CHARACTERISTICS AND DISTRIBUTION OF LAKES 523 
counted by tens of thousands, varying from the size of a tarn to that 
of the great Laurentian lakes above mentioned. A good example of 
this is found in Scotland in the Red Lochan at Tulloch.^ 
(f) Sand drifted into dunes. — It is a well-known fact that sand may 
travel across a country for several miles in the direction of the 
prevailing winds. When these sand-dunes obstruct a valley a lake 
may be formed ; a good example of such a lake is found in Moses 
Lake in the State of Washington. The sand-dunes may also fill up 
or submerge river- valleys and lakes — for instance, in the Sahara, where 
the Shotts are vast lakes filled with sand and water near the point of 
saturation. Indeed, in the afternoon, owing to evaporation, the 
surface is covered with salt crystals. In the morning these have all 
deliquesced, and the surface looks like an ordinary lake. 
(g) Alluvial matter deposited hy lateral streams. — If the current of 
a main river be not powerful enough to sweep away detrital matter 
brought down by a lateral stream, a dam is formed, causing a lake. 
These lakes are frequently met with in the narrow valleys of the 
Llighlands of Scotland. 
(h) Flows of lava. — Lakes of this kind are met with in volcanic 
regions. The marshes round the edges of the Snake River lava-sheets 
seem to be lakes of this sort verging on extinction. In Auvergne, a 
small basin, the Lac d'Aidat, is enclosed by lava from the extinct Puy 
de la Vache, and the Lac de Chambon was formed by the eruption of 
volcanic cones in its valley. The Sea of Tiberias seems to be held back 
by a lava-stream that entered the valley of the Jordan from the east. 
Lake Assal, at the head of the Gulf of Aden, is shut in by a bed of lava. 
3. Organic Basins. — In the vast tundras that skirt the Arctic 
Ocean in both the Old and the New World, a great number of frozen 
ponds and lakes are met with, surrounded by banks of vegetation. 
Snow-banks are generally accumulated every season at the same spots. 
During summer the growth of the tundra vegetation is very rapid, 
and the snow-drifts that last longest are surrounded by luxuriant 
vegetation. When such accumulations of snow finally melt, the 
vegetation on the place they occupied is much less than along their 
borders. Year after year such places become more and more 
depressed, comparatively to the general surface, where vegetable 
growth is more abundant, and thus give origin to lakes. The 
obstructions formed by the " sudd of the Upper Nile region, and by 
the " beaver dams of the North American rivers, may be considered 
as giving rise to lakes of organic origin. 
It is well known that in coral-reef regions small bays are cut off 
from the ocean by the growth of corals, rain and riv^er waters 
accumulate behind these barriers, fresh-water basins being thus 
^ See vol. ii. part i. p. 375. 
