NOT A REGION OF MOST EAPID EROSION. 209 
And yet the conditions necessary to great erosion in the Valley of the 
Colorado are not found to exceed those of many other regions. In fact, the 
aridity of the climate is such that this may be considered a region of lesser, 
rather than greater, erosion. We may suppose that, had this country been 
favored with an amount of rain-fall similar to that of the Appalachian coun 
try, and many other districts on the surface of the earth, that 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 are able to study erosion in this country, and find 
evidences 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, cceteris paribus, as might be 
supposed ; . for, with the increase of rains there will be an increase of vege 
tation, which serves as a protection to the rocks, and distributes erosion 
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 hope to return at some 
future time. 
From the considerations heretofore presented, it is not thought neces 
sary 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 valleys 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 character 
istics of the present topography have existed for a long time. There are 
27 COL 
