GEOLOGIC TIME. 



663 



the continent, towards the close of middle Cambrian time than 

 during subsequent Paleozoic time. It included a portion of the 

 region to the eastward and probably a belt of land extending 

 well towards the Pacific coast of the continental -plateau. The 

 interior (Mississippian) region, west of the 90th meridian, proba- 

 bly drained into the sea to the south, forming a Cambrian Missis- 

 sippi river prior to middle Cambrian time. This limits the 

 Cambrian drainage into the Cordilleran sea to an area estimated 

 at 1,600,000 square miles. The average thickness of mechanical 

 sediments deposited before upper Cambrian time is estimated at 

 from 10,000 to 15,000 feet. Taking the minimum of 10,000 

 feet and the assumed drainage area of 1,600,000 square miles 

 and the rate of denudation at one foot in 1,000 years, it would 

 have required 2,500,000 years to carry to the sea and distribute 

 the 10,000 feet of sediment. This means the deposition of .048 

 of -an inch per year, which is very small if the supposed con- 

 ditions of denudation and transportation were as favorable as the 

 character and mode of occurrence of the sediments indicate. If 

 one-fourth of an inch per year is assumed as the rate of deposi- 

 tion, the 10,000 feet of sediment would have accumulated in 

 480,000 years or, in round numbers, in 500,000 years, which 

 increases the rate of denudation to one foot in 200 years. 1 



CAMBRIAN MECHANICAL SEDIMENTS. 



Rate of erosion over land area 

 of 1,600,000 square miles. 



I foot in 3,000 years, 

 1 foot in 1,000 years, 

 I foot in 200 years - 



Time in years for ero- 

 sion of 2,500 feet. 



7,500,000 



2,500,000 



500,000 



Rate of deposition over sea area of 

 400,000 square miles for strata 10,000 

 feet thick. 



I foot in 750 years, or .016 inch 

 per annum. 



1 foot in 250 years, or .048 inch 

 per annum. 



1 foot in 50 years, or .24 inch 

 per annum. 



In view of the evidence of rapid accumulation contained in the strata themselves 

 the most rapid rate of deposition here stated, namely, .24 inch per annum, is con- 

 sidered as the most probable. 



'By Mr. Willis' method (ante, p. 662, foot note) the mechanical sediments of 

 the Paleozoic age for the area under consideration corresponds to 6 billion mile-feet. 



