CHARACTERISTICS AND DISTRIBUTION OF LAKES 521 
(b) Lakes of the second order have bottom temperatures practically 
constant, but undergoing annual fluctuations. 
(c) Lakes of the third order have bottom temperatures seldom 
very far from the surface temperatures. 
This division into orders corresponds in a general way to the 
characters of lakes — i.e. size, bulk of water, depth — and to the climate 
of the surrounding country. 
In Scotland lakes are sometimes divided into those w^hich are 
covered with ice in winter, and those which never freeze over, the 
former being shallow lakes with a high annual range of temperature, 
and the latter deep lakes with a low annual range of temperature. 
The most generally adopted method of classification of lakes in Classification 
the past is one based on their origin, chiefly from the geological point ^'^ 
of view : — 
I. Rock-Basins. — These have been formed in several ways: — 
(a) Biij slow movements of the eartlis cmst^ during the formation 
of mountains ; the Lake of Geneva in Switzerland and the Lake of 
Aimecy in France are due to the subsidence or warping of part of 
the Alps ; on the other hand. Lakes Stefanie, Rudolf, Albert 
Nyanza, Tanganyika, Leopold,^ and Nyasa, in Africa, and the Dead 
Sea in Syria, are all believed to lie in a great rift or sunken valley. 
(b) 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 I^ake of Issarles 329 feet 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 feet in depth. 
(c) By solution and subsidence due to subterranean channels and caves 
in limestone rocks. — When the roofs of great limestone caves or under- 
ground lakes fall in, they produce at the surface what are called lime- 
stone sinks. Lakes similar to these are also found in regions abounding 
in rock-salt deposits ; the Jura Range offers manv such lakes. 
(d) By glacier erosion. — A. C. Ramsay has shown that innumer- 
able 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. 
^ Also called Rukwa, Hikwa, or Likwa. 
