324 



GEOLOGIC TIME. 



towards the close of middle Cambrian time, than during subsequent 

 Paleozoic time. It included a portion of tlie region to the eastward 

 and i)robably a belt of land extending well toward the Pacific coast 

 of the continental i)lateau. The interior (Mississippian) region, west 

 of the ninetieth meridian, probably drained into the sea to the south, 

 forming a Cambrian Mississippi river prior to middle Cambrian time. 

 This limits the Cambrian drainage into the Cordilleran sea to an area 

 estinmted at 1,000, 000 square miles. Tlie average thickness of mechan- 

 ical sediments deposited before upper Cambrian time is estimated at 

 irom 10,000 to 15,000 feet. Taking the minimum of 10,000 feet and the 

 assumed drainage area of 1,000,000 square miles and the rate of denuda- 

 tion at 1 foot in 1,000 years, it would have required 2,500,000 years to 

 carry to the sea ami distribute the 10,000 feet of sediment. This means 

 the deposition of O-OIS of an inch per year, which is very small if the 

 supposed conditions of denudation and transportation were as favor- 

 able 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 deposition, 

 the 10,000 feet of sediment would have accumulated in 180,000 years 

 or, in round numbers, in 500,000 years, which increases the rate of 

 denudation to 1 foot in 200 years.* 



In dealing with the post-middle Cambrian mechanical sediments we 

 have a somewhat different problem, but, as a whole, rapid deposition 

 is indicated. For instance, the Eureka quartzite of the upj)er Ordo- 

 vician is a bed of sandstone, varying from 200 to 400 feet in thickness, 

 distributed over a wide area, perhaps 50,000 scpuire miles. It is made 

 almost entirely of a white, clean sand that was deposited in so short 

 an interval that the Trenton fauna in the limestone beneath it and in 

 the limestones above it is essentially the same. The sand ai)pears to 

 have been swept rapidly into the sea and distributed by strong cur- 

 rents. The same is true of the 3,000 feet of the lower Carboniferous 



* By Mr. Willis' metliod {ante, p. 323) the nieclianical sedimeuts of tLe Paleozoic 

 a<'0 for the area under consideration correspond to 6,000,000,000 niile-l'eet. Of this 

 total the greater part, namely, two-thirds or 4,000,000,000 mile-feet, are of Cambrian 

 age. Dividing this volume hy the land area Just given, 1,600,000 square miles, we 

 get 2,500 feet as the depth of erosion during the formation of the Camhrian mechan- 

 ical sediments. Assuming different rates of erosion we may obtain times differing 

 as follows : 



Cambrian mechanical sediments. 



Rate of erosion over land area of 

 1,600 000 s(iiiare miles. 



Time in years 

 for erosion of 

 2 500 feet. 



1 foot in 3,000 years. 

 1 foot in 1,000 year.s. 

 1 foot in 200 years . . 



7, 500,000 



2, 500,000 



500,000 



Rate of depo.sition over sea area of 400 000 

 S(iuare miles for strata 10 000 feet thick. 



1 foot in 750 years, or 0-016-iuch per aiuuim. 

 1 foot in 250 years, or 0-048-ineh per annum. 

 1 foot in 50 years, or 0-24-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, 0-24-inch per annum, is con- 

 sidered as the most probable, 



