816 



J. BARBELL MEASUREMENTS OF GEOLOGIC TIME 



one foot in 200 years and a deposition rate of one foot in 50 years. This 

 rate he regards as more probable than slower ones, '^in view of the evi- 

 dence of rapid accumulation contained in the strata themselves." These 

 basal assumptions give 500,000 years for the 10,000 feet of strata.^^ 



"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 Upper Ordovician is a bed of sand- 

 stone, varying from 200 to 400 feet in thickness, distributed over a wide area — 

 perhaps 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 lime- 

 stone 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 probability baselevels of erosion, and chemical denudation was preparing a 

 great supply of mechanical material that, on the raising of the land, was rap- 

 idly 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 

 l)y 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 3,800 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 : 



""Post-Cambrian mechanical Sediments 



Rate of erosion over land area of 

 600,000 square miles. 



1 foot in 3,000 years 

 1 foot in 1,000 years 

 1 foot in 200 years . . 



Time required for re- 

 moval of 3.300 feet. 



9,900,000 years. 



3,300,000 years, 



660,000 years. 



Rate of deposition in sea 

 of 400,000 square miles 

 for 5,000 feet of strata. 



1 foot in 1,980 years, or 

 .006 inch per annum. 



1 foot in 660 years, or 

 .09 inch per annum. 



1 foot in 132 years, or 

 .18 inch per annum. 



"The rate of one foot in 200 years is assumed as the most probable and 

 660,000 years as the time required for the removal and deposition of the 5,000 

 feet of post-Cambrian mechanical sediments." ^' 



SI Loc. clt., p. 663. 



" Loc. clt, pp. 664, 665. 



