nter into the order with a height of
18,000 feet, the list of those surpassing the other mountains of the
globe, might be very much extended.

We shall have occasion hereafter to speak of the volcanic energies still
exerted in this vast stony girdle, and shall therefore confine ourselves
strictly to mere external form.

The arms and branches of mountain chains enclose as has been seen,
basins marked by rivers which convey their surface waters to the ocean.
The rains which fall on the sides of mountains and hills, unite in
torrents and streams, which follow the lines of most rapid slope in
their course to the sea.

The greater rivers mark the lowest part of a principal basin, on each
side of which, at a greater or less distance, are to be found rising
grounds, themselves hollowed out into lateral secondary basins,
containing courses of water less considerable than the first, into which
they cast themselves, and whose branches they are. The borders of these
secondary basins are again hollowed out into basins of a third order,
whose slopes also contain water courses less considerable than the
preceding, into which they in turn discharge themselves. This
ramification continues until we reach the smallest ravines of the
boundary mountains, and the map appears, as it were, covered with a net
work of rivers and lesser streams. The great valley of the Mississippi
and Missouri, forms perhaps the most striking instance of this sort,
upon the surface of our globe.

Rivers and streams are constantly exerting a mechanical action on the
surfaces over which they run; abrading and tearing off fragments even of
the hardest rocks, they roll them in their course until the velocity
becomes insufficient to transport them farther. At diminished velocities
they move fragments of less size, down to the smallest pebbles; at still
less velocities, they transport sand, and finally earthy matter, in the
most minute division. These are deposited in succession in positions
corresponding to the rapidity of the stream, and hence the beds of
rivers present at each of their different sections, materials of
magnitude and quality corresponding to the rate at which the stream
usually flows. The increase in the magnitude of streams, due to violent
rains and the melting of the snows, changes the position of the
substances that compose their bed, and the more easily suspended
materials are often held until the stream actually meets the ocean. In
such sud