32 The American Geologist. July - 1905 
In the usual erosion cycle in a moist climate, deposi- 
tion is a mark of increasing age. There are notable excep- 
tions, but in the typical, normal case a flood-plain is formed 
after a considerable amount of down-cutting has been done 
at the mouth of the stream. In the laccolithic cycle deposi- 
tion takes place at all stages and in all places except the 
uppermost slopes.. For if, after a rain, a flood stream ex- 
tends its course two miles onto the plain and there dries up, 
for the last mile and a half or so it will have been losing 
volume and velocity and will have been depositing its 
material either as a flood plain or as an alluvial fan. If a 
few days later it has shrunk in volume and extends only 
one mile onto the plain, its transporting power will have 
decreased throughout its length and deposition will be tak- 
ing place at the edge of or within the mountains. As the 
stream continues to shrink, its transporting power decreases 
until material is dropped well within the mountains. This 
deposition of material is a normal feature of all stages of 
the erosion cycle, though obviously more material will have 
been deposited when old age is reached than in youth. 
The form taken by the alluvial deposits is somewhat 
different in the two climates. The general process of cut 
and fill is the same for both, but the surface configuration 
differs. An old valley emptying into the sea develops flood 
plains along its lower course and also a delta at its mouth. 
These deposits are laid down in standing or in slowly mov- 
ing water. 
The banks of the river are still higher than its chan- 
nel and the flood plain is a sort of filling dropped into the 
bottom of a curve concave upwards. The laccolithic de- 
posits are as a rule built up on a flat with no pre-existing 
valley, and they take the form ©f alluvial fans. The con- 
fluence of several fans from neighboring streams ma) r 
produce a plain. 
In the normal erosion cycle in a humid region the cross 
section of the valleys changes from a steep sided V in 
youth, to a gentler sloped U in maturity. In arid regions 
the U shape never comes. If the valleys widen, it is by 
the retreat of nearly vertical cliffs. The reason seems to 
be that moisture and its results, soil and vegetation, are at 
