PHYSICAL FEATURES OF COLORADO VALLEY. 673 



Having in view tlie forms which are produced by erosion, it will 

 be convenient to classify the methods of erosion as follows : First, 

 corrasion by running streams, and, second, erosion by rains ; the first 

 producing channels along well-defined lines, the second producing tie 

 general surface features of the landscape. 



Of the fii'st class we have two varieties : 



A. The corrasion of water-gaps. 



J^. The corrasion of caiions. 



Of the second class we have three varieties : 



A. Cliff erosion, where the beds are slightly inclined, and are of 

 heterogeneous structure, some soft and others hard ; and, for the pro- 

 duction of the best-marked forms, the climate should be arid. Here 

 the progress of erosion is chiefly by undermining. 



D. Hog-back erosion, where the beds have a greater inclination, 

 but are still of heterogeneous structure. Here the progress of erosion 

 is by undermining and surface-washing, and the typical forms would 

 require an arid climate. 



(7. Hill and mountain erosion, where the beds may lie in any shape, 

 and be composed of any material not included in the other classes, 

 and the progress of erosion is chiefly by surface-washing. The typi- 

 cal forms are found in a moist climate. 



There is still another agency in the production of topographic feat- 

 ures, viz., the eruption of molten matter from below the general sur- 

 face. The beds formed are soon modified by erosion, and then the 

 forms produced are due to that agency, and fall under the general 

 series. But there is a time, immediately after the eruption, when 

 these beds lie in forms due to igneous dynamics, and the most im- 

 portant features produced are cones. These cones are very conspic- 

 uous features of the landscape over much of the region drained by the 

 Colorado River. 



The district of country drained by the Colorado and its tributaries 

 is divided into two parts, by a well-marked line of displacements. 

 The lower third of the valley, which lies southward from this line, is 

 but little above the level of the sea, except that here and there ranges 

 of mountains are found. From this region, there is usually a bold 

 step to a higher. 



The upper two-thirds of the area drained by the Colorado is from 

 4,000 to 8,000 feet above the level of the sea, with mountain-ranges on 

 the east, north, and west, of greater altitude. The bold step from the 

 lower country to the table-lands is usually an escarpment in rocks of 

 the Carboniferous Age, marked, here and there, by beds of lava, and 

 along its margin stand many volcanic cones. San Francisco Mountain 

 is made up of a group of these beds of eruptive matter, covering strat- 

 ified rocks. This higher region is the one to which we have given 

 especial attention in the previous discussion. 



The principal condensation of moisture occurs on and about the 

 VOL. VII. 43 



