756 J. BAREELL— MEASUREMENTS OF GEOLOGIC TIME 



arguments against the peneplain.^' One of these rested on the fact that 

 no extensive peneplains, not uplifted or dissected^ are known to exist at 

 tire present time in any part of the earth, although peneplains were said 

 by Davis and his school to have been produced again and again in the 

 past. Therefore, argued Tarr, we need as a fundamental assumption to 

 believe that during a part of the remote past the conditions have been 

 different from these that have prevailed in any part of the known earth 

 during the present and the immediate past.^^ 



The following year Davis published a rejoinder/^ and, in regard to the 

 contention cited above, he stated that here Tarr and he were in agreement, 

 except that he should prefer to replace "fundamental assumption" by 

 "necessary corollary." Davis notes that it was as a very surprising corol- 

 lary that he came on the difference between the present and certain parts 

 of the past with respect to peneplanation.^* 



Davis's position is now regarded as established, but the significance of 

 this fact of difference between the present and the past is not fully appre- 

 ciated outside of the field of physiography. The earth, has rested time 

 and again in repose; shallow seas have spread far and wide over base- 

 leveled lands: over the greater part of the continental surfaces whicli 

 remained above the sea erosion must have be'come reduced to a small part 

 of its present rate. This conclusion, which is fundamental in stratig- 

 raphy and in the stratigraphic measures of geologic time, will be developed 

 from other points of view in the following topics. 



It is clear that epochs of diastrophism are more or less closely corre- 

 lated in widely different regions. Changes in sealevel are necessarily felt 

 over the whole earth, increasing or decreasing by relativity the mean 

 height of the lands. But beside this the lands themselves are periodically 

 broadly warped and, more locally, mountain growth takes place. In so 

 far as the relief of the land is simultaneously modified, the stages of ero- 

 sion cycles in different regions tend to be correlated and the mean rate of 

 denudation for broad regions and even for the whole earth may vary in 

 the same direction. 



The rate of denudation increases through the stage of topographic 

 youth, reaching a maximum when all of a drainage basin has become dis- 

 sected and given a maximum of sloping surface. From this mature stage 

 the rate decreases as the elevation of the interstream areas are lowered, 

 and finally nearly ceases in topographic old age. It is important, how- 

 ever, that this law of variation should be examined in more detail, for 



11 R. S. Tarr : The peneplain. American Geologist, vol. xxi, 1898, pp. 351-370. 

 '^ Loc. cit., pp. 353, 354. 



13 W. M. Davis : The peneplain, American Geologist vol. xxiii, 1899, pp. 207-239. 

 i*Loc. cit, p.. 221. 



