WEAKNESS OF FLUVIATlLE HYPOTHESIS 713 



Tlie comparison of the vast and even surface of the Great Plains with 

 that formed by certain great rivers — as the Po, the Ganges, or the 

 Hwang-ho — appear to afford evidences strongly against rather than in 

 support of the hypothesis. In the last mentioned cases it is the mouths 

 of the rivers that are considered. In the Great Plains area it is really 

 the headwaters of the streams tliat are taken into account. Clearly there 

 can not be any valid comparison. The tendency of rivers to form exten- 

 sive deposits of fine texture and even stratification appertains to their 

 lower reaches and not to their sources. 



A lofty mountain background capable of supplying enormous amounts 

 of rock-waste seems to be afforded on a grand scale by the Rockies. In 

 moist climates generally, and with similar features, such supplies of 

 coarse materials are usually at hand. Even the coarse piedmont deposits 

 in many desert regions are important. The case of the Great Plains is 

 not in accord with such rule. So far as personal observation goes, and 

 so far as the most trustworthy literature indicates, coarse piedmont de- 

 posits do not extend forward everywhere along the Rocky Mountain 

 front. Deposits of fine homogeneous materials of the nature of loess 

 begin to appear on the great piedmont, and extend in enormous volume 

 indefinitely eastward. Coarse materials are generally wanting. Only 

 along two streams in Kansas and Nebraska, for instance, do coarse ma- 

 terials appear beyond the piedmont belt. 



Recapitulation 



From the foregoing record of observations and the consideration of the 

 Great Plains, it is argued that — 



(1) The dominant relief feature is to be regarded as fashioned mainly 

 by eolative processes, and as infinitely smoother than it is possible to 

 attain through planation effects of water action. 



(2) The Plains deposits are chiefiy wind-borne dusts derived from 

 the deflation of the arid regions, and extend in full force to the Missouri 

 River. 



(3) Eolation, both of destructive and constructive character, is still 

 in progress on a large scale, deflation prevailing in a broad western belt, 

 aeroposition predominating in the wide median belt, while in the eastern 

 belt extensive eolic deposition is greatly obscured by the conditions pre- 

 sented by moist climate. 



XLVII— Bull. Geol. Soc. Am., Vol. 22, 1910 



