GEOLOGIC TIME. 313 



deposits of California at the i)resent time, it appears that this land 

 varied from 100 to 150 miles in width and was practically continuous 

 along the western side of the Cordilleran sea. This view is further 

 strengthened by the fact that the Carboniferous fauna of California has 

 certain characteristics which are not found in the Carboniferous of the 

 Cordilleran area. Our knowledge of the conditions north of the fifty - 

 fifth pai-allel is limited by the want of accurate geologic data. If Cam- 

 brian and Carboniferous rocks were not deposited in the Mackenzie 

 Eiver basin and also on the eastern side of the area now covered by 

 Cretaceous strata, the inference is that during Cambrian and Car- 

 boniferous time there was a laud area to the east and north of the 

 northern Cordilleran sea that may have been tributary to the latter. 



SOURCE OF SEDIMENTS DEPOSITED IN THE CORDILLERAN SEA. 



The sediments deposited in every sea or lake are derived from laud 

 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 laud. 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 1 foot in 3,000 years. 

 This varies locally, according to Sir Archibald Geikie, from 1 foot in 

 750 years to 1 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 Car- 

 boniferous. The material forming the argillaceous shales of the Cam- 

 brian and Devonian was supplied to the sea nuire slowly. These con- 

 clusions are sustained by the slight change in the character of the 

 faunas where interrupted hy 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 

 assuming a minimum rate of mechanical denudation — of considerably 

 less than 1 foot in 1,000 years — for the area tributary to the Cordilleran 

 sea. 



Chemical denudation is the removal of material taken into solution 

 by water. Mr. T. Mellard Keade has discussed this phase of denuda- 

 tion in an adniiiable manner.t He came to the conclusion, from what 

 was known of the water discharged into the ocean per year, the aver- 

 age amount of material in chemical solution, and the area of laud sur- 

 face drained by the rivers, tliat an average of 100 tons of rocky matter 

 is dissolved per English square mile per annum. Of this he says: "If 



*Brit. Assoc. Adv. Sci., sixty-second meeting, 1892, p. 21. 



t P»-«c'. Liverpool Geol. Snc, 1877, vol. in, pt. 3; pp. 212-235. Chemical Denudation 

 in relation to Geological Time, 1879, pp. 1-61. 



