554 THE FBESH- WATER LOCHS OF SCOTLAND 
purposes ; its density is slightly above that of fresh water, and the 
proportion of soluble salts about one-fourth of that in the ocean. At 
the west end of the lake, where the concentration is greatest, owing to 
the distance from the feeder canals, the proportion of soluble salts 
is 1"34 per cent., of which 0*92 per cent, is sodium chloride. 
Schweinfurth ^ shows that the degree of concentration of salt in a 
lake the volume of wliich has been continually reduced, and to which 
salt has constantly been added, should be many times greater than 
this amount, and concludes that the lake must have a subterranean 
outlet. Temperature observations gave a maximum of 94° "2 Fahr. 
(34° '5 C.) in shallow water close to the shore about 2 o'clock on an 
afternoon in May 1907, and a minimum of 54°-8 Fahr. (12°-2 C.) at 
the surface in the early morning. One series gave a difference in 
temperature of 12° '4 Fahr. between the surface and a depth of 3 
fathoms; another gave a difference of 8° "8 Fahr. between the surface 
and a depth of only 1 fathom.^ There is a great quantity of life in 
the lake, belonging to comparatively few species. 
The lakes occupying the so-called Great Rift Valley in East Africa, 
an inland drainage area extending from the Red Sea to south of the 
equator and covering an area of about 50,000 square miles, should be 
referred to here, but it has been found more convenient to deal with them 
after describing the lakes of the Zambesi basin (see pp. 606 and 618). 
North The inland drainage areas of North America (Great Salt Lake 
area. Central America, and Mexico) are estimated by Murray to cover 
278,000 square miles (see fig. 66). 
West of the Wasatch Range and the Colorado plateaus, south 
of the Columbia plateaus, and east and south of the Sierra Nevada, 
there is an arid region embracing all of Nevada, part of Utah and 
Arizona, and the south-eastern corner of California. Humid air- 
currents travelling eastward from the Pacific suffer a condensation of 
their vapour before reaching the basin, so that they arrive as drying 
winds. This region is diversified by many independent mountain 
ranges of north-and-south trend and of varied structure, uniting to 
form troughs, the floors of which sometimes stand at altitudes of 
Great Salt from 4000 to 6000 feet, as in Utah and Northern Nevada. In the 
a -e area. gQ^-^j^_^^gg^ ^^le floors of the depressions are at moderate altitudes; 
and in two localities, one in southern Nevada — Death Valley — 
1 See note by Dr Schweinfurth on The Salt in the Wadi Ryan " in Willcocks, 
Egy2Jtian Irrigation, Apj^endix II., pp. 460-465. 
2 W. A. Cunnington, " Description of a Biological Expedition to the Birket el 
Qurun, Faytim Province of Egypt," Proc. Zool. Soc. London, 1908, p. 3. Hitherto 
this lake has been known as the Birket el Qurun, but Dr Cunnington informs us 
that according to Captain Lyons the official spelling is now Birket Qaruri. 
