1. _By slow movements of the earth's crust_, during the formation of
mountains; the Lake of Geneva in Switzerland and the Lake of Annecy in
France are due to the subsidence or warping of part of the Alps; on
the other hand, Lakes Stefanie, Rudolf, Albert Nyanza, Tanganyika and
Nyasa in Africa, and the Dead Sea in Asia Minor, are all believed to
lie in a great rift or sunken valley.
2. _By Volcanic Agencies._--Crater-lakes formed on the sites of
dormant volcanoes may be from a few yards to several miles in width,
have generally a circular form, and are often without visible outlet.
Excellent examples of such lakes are to be seen in the province of
Rome (Italy) and in the central plateau of France, where M. Delebecque
found the Lake of Issarlès 329 ft. in depth. The most splendid
crater-lake is found on the summit of the Cascade range of Southern
Oregon (U.S.A.). This lake is 2000 ft. in depth.
3. _By Subsidence due to Subterranean Channels and Caves in Limestone
Rocks._--When the roofs of great limestone caves or underground lakes
fall in, they produce at the surface what are called _limestone
sinks_. Lakes similar to these are also found in regions abounding in
rock-salt deposits; the Jura range offers many such lakes.
4. _By Glacier Erosion._--A. C. Ramsay has shown that innumerable
lakes of the northern hemisphere do not lie in fissures produced by
underground disturbances, nor in areas of subsidence, nor in synclinal
folds of strata, but are the results of glacial erosion. Many flat
alluvial plains above gorges in Switzerland, as well as in the
Highlands of Scotland, were, without doubt, what Sir Archibald Geikie
calls glen-lakes, or true rock-basins, which have been filled up by
sand and mud brought into them by their tributary streams.