30 The American Geologist. JulJ - 1905 
transporting power by more than a simple ratio. These 
facts were brought out thirty years ago by Gilbert in his 
classic memoir on the Henry mountains. 
The amount of corrasion which a stream can perform 
depends upon its load. The transported detritus forms the 
tool with which it cuts, but an excess of material prevents 
corrasion. When a stream has all the load it can carry, the 
entire energy is used in transportation, and there is none 
for corrasion. If there is an excess of detritus, the trans- 
porting power is insufficient and deposition takes place. 
When a stream empties into a body of standing water its 
velocity is checked, material is deposited and further corra- 
sion is impossible. 
The ordinary peneplain, of Powell's type, is produced 
as a result of checked velocity. On emptying into the sea 
a stream's velocity diminishes, it deposits material, and its 
valley widens by weathering. When several valleys widen 
at the expense of the interstream areas a flat is formed, and 
this gradually extends upstream, until a peneplain is pro- 
duced. But the initial cause of these results is checked 
velocity and that alone. 
The energy of a stream depends not only upon velocity 
but also iron volume. Obviously a decrease in volume 
would also lead to deposition and to a cessation of corra- 
sion. Such a decrease in volume might take place in vari- 
ous ways; but the common way in the plateau region is 
when 'a stream in its course passes from a less arid to a more 
arid climate. In the case of our ideal laccolith the rain 
would all be caught near the summit, streams would become 
established which would flow down the slopes, and on 
reaching the arid surrounding plain these streams would 
speedily dry up. This result would be accomplished partly 
by evaporation and partly by soaking in, as a result of the 
lower ground water level. 
In the case of the laccolith, the process is aided by 
lessened declivity. The form being a constructional one, 
pushed up out of a previously existing plain, there would 
be a change in grade in passing away from the slopes of the 
mountains. This decrease in declivity would produce a 
corresponding decrease in velocity. Hence lessened volume 
