676 THE POPULAR SCIENCE MONTHLY. 



the surface of the earth, the base-level of erosion of the entire area 

 would have been the level of the sea ; and, under such circumstances, 

 though the erosion would have been much greater than we now find, 

 the evidences of erosion would have been more or less obliterated. 

 As it is, we ai"e able to study erosion in this country, and find evi- 

 dences of its progress and its great magnitude, from the very fact that 

 the conditions of erosion have been imperfect. 



It is proper to remark here that erosion does not increase in ratio 

 to the increase of the precipitation of moisture, as might be supposed ; 

 for, with the increase of rains there will be an increase of vegeta- 

 tion, which serves as a protection to the rocks, and distributes ero- 

 sion more evenly, and it may be that a great increase of rains in this 

 region would only produce a different series of topographic outlines, 

 without greatly increasing the general degradation of the valley of 

 the Colorado. 



To a more thorough discussion of this subject I hoj^e to return at 

 some future time. 



From the considerations heretofore presented, it is not thought 

 necessary to refer the exhibition of erosion shown in the canons and 

 cliffs to a more vigorous action of aqueous dynamics than now exists, 

 for, as I have stated, a greater precipitation of moisture would have 

 resulted in a very different class of topographic features. Instead of 

 canons, we should have had water-gaps and ravines ; instead of val- 

 leys with cliff-like walls, we should have had valleys bounded by hills 

 and slopes ; and if the conclusions to which we have arrived are true, 

 the arid conditions now existing must have extended back for a period 

 of time of sufficient length to produce the present canons and cliffs. 

 But there are facts which seem to warrant the conclusion that this 

 condition has existed for a much longer period than that necessary 

 for the production of the present features ; that is, the characteristics 

 of the present topography have existed for a long time. There are 

 evidences that the lines of cliffs themselves have been carried back for 

 great distances as cliffs by undermining, which is a process carried on 

 only in an arid region. 



The evidence is of this character : I have stated that the drainage 

 of the inclined plateaus is usually from the brink of the cliffs back- 

 ward ; i. e., the water falling on the plateau does not find its way im- 

 mediately over the cliffs, but runs from the very brink or edge of the 

 plateau back toward the middle or farther side, which is usually found 

 against the foot of another line of cliffs, and here the waters are turned 

 toward some greater channel, which runs against the dip and cuts 

 through the cliffs. Now, the water-ways at the heads of these streams 

 that have their sources near the brink of the cliffs would always be 

 small, shallow, and ramifying into many minute branches if the line 

 of cliffs were a fixed or immoA'^able line, but we often find that the 

 cliffs have been carried back by the undermining process until all 



