311 



[Stevenson, 



But the character of the deposit on these benches shows that it could not 

 have accumulated under such conditions as must have existed had the 

 plains resulted from lateral corrasions by streams with but slight fall. 

 Under such circumstances, as has well been shown by Maj. Powell, the 

 debris torn away by tributary streams would be distributed over the wid- 

 ened flood-plain of the main stream and would not be washed away by 

 that stream. Such a deposit would contain many transported fragments, 

 fragments, indeed, of rocks belonging not far away, but showing distinct 

 traces of water-wear, for their motion would be slow and their exposure 

 long-continued, owing to the slight fall of the transporting streams. No 

 small period would elapse from the time of tearing them from the rock to 

 that of depositing them on the flood-plain ; and during all this period they 

 would be subjected to the action of water. 



This condition is well shown on the great plains stretching eastward 

 from the Rocky Mountains for hundreds of miles and finally merging into 

 the prairies of the Missouri-Mississippi Valley. These plains are covered 

 with a deposit which originated certainly according to Maj. Powell's doc- 

 trine. The character of the deposit is shown in the channel-ways of all 

 the streams. On top is an irregular layer of fine silt, which rests on a 

 mixed mass of fine silt, gravel and large water-worn fragments; while, 

 above all, over the whole surface, water-worn fragments are freely stiewn. 

 The higher benches toward the mountains as well as those on hills far out 

 in the plains show a similar covering. The coarser beds of gravel are 

 often cemented by carbonate of lime into firm conglomerates. But 

 this is not the character of the deposit on the horizontal benches of the 

 region under consideration. As has been said so often in this paper, the 

 deposits on these benches contain no water- worn, transported fragments ; 

 the only fragments found belong clearly enough to the underlying rocks, 

 and show no signs of having been subjected to the action of running 

 water. 



The doctrine of base-levels of erosion, though adequately and beautifully 

 explaining the conditions existing in the arid regions of the far- west, fails to 

 account for these horizontal benches in the Alleghany region, one of great 

 rainfall. These plains are too widespread, too nearly horizontal and par- 

 allel, too recent and too nearly free from traces of running water, to be re- 

 garded as marking base lines of erosion referable to stream beds. 



It is impossible to account for the phenomena on the hypothesis of a 

 great flood's sweeping down over the whole region, for the action of such 

 a flood would be too violent to produce effects such as have been described 

 in this paper. 



The benches bear much resemblance to beach lines, marking successive 

 stages of emergence from a body of water ; but they are not due to base- 

 level erosion in the full sense of that term in this connection. The areas 

 of the benches are so insignificant in many places that they could not have 

 been leveled by water falling on and flowing off the surface. Such a process 

 would require a vast length of time, altogether too vast in view of the 



