664 THE JOURNAL OF GEOLOGY. 



In dealing with the post-middle Cambrian mechanical sedi- 

 ments we have a somewhat different problem, but, as a whole, 

 rapid deposition is indicated. For instance, the Eureka quart- 

 zite of the upper Ordovician is a bed of sandstone, varying from 

 200 to 400 feet in thickness, distributed over a wide area, — per- 

 haps 50,000 square 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 appears to have been 

 swept rapidly into the sea and distributed by strong currents. 

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

 sand and the 2,000 feet in the upper portion of the Carboniferous, 

 while the shales of the upper Devonian accumulated more slowly. 

 In this connection we must bear in mind that during the long 

 periods in which the calcareous sediments forming the limestones 

 were being deposited, the tributary land areas were in all proba- 

 bility base-levels of erosion, and chemical denudation was pre- 

 paring a great supply of mechanical material that, on the raising 

 of the land, was rapidly swept into the sea and distributed. In 

 this manner the time period of actual mechanical denudation was 

 materially shortened, yet, on account of the manifestly slower 

 deposition of the Devonian shales, the rate of denudation should 

 be assumed as less than during Cambrian time. 



In post-Cambrian time the area of the land surface was 

 materially reduced by subsidence, which did not, however, greatly 

 extend the Cordilleran sea, and it may fairly be estimated at 

 600,000 square miles. The depth of mechanical sediments 

 already estimated is 5,000 feet, and their volume at two billion 

 mile-feet. Dividing the volume by the area of erosion we get 

 3,300 feet as the depth of erosion required. 



Again, applying different rates of erosion, with allowance for 



slow progress of degradation during Devonian time, we have : 



Of this total the greater part, namely, two-thirds or 4 billion mile-feet, are of Cam- 

 brian age. Dividing this volume by 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 Cambrian 

 mechanical sediments. Assuming different rates of erosion we may obtain times dif- 

 fering as follows : 



