648 THE JOURNAL OF GEOLOGY. 



according to Sir Archibald Geikie, from one foot in 750 years to 

 one foot in 6,000 years. 1 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 argil- 

 laceous 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 

 assuming a minimum rate of mechanical denudation — of con- 

 siderably less than one foot in 1,000 years — for the area tribu- 

 tary 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. 2 He came to the 

 conclusion, from what was known of the volume of water dis- 

 charged into the ocean per year, the average amount of material 

 in chemical solution and the area of land surface drained by the 

 rivers, that an average of 100 tons of rocky matter is dissolved 

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

 allot 50 tons to carbonate of lime, 20 tons to sulphate of lime, 

 7 to silica, 4 to carbonate of magnesia, 4 to sulphate of mag- 

 nesia, 1 to peroxide of iron, 8 to chloride of sodium, and 6 to 

 the alkaline carbonates and sulphates we shall probably be as 

 near the truth as present data will allow us to come." 3 By the 

 use of the data given by Mr. John Murray, in a paper on the 

 total annual rainfall on the land of the globe, and the relation of 

 rainfall to the discharge of rivers, 4 I obtain 113 tons as the total 



'Brit. Assoc. Adv. Sci., Sixty-second Meeting, 1893, P- 21. 



2 Proc. Liverpool Geol. Soc, Vol. III., pt. 3, 1877, pp. 212-235. Chemical Denuda- 

 tion in Relation to Geological Time, 1879, pp. 1-61. 

 3 Loc. cit., p. 229. 

 "Scottish Geol. Mag., Vol. III., 1887, pp. 65-77. 



