INFLUENCE OF STRATA UPON SCENERY. 
259 
filters through innumerable crevices, dissolving some 
substances, especially when it is charged with car- 
bonic acid, and leaving others. It also acts mechan- 
ically, for as it expands when freezing, it splits up 
even the toughest rocks, if only there are any crevices 
into which it can enter. In a dry climate, therefore, 
the slopes will generally be steeper than in a more 
rainy region. Even in the absence of water, changes 
of temperature have a considerable effect owing to 
the fractures which they produce by the successive 
contractions and expansions to which they give rise. 
These, however, though the principal, are by no 
means the only factors in denudation. The roots of 
plants, for instance, have a considerable effect, in- 
sinuating themselves into the smallest crevices and, 
as they expand with growth, enlarging them by de- 
grees. Yet, on the whole, the action of vegetation 
is conservative. It absorbs much of the rainfall, 
and the formation of torrents is thus greatly checked. 
Some of the French Alpine districts, and much of 
Northern Africa, have suffered terribly, and in fact 
been reduced almost to deserts, by the reckless de- 
struction of forests. 
Different kinds of rocks are very differently af- 
fected by atmospheric influences. 
Siliceous rocks are liable to disintegration by 
17* 
