- 

 sition of the mechanical sediments, we have a total of 17,500,000 years 

 as the duration of Paleozoic time. 



In reviewing the preceding estimates we must consider that through- 

 out I have increased the various factors above those usually accepted — 

 thus for mechanical sedimentation the erosion of 1 foot in 200 years is 

 used. If the usually accepted average of 1 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 S(piare mile per annum — and 

 the assumption made that all calcium derived from the adjoining drain- 

 age area was deixjsited within the Cordilleran sea. Again, the total 

 supply provided i>er 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 S(iuare mile in the Cordilleran sea 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 000,000, 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 — 5 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.* I*rof. 

 Henry S. Williams applies the term {/eochronology, giving the standard 

 time unit used the name geocJirone. The geochrone used by him in 

 obtaining a standard scale of geochronology is the period represented 

 by the Eocene. His time scale gives 15 for the Paleozoic, 3 for the 

 Mesozoic, and 1 for the Cenozoic, including the Quaternary and the 

 Eecent. t 



* Manual of Geology, 1875, p. 586. 



\ Journal of Geology, Chicago, 1893, vol. i, pp. 294,295. 



