360 
The American Geologist. 
Decern 1 xr, 1893 
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 sandstone, varying from 200 to 400 feet in thickness, distrib- 
uted 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 inter- 
val that the Trenton fauna in the limestone beneath it and in the lime- 
stones 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 Bhales 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 base-levels of erosion, and chemical 
denudation was preparing 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 2,000,000,000 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: 
Post-Cambhian Mechanical Sediments. 
Rate of erosion over land 
area of 000,000 square 
miles. 
1 foot in 8,000 years. 
1 foot in 1,000 years. 
1 foot in -00 \ 
Time required for 
removal of 3,300 
feet. 
9,900,000 years. 
3,300,000 ! i 
6150,000 years. 
Rate of deposition in sea of 
400,000 square miles, for 
5,000 feet of strata. 
1 foot in 1980 years, or .006 
i Qch per annum. 
. foot in 660 years, or .09 inch 
per annum. 
1 foot in 182 years, or .18 inch 
per annum. 
The rate of one foot in 200 years is assumed as the most probable 
and 060,000 years as the time required for the removal and deposition 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 pre?-Cambrian of the northern portion 
of the Cordilleran sea that would materially lengthen the period. 
Dr. George M. Dawson describes the Nisconlith series, especially 
in the Selkirk range of British Columbia, as composed of "black- 
ish argillite-schists and phyllites, generally calcareous, with some beds 
