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 strea1ns 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 1% m., formed by the Lochlarig river and its tributaries. The long stretch of alluvium that separates Loch Voil from Loch Lubnai g 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 Q 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 IO 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 eliects of evaporachange 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 La/ees.-f(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 Sénlidahy Evapoiiation

€XpI'eS5S in YZITIS D ltl' .

Great Salt Lake (Russell) .... 238-12 er e 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 . 3925 KZSO-4 8756

IM O'§ 2I ll;/P534 0°166

E - - 4' 44 8 2 - I '0

Ca 2-424 MgSO4

Cl 128-278 CaSO4 8-240

S03 . . I2'522 F€2O3+Al2O3 O'OO4

O in sulphates 2-494 SiO2 0-018

F e203 -l-A1203 0-004 Surplus S03 0-051 S102 O'OI8

Bo2Oa . trace

Bra . 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. Urmia Sea. Dead Sea. Lake Van. Suez C§ 1}1al» Open. Karabugas. ISma11lf1-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 U Iron 0-0053 0-0014 0-0069

H Magnesia 0-6598 0-4031

Carbonate of Soda 5-3976

Phosphate of Lime 0-0028 0-0021 0-0029 Sulphate of Lime 1-3499 0-9004 0-7570 0-8600 1-8593 Magnesia 0-9324 2-9799 3-0855 61-9350 13-5460 0-2595 3-2231 Soda 1-7241 2-5673

Potash 0-5363

Chloride of Sodium 6-9008 6-2356 8-1 163 83-2840 192-4100 76-5000 8-0500 40-4336 Potassium 0-2209 0-1 145 0-1339 9-9560 23-3000 0-6231 Rubidium 0-0055 0-0034 0-2510 0-0265 Magnesium 0-0003 0-6115 129-3770 15-4610 95-6000 4-7632 Calcium . 0-5990 22-4500

Bromide of Magnesium 0-0045 O'OO8I 0-1930 2-3100 0-0779 Silica . 0-0098 0-0024 0-2400 0-0761 0-0027 Total Solid Matter 1 1-1463 10-8987 12-9773 284-9960 222-7730 221-2600 17-2899 51-0264 tion 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 inliowing 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 infiow 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 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:- I Tanay. Bleu. Marjelen St Gothard.

SiO2 . 0-003 0-0042 0-0014 0-0008

Fe2O3-i-A1203, 0-0012 0-0006 0-0008 trace NaCl 0-0017

Na2SO4 0-001 1 0-0038 0-0031 0-00085 Na2CO3 0-00128

KQSO4 0-0021 O'OO28 0-0044

KQCO3 0-0003 0-00130

MgS04 0-006 0-0305

MgCOa 0-0046 0-0158 0-0008 0-00015

CaCO3 0-107 0-1189 O-0061 0-00178

MnO 0-001 .