TORRENTS EXTINGUISHED. 269 
their own beds and the rapidity of their flow, and thus tending 
to become river-like in character. 
We cannot measure the share which human action has had 
in augmenting the intensity of causes of mountain degradation, 
and of the formation of plains and marshes below, but we know 
that the clearing of the woods has, in some cases, produced, 
within two or three generations, effects as blasting as those 
generally ascribed to geological convulsions, and has laid waste 
the face of the earth more hopelessly than if it had been buried 
by a current of lava or a shower of volcanic sand. New tor- 
rents are forming every year in the Alps. Tradition, written 
records, and analogy concur to establish the belief that the 
ruin of most of the now desolate valleys in those mountains 
is to be ascribed to the same cause, and authentic descrip- 
tions of the irresistible force of the torrent show that, aided by 
frost and heat, it is adequate to level Mont Blane and Monte 
Rosa themselves, unless new upheavals shall maintain their 
elevation. 
There are cases where torrents cease their ravages of them- 
selves, in consequence of some change in the condition of the 
basin where they originate, or of the face of the mountain at a 
higher level, while the plain or the sea below remains in sub- 
stantially the same state as before. If a torrent rises in a small 
valley containing no great amount of earth and of disintegrated 
or loose rock, it may, in the course of a certain period, wash 
out all the transportable material, and if the valley is then left 
with solid walls, it will cease to furnish débris to be carried 
down by floods. If, in this state of things, a new channel be 
formed at an elevation above the head of the valley, it may 
divert a part or even the whole of the rain-water and melted 
snow which would otherwise have flowed into it, and the once 
furious torrent now sinks to the rank of a humble and harmless 
brooklet. “In traversing this department,” says Surell, “ one 
often sees, at the outlet of a gorge, a flattened hillock, with a 
fan-shaped outline and regular slopes; it is the bed of dejec- 
tion of an ancient torrent. It sometimes requires long and 
