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various stages of development a fairly good idea of the course they run can be obtained.

In humid regions two processes tend to the extinction of a lake, viz. the deposition of detrital matter in the lake, and the lowering of the lake by the cutting action of the outlet stream on the barrier. These outgoing streams, however, being very pure and clear, all detrital matter having been deposited in the lake, have less eroding power than inflowing streams. One of the best examples of the action of the filling-up process is presented by Lochs Doine, Voil and Lubnaig in the Callander district of Scotland. In post-glacial times these three lochs formed, without doubt, one continuous sheet of water, which subsequently became divided into three different basins by the deposition of sediment. Loch Doine has been separated from Loch Voil by alluvial cones laid down by two opposite streams. At the head of Loch Doine there is an alluvial flat that stretches for 11/2 m., formed by the Lochlarig river and its tributaries. The long stretch of alluvium that separates Loch Voil from Loch Lubnaig has been laid down by Calair Burn in Glen Buckie, by the Kirkton Burn at Balquhidder, and by various streams on both sides of Strathyre. Loch Lubnaig once extended to a point 3/4 m. beyond its present outlet, the level of the loch being lowered about 20 ft. by the denuding action of the river Leny on its rocky barrier.

In arid regions, where the rainfall is often less than 10 ins. in the year, the action of winds in the transport of sand and dust is more in evidence than that of rivers, and the effects of evaporation greater than of precipitation. Salt and bitter lakes prevail in these regions. Many salt lakes, such as the Dead Sea and the Great Salt Lake, are descended from fresh-water ancestors, while others, like the Caspian and Aral Seas, are isolated portions of the ocean. Lakes of the first group have usually become salt through a decrease in the rainfall of the region in which they occur. The water begins to get salt when the evaporation from the lake exceeds the inflow. The inflowing waters bring in a small amount of saline and alkaline matter, which becomes more and more concentrated as the evaporation increases. In lakes of the second group the waters were salt at the outset. If inflow exceeds evaporation they become fresher, and may ultimately become quite fresh. If the evaporation exceeds the inflow they diminish in size, and their waters become more and more salt and bitter. The first lake which occupied the basin of the Great Salt Lake of Utah appears to have been fresh, then with a change of climate to have become a salt lake. Another change of climate taking place, the level of the lake rose until it overflowed, the outlet being by the Snake river; the lake then became fresh. This expanded lake has been called Lake Bonneville, which covered an area of about 17,000 sq. m. Another change of climate in the direction of aridity reduced the level of the lake below the level of the outlet, the waters became gradually salt, and the former great fresh-water lake has been reduced gradually to the relatively small Great Salt Lake of the present day. The sites of extinct salt lakes yield salt in commercial quantities.

The Water of Lakes.—(a) Composition.—It is interesting to compare the quantity of solid matter in, and the chemical composition of, the water of fresh and salt lakes:—

	Total Solids by Evaporation expressed in Grams per Litre.
Great Salt Lake (Russell)	238.12
Lake of Geneva (Delebecque)	0.1775

The following analysis of a sample of the water of the Great Salt Lake (Utah, U.S.A.) is given by I. C. Russell:—

	Grams per Litre.		Probable Combination.
Na	75.825	NaCl	192.860
K	3.925	K2SO4	8.756
Li	0.021	Li2SO4	0.166
Mg	4.844	MgCl2	15.044
Ca	2.424	MgSO4	5.216
Cl	128.278	CaSO4	8.240
SO3	12.522	Fe2O3 + Al2O3	0.004
O in sulphate	2.494	SiO2	0.018
Fe2O3 + Al2O3	0.004	Surplus SO3	0.051
SiO2	0.018
Bo2O3	trace
Br3	faint trace

The following analyses of the waters of other salt lakes are given by Mr J. Y. Buchanan (Art. “Lake,” Ency. Brit., 9th Ed.), an analysis of sea-water from the Suez Canal being added for comparison:—

	Koko-nor.	Aral Sea	Caspian Sea.		Urmia Sea.	Dead Sea.	Lake Van.	Suez Canal, Ismailia.
			Open.	Karabugas.
Specific Gravity	1.00907	..	1.01106	1.26217	1.17500	..	1.01800	1.03898
Percentage of Salt	1.11	1.09	1.30	28.5	22.28	22.13	1.73	5.1
Name of Salt.	Grams of Salt per 1000 Grams of Water.
Bicarbonate of Lime	0.6804	0.2185	0.1123	..	..	..	..	0.0072
Bicarbonate of Iron	0.0053	..	0.0014	..	..	..	..	0.0069
Bicarbonate of Magnesia	0.6598	..	..	..	..	..	0.4031	..
Carbonate of Soda	..	..	..	..	..	..	5.3976	..
Phosphate of Lime	0.0028	..	0.0021	..	..	..	5.3976	0.0029
Sulphate of Lime	..	1.3499	0.9004	..	0.7570	0.8600	..	1.8593
Sulphate of Magnesia	0.9324	2.9799	3.0855	61.9350	13.5460	..	0.2592	3.2231
Sulphate of Soda	1.7241	..	..	..	..	..	2.5673	..
Sulphate of Potash	..	..	..	..	..	..	0.5363	..
Chloride of Sodium	6.9008	6.2356	8.1163	83.2840	192.4100	76.5000	8.0500	40.4336
Chloride of Potassium	0.2209	0.1145	0.1339	9.9560	..	23.3000	..	0.6231
Chloride of Rubidium	0.0055	..	0.0034	0.2510	..	..	..	0.0265
Chloride of Magnesium	..	0.0003	0.6115	129.3770	15.4610	95.6000	..	0.0027
Chloride of Calcium	..	..	..	..	0.5990	22.4500	..	..
Bromide of Magnesium	0.0045	..	0.0081	0.1930	..	2.3100	..	0.0779
Silica	0.0098	..	0.0024	..	..	0.2400	0.0761	0.0027
Total Solid Matter	11.1463	10.8987	12.9773	284.9960	222.2600	221.2600	17.2899	51.0264

This table embraces examples of several types of salt lakes. In the Koko-nor, Aral and open Caspian Seas we have examples of the moderately salt, non-saturated waters. In the Karabugas, a branch gulf of the Caspian, Urmia and the Dead Seas we have examples of saturated waters containing principally chlorides. Lake Van is an example of the alkaline seas which also occur in Egypt, Hungary and other countries. Their peculiarity consists in the quantity of carbonate of soda dissolved in their waters, which is collected by the inhabitants for domestic and commercial purposes.

The following analyses by Dr Bourcart give an idea of the chemical composition of the water of fresh-water lakes in grams per litre:—

	Tanay.	Bleu.	Märjelen.	St Gothard.
SiO2	0.003	0.0042	0.0014	0.0008
Fe2O3 + Al2O3	0.0012	0.0006	0.0008	trace
NaCl	0.0017	..	..	..
Na2SO4	0.0011	0.0038	0.0031	0.00085
Na2CO3	..	..	..	0.00128
K2SO4	0.0021	0.0028	0.0044	..
K2CO3	..	..	0.0003	0.00130
MgSO4	0.006	0.0305	..	..
MgCO3	0.0046	0.0158	0.0008	0.00015
CaSO4	..	..	..	..
CaCO3	0.107	0.1189	0.0061	0.00178
MnO	0.001	..	..	..