SOIL: ITS ORIGIN AND NATURE. 181 
These silicates, acted upon by rain-water, are eventually decom- 
posed and converted into certain soluble and insoluble com- 
pounds. The soluble matter, consisting of bi-carbonates, are 
leached out, while the insoluble compounds may remain behind 
in the decomposing rock, or be washed away by the mechanical 
action of the rain. Such chemical changes are not confined 
merely to the superficial portions of rocks, but take place at all 
depths to which water can percolate. Rocks may in this way 
become “rotted” for hundreds of feet below the surface of the 
ground. 
Living plants play an important réle in the disintegration of 
rocks. By means of the acids in their roots they dissolve out the 
mineral substances they require. Further, their roots enter the 
natural division-planes of rocks, and gradually wedge these 
asunder, and thus, by allowing freer percolation of water, they 
prepare the way for more rapid rock-disintegration. The 
reduction of rocks is likewise helped on in many places by the 
action of tunnelling and burrowing animals—by worms and 
moles, for example, and by different kinds of insects. All these 
not only take part in the process of soil-circulation, but are the 
means of introducing much organic matter, and hence they aid 
the chemical action of percolating water. 
The rate at which disintegrated rock-material travels over the 
surface is determined not only by the form of the ground, but 
largely by the character of the climate. In regions practically - 
devoid of any vegetable covering, as desert-tracts, loose 
materials may be removed from the steeper slopes as rapidly as 
they appear. In such countries mountains and hills, however 
rapidly their rocks may be crumbling down, are not mantled 
with sheets of disintegrated materials, but are kept bare by the 
wind, which sweeps everything away to the low grounds, which in 
time, therefore, become more or less concealed under accumula- 
tions of drifting sand. In Arctic regions, again, where vegetation 
is meagrely developed, rocks are broken up with astonishing 
rapidity, and the disintegrated débris, under the influence of 
alternate freezing and thawing, readily descend declivities— 
occasionally moving ez masse down relatively gentle slopes. 
Under ordinary conditions such rapid transference of rock- 
materials is quite exceptional in temperate lands. Only amongst 
our mountains does anything of the kind become conspicuous— 
where, owing to the inclemency of the climate, and the steep 
