PHYSICAL FEATURES OF COLORADO VALLEY. 535 



district is traversed by brooks and creeks and rivers of mud. A clear 

 stream is never seen witbout going up to a moistcr region on some 

 high mountain, and no permanent stream is found, unless it has its 

 source in such a mountain. In a country well supplied with rains, so 

 that there is an abundance of vegetation, the water slowly penetrates 

 the loose soil, and gradually disintegrates the underlying solid rock, 

 quite as fast as, or even faster than, it is carried away by the wash of 

 the rains, and the indurated rock has no greater endurance than the 

 more friable shales and sandstones; but, in a dry climate, the softer 

 rocks are soon carried away, while the harder rocks are washed naked, 

 and the rains make but slow progress in tearing them to pieces. 



When a great fold emerges from the sea, or rises above its base 

 level of erosion, the axis appears above the water (or base-level) first, 

 and is immediately attacked by the rains, and its sands are borne off 

 to form new deposits. It has before been explained that the emer- 

 gence of the fold is but little faster than the degradation of its surface, 

 but, as it comes up, the wearing away is extended still farther out on 

 the flanks, and the same beds are attacked in the ncAV land which have 

 already been carried away nearer the centre of the fold. In this way 

 the action of erosion is continued on the same bed from the upturned 

 axis toward the down-turned axis, and it may and does often liapj^en 

 that any particular bed may be entirely carried away, with many un- 

 derlying rocks, nearer the former line, before it is attacked near the 

 latter. Now, as the beds ai'e of heterogeneous structui*es, some hard 

 and others soft, the harder beds withstand the action of the storms, 

 while the softer beds are rapidly carried away. 



The manner in which these beds are degraded is very different. 

 The softer are washed from the top, but the harder are little affected 

 by the direct action of the waters they are torn down by another 

 process. As the softer beds disappear, the harder are undermined, 

 and are constantly breaking down ; are crushed, more or less, by the 

 fall, and scattered over, and mingled with the softer beds, and are 

 carried away with them. But the progress of this undermining and 

 digging down of the cliff is parallel with the upturned axis of the 

 fold, so that the cliffs face such an axis. 



When the fold is abrupt, so that the rocks on either side are made 

 to incline at a great angle, ridges are formed, and this topographic 

 structure of a country may be found even in a land of rains, though 

 the ridges will usually be low, rounded, and more or less irregular, 

 while in a dry climate they will be steep and regular, and will usually 

 culminate above in a sharp edge ; but where the rocks are slightly in- 

 clined, terraces will be formed, with well-defined escarpments. 



It is interesting to note the manner in which the textures of these 

 hard capping rocks affect the contours of the cliffs. When the hard 

 rocks are separated into well-defined layers, or beds, the cliffs will be 

 more or less terraced, as the strata vary in hardness. This is well 



