AND ITS INHABITANTS 59 



mile per year, the northern Atlantic basin being next with 130 

 tons. The rate for the Hudson Bay basin, where the topog- 

 raphy is nearly flat and the rocks are mainly granites, is lowest, 

 with 28 tons. 



Holmes states 4 that the known denudation rates vary be- 

 tween i foot in 400 years (Irawadi basin) to i foot in 47,000 

 years (Hudson Bay region). For North America the rate 

 for solvent denudation is i foot in 30,000 years, and for 

 mechanical removal i foot in 12,000 years. "Taking both 

 together, the average rate of denudation is found to be i foot 

 in 8,600 years." 



The ancient life is known to us in the countless fossils of 

 the rocks, for they are the remains of plants and animals 

 living on the lands and in the seas at the time when the sedi- 

 ments were accumulating. Most of them, like the shellfish, 

 lived on or in the bottom of the seas, other active forms swam 

 about in the water, while the remains of the plants and animals 

 of the lands were drifted by the streams into the marine areas, 

 where they eventually sank to the bottom and came to be 

 covered over buried by the accumulating sands, muds, or 

 limestones. It should, however, be added that but little of 

 the life of any time is preserved as fossils, for much of it is 

 too soft to be preservable, or it is eaten or destroyed by other 

 organisms or reduced by the solvent powers of bacteria and 

 the acids of the waters. 



The record of the physical forces which automatically en- 

 tomb the successive upwellings of life is found upon all the 

 continents, but is woefully incomplete in any one. For many 

 cycles the marine record is best in North America, at other 

 times in Europe or the Mediterranean countries or in Asia. 

 The land or fresh-water record is even more scattered and 

 imperfect, with the longest deposition in Africa, a continent 

 that, south of the Sahara desert, has been invaded but little 



4 Holmes, A., op. cit., p. 57. 



