RHYTHMS IN DENUDATTOX i bO 



miles, shoAvs the much lower denudation rate of one foot in 21/UH) years, 

 but an important (?orrection niay be needed in the arid and semi-arid 

 regions for eoliau trans])ortatioH. The amoant of denudation increases 

 with high rainfall, but the Mississippi basin shows, nevertheless, a liigiier 

 rate than the southern Atlantic M^atershed. 



Very high rates have heen found in mountain regions in other countries, 

 the Po being estimated to lower its basin one foot in 729 years, the Ganges 

 oue foot in 823 years. Geikie devotes a special heading to "The un- 

 equal erosion of land."'' He calls attention to the dependence of erosion 

 upon slope, and that within each river basin the rate varies greatly in its 

 different parts. It seems likely that over the more rugged portions of 

 the mountains in the Po and Ganges drainage area the rate of erosion 

 may be two or five times the mean for the whole of those areas. 



Using Clarke's estimate for chemical denudation for all the continents 

 as one foot in 30,000 years and taking this as 30 per cent of the total 

 denudation gives a mean rate of total denudation at the present time of 

 one foot in 8,600 years. This, although reliable as the present rate, per- 

 haps to within 10 per cent, is seen to depend upon the average of a great 

 range in values, both locally and regionall}^, from ten to twenty or fifty 

 times the mean as one limit to zero as the other limit, or even beyond it 

 to actual deposition. Any condition in the past lohicli would cliaiige the 

 aspects and areas of the lands in the same direction over various con- 

 tinents v)ould chcvnge the mean for such times enormously . Therefore if 

 such a figure, obtained for the present, is used blindly, it is likely to be- 

 come an extremely misleading unit for the measurement of geologic time. 

 Harker has recently called attention to the lack of value of mean figures 

 Avhich are based on such widely varying data.^^ 



RELATIONS OF RATE TO THE CYCLE OF EROSION 



Davis in his publications from 1889 to 1892, extending the ideas of 

 baseleveling formulated by Powell and Dutton, showed that the lands had 

 been baseleveled recurrently in former geologic periods. To the result- 

 ing topographic form he gave the name of peneplain. Many ancient pene- 

 plains are now buried and preserved as surfaces of unconformity; others 

 have been elevated and more or less destroyed by new cycles of erosion as 

 yet uncompleted. In topographic old age the rate of erosion he showed 

 must become indefinitely low. 



In 1898 Tarr published a paper in which he brought forward various 



A. Geikie : Text-book oC CTeology, 1903, pp. 591-593. 



^" A. Harker: Geology in relation to the exact sciences, witb an excursus on .yeological 

 time. Nature, vol. 05, 1915, pp. 105-109. 



