GEOLOGIC TIME. 



825 



sand ami the 13,000 feet in tlie upper portion of tlic 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 liuiestones were being de- 

 l)osited, the tributary land areas were, in all probability, base levels of 

 erosion, and chemical denudation was pieparing 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 nnxterially shortened, yet, on account of 

 the manifestly slower depositions 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 subsid<?nce, which did not, however, greatly exteiul the 

 Cordilleran sea, and it may fairly be estimated at 000,000 square miles. 

 The depth of mechanical sediments already estimated is 5,000 feet and 

 their volume 2,000,000,000 mile feet. Dividing the volume by the area 

 of erosion we get 3,300 feet as the dei>th of erosion re(|uired. 



Again, applying different rates of erosion with allowance for slow 

 progress of degradation during Devonian time, we have: 



rost-Camhrian mechanical sediments. 



Eato of erosion over laud area of 

 600,000 square miles. 



1 foot in 3,001) years 

 1 foot in ],(ino years 

 1 foot ill 200 years. - 



Time required for 

 removal of 3 300 

 feet. 



9,900,000 years . . 

 3,300,000 years .. 

 060,000 years.. .. 



Rate of deposition in sea of 400,000 

 square miles, for 5,000 feet of strata. 



\ foot ill 1,080 years, or 0. 000 inch per an- 

 num. 



1 foot ill (100 years, or 0.018 iiicli ]ier an- 

 num. 



1 foot in 132 years, or 0.0'J iiieli i)cr an- 

 num. 



The rate of 1 foot in 200 years is assumed as the most probable and 

 660,000 years as the time required for the removal and dei)osition of 

 the 5,000 feet of post-Cambrian mechanical sediments. 



There is one factor that may need to be taken into consideration in 

 estimating the time duration of the deposition of the mechanical sedi- 

 ments of the Cambrian and i»re(?)-( Cambrian of the northern i)ortioii of 

 the Cordilleran sea that w^ould materially lengthen the period. Dr. 

 George ]\I. Dawson describes the Nisconlith series, especially in the 

 Selkirk range of British Columbia, as composed of ^'blacki.sh argillite- 

 schists and phyllites, generally calcareous with some beds of limestone 

 and quartzite, 15,000 feet."* It is correlated with the Bow KMver 

 series, which contains, in theup])er portion, the lower Cambrian fauna. 

 The presence of these calcareous beds indicates a slower rate of depo- 

 sition than we have estimated for the lower portion of the Cambrian 



* lUdl. deol. Soc. Jm., ISitl, vol., ii, p. 168. 



