204 EXPLORATION OF THE CANONS OF THE COLORADO. 
either side of the river, though exceedingly friable, cannot be degraded. In 
these districts of country, the first work of rains and rivers is to cut chan 
nels, and divide the country into hills, and, perhaps, mountains, by many 
meandering grooves or water-courses, and when these have reached their 
local base levels, under the existing conditions, the hills are washed down, 
but not carried entirely away. 
With this explanation I may combine the statements concerning eleva 
tion and inclination into this single expression, that the more elevated any 
district of country is, above its base level of denudation, the more rapidly 
it is degraded by rains and rivers. 
The second condition in the progress of erosion, is the character of the 
beds to be eroded. Softer beds are acted upon more rapidly than the harder. 
The districts which are composed of softer rocks are rapidly excavated, so 
as to become valleys or plains, while the districts composed of harder rocks 
remain longer as hills and mountains. 
Where the beds are of stratified material, so that the change from harder 
to softer materials is from bed to bed, rather than from district to district, and 
in a vertical or inclined direction, rather than a horizontal, the topographic 
features, which I have described as hog-backs and cliffs of erosion, are pro 
duced. The difference between hog-backs and cliffs of erosion is chiefly 
due to the amount of dip or inclination of the beds. 
But there is another condition necessary to the production of cliffs and 
hog-backs in their typical forms. The country must be arid, for where there 
is a great amount of rain-fall, the water penetrates and permeates the rocks, 
and breaks them up, or rots them, to use an expression which has been 
employed with this meaning; and the difference between the durability of 
the harder beds and that of the softer, is, to some extent, compensated for 
by this agency, though doubtless ridges and cliffs may be produced in less 
arid climates, as we find them in the Appalachian System, but not so well 
marked. In a region of country where there is a greater amount of rain 
fall, the tendency is to produce hills and mountains, rather than plateaus 
and ridges, with escarpments. 
Now let us examine the character of the channels which running 
streams carve. Where the rocks to be carved are approximately horizontal, 
