Geologic Time.— Walcott 349 
sotaare along the eastern border of the area, next to the Archean rocks, 
— where it is probable that the Cretaceous overlaps the Paleozoic to 
the Archean. 
The western side of the Cordilleran sea seems to have been bounded 
by a land area that separated it from the Paleozoic sea, which extend- 
ed through central California and the Pacific border of British Colum- 
bia and Vancouver's Island. From the position of the Carboniferous 
deposits of California at the present time it appears that this land va- 
ried from 100 to 150 miles in width and was practically continuous along 
the western side of the Cordilleran sea. This view is further strength- 
ened by the fact that the Carboniferous fauna of California has cer- 
tain characteristics which are not found in the Carboniferous of the 
Cordilleran area. Our knowledge of the conditions north of the 55th 
parallel is limited by the want of accurate geologic data. If Cambrian 
and Carboniferous rocks were not deposited in the Mackenzie river 
basin and also on the eastern side of the area now covered by Creta- 
ceous strata, the inference is that during Cambrian and Carboniferous 
time there was a land area to the east and north of the northern Cor- 
dilleran sea that may have been tributary to the latter. 
Source op Sediments Deposited in the Cordilleran Sea. 
The sediments deposited in every sea or lake are derived from land 
areas either by mechanical or chemical denudation. 
Mechanical denudation results from the action of the waves and cur- 
rents along the shore and the agency of rain, frost, snow, ice, wind, 
heat, etc., on the land. Rain is the most important factor and the 
result depends mainly upon its amount and the slope or the gradient 
of the land. The general average of denudation for the surface of the 
land areas of the globe, now usually accepted, is one foot in 3,000 years. 
This varies locally, according to Sir Archibald Geikie, from one foot in 
750 years to one foot in 6,000 years*. Of the rate of denudation during 
Paleozoic time about the Cordilleran sea we know very little, but I 
think that it was relatively rapid in early Cambrian time and during 
the deposition of the arenaceous sediments of the Ordovician and 
Carboniferous. The material forming the argillaceous shales of the 
Cambrian and Devonian was supplied to the sea more slowly. These 
conclusions are sustained by the slight change in the character of the 
faunas where interrupted by the sands and pebbles of the Ordovician 
and Carboniferous and the marked change between the base and summit 
of the argillaceous shales. As a whole I think we are justified in as- 
suming a minimum rate of mechanical denudation — of considerably 
less than one foot in one thousand years — for the area tributary to the 
Cordilleran sea. 
Chemical denudation is the removal of material taken into solution 
by water. Mr. T. Mellard Reade has discussed this phase of denudation 
in an admirable manner.t He came to the conclusion from what was 
*Brit. Assoc. Adv. Sci., Sixty-Second Meeting, 1898, p. 21. 
tPi-oc. Liverpool Geo], Hoc. vol. 3. pt. 8, 1*77, pp. 212-285. ( 'liemical Denudation in 
relation to Geological time. 1879, pp. 1-01. 
