sed above the level of the sea. It is
therefore not surprising, since the atmosphere becomes colder in
proportion as we ascend in it, that there should be heights, even in
tropical countries, where the snow never melts. The lowest limit to
which the perpetual snow extends downwards, from the tops of mountains
at the equator, is an elevation of not less than 16,000 feet above the
sea; while in the Swiss Alps, in lat. 46 degrees N. it reaches as low as
8,500 feet above the same level, the loftier peaks of the Alpine chain
being from 12,000 to 15,000 feet high. The frozen mass augmenting from
year to year would add indefinitely to the altitude of alpine summits,
were it not relieved by its descent through the larger and deeper
valleys to regions far below the general snow-line. To these it slowly
finds its way in the form of rivers of ice, called glaciers, the
consolidation of which is produced by pressure, and by the congelation
of water infiltered into the porous mass, which is always undergoing
partial liquefaction, and receiving in summer occasional showers of rain
on its surface. In a day of hot sunshine, or mild rain, innumerable
rills of pure and sparkling water run in icy channels along the surface
of the glaciers, which in the night shrink, and come to nothing. They
are often precipitated in bold cascades into deep fissures in the ice,
and contribute together with springs to form torrents, which flow in
tunnels at the bottom of the glaciers for many a league, and at length
issue at their extremities, from beneath beautiful caverns or arches.
The waters of these streams are always densely charged with the finest
mud, produced by the grinding of rock and sand under the weight of the
moving mass. (See fig. 18.)

[Illustration: Fig 18.

Glacier with medial and lateral moraines and with terminal cave.]

The length of the Swiss glaciers is sometimes twenty miles, their width
in the middle portion, where they are broadest, occasionally two or
three miles; their depth or thickness sometimes more than 600 feet. When
they descend steep slopes, and precipices, or are forced through narrow
gorges, the ice is broken up, and assumes the most fantastic and
picturesque forms, with lofty peaks and pinnacles, projecting above the
general level. These snow-white masses are often relieved by a dark
background of pines, as in the valley of Chamouni; and are not only
surrounded with abundance of the wild rhododendron in full flower,