sphere being colder is capable of holding less water in suspension,
a diminished fall of rain takes place. Thus at St. Petersburg, the
amount is only 16 inches, and at Uleaborg in the Gulf of Bothnia (N.
lat. 65 degrees), only 13-1/2 inches, or less than half the average of
England, and even this small quantity descends more slowly in the
temperate zone, and is spread more equally over the year than in
tropical climates. But in reference to geological changes, frost in the
colder latitude acts as a compensating power in the disintegration of
rocks, and the transportation of stones to lower levels.

Water when converted into ice augments in bulk more than one-twentieth
of its volume, and owing to this property it widens the minute crevices
(or _joints_) of rocks into which it penetrates. Ice also in various
ways, as will be shown in the next chapter, gives buoyancy to mud and
sand, even to huge blocks of stone, enabling rivers of moderate size and
velocity to carry them to a great distance.

The mechanical force exerted by running water in undermining cliffs, and
rounding off the angles of hard rock, is mainly due to the intermixture
of foreign ingredients. Sand and pebbles, when hurried along by the
violence of the stream, are thrown against every obstacle lying in their
way, and thus a power of attrition is acquired, capable of wearing
through the hardest siliceous stones, on which water alone could make no
impression.

_Newly formed valleys._--When travelling in Georgia and Alabama, in
1846, I saw in both those States the commencement of hundreds of valleys
in places where the native forest had recently been removed. One of
these newly formed gulleys or ravines is represented in the annexed
woodcut (fig. 14), from a drawing which I made on the spot. It occurs
three miles and a half due west of Milledgeville, the capital of
Georgia, and is situated on the farm of Pomona, on the direct road to
Macon.[268]

Twenty years ago, before the land was cleared, it had no existence; but
when the trees of the forest were cut down, cracks three feet deep were
caused by the sun's heat in the clay; and, during the rains, a sudden
rush of water through the principal crack deepened it at its lower
extremity, from whence the excavating power worked backwards, till, in
the course of twenty years, a chasm, measuring no less than 55 feet in
depth, 300 yards in length, and varying in width from 20 to 180 feet,
was the result. The high r