CHAMBERLTN— THE AGE OF THE EARTH. 253 



The periods occupied in the process of lowering the surface by 

 denudation are held with good reason to be greater than those occu- 

 pied in its elevation by deformative action. It is needless to say that 

 elevation increases the velocity of the run-ofif, and that this velocity 

 greatly increases the transporting power. ^^ It is generally agreed 

 that the present altitude of the continents is greater than their mean 

 elevation during geologic history. Geologists recognize at least two 

 stages in which the continents were exceptionally high and broad : 

 that which attended the transition from the Paleozoic to the Mesozoic 

 Era, and that which attended the transition of the Tertiary to the 

 present epoch. The existing stage thus falls in one of the most 

 notable stages when continental elevation and breadth were greatest, 

 though perhaps not at its climax. Geikie estimates the present mean 

 elevation of the land at 2,441 feet.^- The mean elevation of the great 

 peneplains is a matter of judgment rather than of knowledge, but no 

 one would probably put the elevation at much more than a third of 

 this. Probably a third is too high. The mean elevation for all the 

 ages, high and low, quite surely falls somewhere between 2,400 and 

 800 feet, and probably nearer the 800 than the 2,400. There can be 

 little doubt, then, that the present rate of denudation and deposition 

 is much above the mean rate. 



There are incidental conditions attending high relief which add 

 appreciably to the immediate effects of the steep declivities to which 

 it gives rise. Relief of the surface increases the vertical air currents 

 and these favor precipitation; they also tend to concentrate the pre- 

 cipitation and give it enhanced effect. High relief often induces 

 sharp showers and distinctly rapid run-off. The smooth surfaces of 

 the stages of lower elevation, on the other hand, favor a more even 



phere," ibid., Vol. VI. (1898), pp. 609-22; and specifically as applied to the 

 question of age ; " Rhythm and the Measurement of Time," J. Barrell, Bull. 

 Geol. Soc. Am., Vol. 28 (1917), pp. 745-904. The argument used in the pres- 

 ent paper will be found stated as a quotation from Chamberlin in Holmes' 

 " The Age of the Earth," cited above (1913), pp. 79-8i. See also T. C. Cham- 

 berlin, " Diastrophism as the Ultimate Basis of Correlation," in " Outlines of 

 of Geologic History," compiled by B. Willis and R. D. Salisbury (1910), The 

 University of Chicago Press, pp.' 298-306. 



11 See the special investigation of G. K. Gilbert, " The Transportation of 

 Debris by Running Water," Prof. Paper 86, U. S. Geol'. Surv. (1914). 



""Textbook of Geology," 4th ed. Vol. I. (1903), p. 49. 



