METHODS OF EROSION, 'CLASSIFIED. 205 
* 
and composed of stratified beds of varying thickness, the tendency 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 tendencies are more clearly defined when 
the ineteorologic 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 stream runs through unstratified rocks, in a district well supplied with 
rains, the walls or banks of the stream are cut down in more gentle slopes. 
For purposes of discussion, it will be convenient to call the deep chan 
nels of streams through table-lands, in arid climates, canons; and the deep 
channels of streams through heterogeneous beds, in a moist climate, water- 
gaps, or narrows, and ravines. 
Having in view the 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 the general surface features 
of the landscape. 
Of the first class we have twd varieties: 
A. The corrasion of water-gaps. 
B. The corrasion of canons. 
. Of the second class we have three varieties: 
A. Cliff erosion, where the beds are slightly inclined, and are of hete 
rogeneous structure, some soft and others hard; and for the production 
of the best marked forms, the climate should be arid. Here the progress 
of erosion is chiefly by undermining. 
B. 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. 
C. 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 typical forms are 
found in a moist climate. 
