U8 A M A N I A L OF TOPOGRAP HIC METHODS. 



more rapid in the moister climate, and consequently that, finding an 

 abundance of material in the bed of soil, a larger proportion of the ener- 

 gies of corrasion are devoted to removing it, while proportionately less is 

 devoted to rock work. Still the effect of structure is by no means absent 

 in the East, 



Since disintegration and corrasion of hard or insoluble rocks go on 

 slowly, and of soft or soluble rocks rapidly, the elevated areas are conse- 

 quently, as a rule, composed of the former, while the depressed areas are 

 commonly of the latter class of rocks. It is the survival of the hardest. 



When erosion has "left a peak, a projection, spur or boss, a butte or 

 mesa, a neck or dike, it is commonly because the material is harder than 

 that adjoining. The valleys of the Appalachian region are almost without 

 exception cut in soluble limestone, while the ridges are mainly, and the 

 higher ones entirely, of sandstone. 



Streams usually make their channels along lines of least resistance. 

 They accommodate themselves to the softness of the rocks and avoid 

 obstacles. The more rapid the stream, however, the less does it care for 

 obstacles, while gentle streams are most easily diverted. 



The level surface of a plateau is generally the summit of a hard bed, 

 from which, it may be, softer beds have been washed away and on which 

 erosion lias comparatively come to a standstill. 



Where rocks of different hardness are subjected for the same time to 

 an equal intensity of corrasion, since the effect upon the softer rock is 

 greater than that upon the harder, it will be brought down to gentler 

 slopes; in other words, other things being equal, the harder the rock the 

 steeper the slope, the softer the rock the more gentle the slope. Now, let 

 this proposition be applied to the cross sections of stream beds. Suppose 

 two stream beds, one in soft rock, another in hard rock, both of them sub- 

 jected to the same climatic agencies and the same corrasive action for the 

 same time. In these two rocks the stream beds will be carved somewhat as 

 shown in Nos. 1 and 2, in Figure 13, indicating progressive stages of opera- 

 tion. 



The simplest case for consideration and a very common one is that of 

 horizontal beds, alternately hard and soft, such as are represented in Fig- 



