Page:The Journal of geology (IA journalofgeology11893univers).pdf/222

This page has been proofread, but needs to be validated.
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 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 assuming a minimum rate of mechanical denudation—of considerably less than one 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 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 discharged 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 magnesia, 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

  1. ↑ Brit. Assoc. Adv. Sci., Sixty-second Meeting, 1893, p. 21.
  2. ↑ Proc. Liverpool Geol. Soc., Vol. III., pt. 3, 1877, pp. 212-235.Chemical Denudation in Relation to Geological Time, 1879, pp. 1-61.
  3. ↑ Loc. cit., p. 229.
  4. ↑ Scottish Geol. Mag., Vol. III., 1887, pp. 65-77.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.