Topographic Form*. — Perry. 157 
be so altered by erosion that it naturally falls among erosion 
products and can not be placed in this division. 
The destructive processes are more important than those of 
construction, for they have produced all the greater features 
of the earth except volcanic mountains, and all forms, of what- 
ever origin, arc subject to modifications by their action. A- 
Boon as the land emerges from the water the various destruc- 
tive processes begin their work and tend to reduce it once 
more to the ocean level. We find marking the stages of this 
action an infinite gradation of forms, from the rugged moun- 
tain system to the featureless plain that marks the approxi- 
mate base level of erosion. 
Destructive action maybe classified under four divisions 
The first is erosion by rivers and streams. This action 
tends to form canons and gorges which subsequently broaden 
into valley- through the action of subaerial degradation. 
It- effects are most marked in a country that is topographi- 
cally young, for there it works with the greatest energy and 
subaerial degradation has not obliterated the evidences of it- 
action. Landslides resulting directly or indirectly from 
stream erosion may produce local forms of considerable im- 
portance. They may also result from subaerial degrada- 
tion. 
Subaerial degradation form- the Becond division. It com- 
prise- the action chiefly of the rain and frost. With the help 
of streams to carry away the detritus, it broadens canons into 
valleys, carves mountain masses into peaks and bad-lands, and 
finally reduces the whole surface to a flat or gently undulating 
peneplain. All mountains and mountain systems not of vol- 
canic origin are due to such action ; also isolated mountains 
and hills, outliers, buttes, plateaus, mesas, escarpments and 
cliff's of recession, and plains not formed directly by construc- 
tive processes. The work of subaerial degradation i- insep- 
arably connected with stream erosion and bo there i- no sharp 
line separating this division from the preceding. The action 
of the smallest rill differs from that of a river only in extent. 
For convenience, however, we class the action of rills with sub- 
aerial degradation and restrict stream erosion or corrasion to 
to the action of stream- of some magnitude. 
The third division comprises the action of wave-. The only 
