PHYSICAL FEATURES OF COLORADO VALLEY. 671 



and thus we may have a series of base-levels of erosion, below wbich 

 the rocks on either side of the river, though exceedingly friable, can- 

 not be degraded. In these districts of country, the first work of rains 

 and rivers is to cut channels, and divide the country into hills, and 

 perhajDS mountains, by many meandering grooves or water-courses, 

 and when these have reached their local base-levels, under the ex- 

 isting conditions, the hills are washed down, but not carried entirely 

 away. 



With this explanation I may combine the statements concerning 

 elevation 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 dis- 

 trict to district, and in a vertical or inclined direction, rather than an 

 horizontal, the topographic features, which I have described as hog- 

 backs and cliffs of erosion, are produced. The difierence between hog- 

 backs and cliffs of erosion is chiefly due to the amount of dip or in- 

 clination 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 rainfall, the water penetrates and 

 permeates the rocks, and breaks them up, or rots them, to use an ex- 

 pression which has been employed with this meaning ; and the differ- 

 ence 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 rainfall, 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 hori- 

 zontal, and composed of stratified beds of varying thickness, the ten- 

 dency is to cut channels with escarpments or cliffs ', but if the beds 

 are greatly inclined, or composed of unstratified material, the tendency 

 is to cut channels with more flaring and irregular walls. These tenden- 

 cies are more clearly defined when the meteorologic conditions are 

 favorable that is, if a stream cuts through stratified rocks, in an 

 arid region, and carries the waters from a district more plentifully 

 supplied, the cliff character of the walls is increased ; and where a 



