nd of
boulders thrown violently against each other in the beds of torrents. By
such wear and tear rocky fragments swept down from the hills are in part
converted into sand and fine mud; and the turbid Ganges, during its
annual inundation, derives more of its sediment from this source than
from the waste of the fine clay of the alluvial plains below.[265]

On the verge of the tropics a greater quantity of rain falls annually
than at the equator. Yet parts even of the tropical latitudes are
entirely destitute of rain: Peru, for example, which owes its vegetation
solely to rivers and nightly dews. In that country easterly winds
prevail, blowing from the Pacific, and these being intercepted by the
Andes, and cooled as they rise, are made to part with all their moisture
before reaching the low region to the leeward. The desert zone of North
Africa, between lat. 15 degrees and 30 degrees N., is another instance
of a rainless region. Five or six consecutive years may pass in Upper
Egypt, Nubia, and Dongola, or in the Desert of Sahara, without rain.

From the facts above mentioned, the reader will infer that in the course
of successive geological periods there will be great variations in the
quantity of rain falling in one and the same region. At one time there
may be none whatever during the whole year; at another a fall of 100 or
500 inches; and these two last averages may occur on the two opposite
flanks of a mountain-chain, not more than 20 miles wide. While,
therefore, the valleys in one district are widened and deepened
annually, they may remain stationary in another, the superficial soil
being protected from waste by a dense covering of vegetation. This
diversity depends on many geographical circumstances, but principally on
the height of the land above the sea, the direction of the prevailing
winds, and the relative position, at the time being, of the plains,
hills, and the ocean, conditions all of which are liable in the course
of ages to undergo a complete revolution.

_Recent rain-prints._--When examining, in 1842, the extensive mud-flats
of Nova Scotia, which are exposed at low tide on the borders of the Bay
of Fundy, I observed not only the foot-prints of birds which had
recently passed over the mud, but also very distinct impressions of
rain-drops. A peculiar combination of circumstances renders these
mud-flats admirably fitted to receive and retain any markings which may
happen to be made on their surface. The