Lake

e. Lakes due to Solution in Calcareous Rocks. The solvent ac tion of water in limestone regions causes depressions to be formed in various ways and with varying shapes. If such regions have been glaciated within recent times, an impervious ground moraine may line the depressions thus forming a lake basin and preventing further solvent action. Again underground streams cause subsidence in limestone areas producing a "limestone sink" and limestone soils being usually clayey, a lake is formed. A similar action is seen in regions underlain by salt beds.

f. Lakes on Ice occur in depressions. They may be completely on the ice or partly on the ice and partly on solid rock.

g. Lakes frequently occur in hollows and depressions in water bearing strata, owing to the rise of the water-table. They have no apparent inlet or outlet and will show periodic changes in level owing to variations in the height of the water-table. Desert oases and lakes on moraines are often of this type.

h. Organic Lakes. In the tundras around the Arctic Circle, a great number of frozen ponds and lakes occur, surrounded by banks of vegetation. Snow banks generally accumulate in the same place every season. During the summer the growth of vege tation is rapid thus quickly surrounding lingering snowdrifts. When these latter melt, their sites are occupied by a thinner growth of vegetation than the surrounding regions and thus, year after year these places become more and more depressed and give rise to lakes.

Life History of Lakes.

From the time of its formation a lake is destined to disappear. The historical period has not been long enough to enable man to investigate the life history of any lake of considerable size, but much information is now available upon the various stages in the development. In humid climates two processes tend to the extinction of a lake, e.g., the deposition of detrital matter and the lowering of the lake by the downward cutting of the effluent stream. These outgoing streams, how ever, being very pure and clear, all detrital matter having been deposited in the lake, have less eroding power than the inflowing streams. The accumulation of organic deposits plays an important part under certain conditions in filling up lakes. Such deposits are known as cumulose deposits.

In arid regions where the rainfall is often less than io in. per annum, the effects of evaporation are greater than those of pre cipitation, so that salt and bitter lakes prevail in these regions.

The occurrence of salt and bitter lakes generally indicates a change from humid to arid conditions, for many salt lakes, e.g., the Dead Sea and the Great Salt Lake, are descended from fresh water ancestors, whilst others, e.g., the Caspian and Aral Seas, are isolated parts of the ocean. In lakes of the first group evapo ration exceeds the inflow. The inflowing waters bring in a small amount of saline and alkaline matter, which becomes more and more concentrated. In lakes of the second group the waters were

salt at the outset.

The Water of Lakes.

The composition of lake water is im portant for on it depends not only the type of fauna that the lake can maintain but the possibility of it maintaining any fauna at all. Also lakes rich in saline and alkaline compounds, are of commercial importance. As already shown the relation between the rates of evaporation and inflow are the determining factors of the composition, but the time element also enters in. River water may contain a variety of salts in solution, the most im portant being calcium carbonate. These salts are not equally soluble in water and the addition of a more soluble salt to an al ready saturated solution will cause the precipitation of the less soluble salt. Moreover, as evaporation proceeds and the water becomes more saturated, salts are precipitated in the following order :—calcium and magnesium carbonates, calcium sulphate, sodium carbonate and sodium sulphate, sodium chloride and magnesium sulphate whilst magnesium chloride and calcium chloride remain in solution. Slight variations in order may occur, but the important fact remains that the longer the process of evaporation has gone on, the richer the water becomes in various chlorides. The composition of the water of inflowing streams de pends upon the mineralogical constitution of the rocks in the drainage area of the inflowing rivers. The composition is also dependent on whether the rivers are glacial streams. This is very marked in small fresh water lakes.

F. W. Clarke (Data of Geochemistry, Washington, 1916), di vides salt lakes into nine classes according to their chemical com position [for additional information see also L. W. Collet, Les Lacs (1925) and Chamberlin and Salisbury, Geology (1909)1 The amount of dissolved matter varies considerably in salt and bitter lakes, as the following list will show (the figures are in grammes per litre) :—Dead Sea (Terriel) 245.732, Caspian Sea (Gobel) 6.294; Elton Lake (Erdman) 264.98; Great Salt Lake (Allen) 149.936; Pyramid Lake, Nevada (Clarke) 3.4861; Van Lake (Chancourtois) 22.6; Aral Sea 10.8416; and the Atlantic Ocean (Thoulet) 35.0631. With these may be compared the corn position of fresh water lakes, e.g., Katrine (Harris) 0.029.

Movements and Temperature of Lake-waters.—Winds cause a transference of water which results in raising the level at the end to which the wind is blowing. In addition to the well known progressive waves there are also stationary waves or "seiches" which are less apparent. A seiche is a standing oscilla tion of a lake, usually in the direction of the longest diameter, but occasionally transverse. Seiches were first discovered in 173o and they were first systematically investigated upon Lake Geneva. Subsequently a large number of observations have been made on lakes throughout the whole of the world.