312 PHYSICAL GEOGRAPHY. 



to the season. This is the case also with the Ngami, in Southern Africa, the first of the 

 discoveries of the intrepid Dr. Livingstone. The northern part of the western world is pre 

 eminently the country of lakes, presenting the largest masses of fresh water to be found upon 

 the surface of the globe. The following statement shows the area of the principal : 



Sq. Miles. Sq. Miles. 



Lake Superior - - 40,OOO 



Huron - 25,000 



Michigan - - 25,000 



Erie - 11,000 



Ontario .... 10,000 



Winnipeg .... 9,000 



Athabasca - . 3,000 



Great Slave Lake - - - 12,000 



Great Bear Lake - - 8,000 



Champlain - - 500 



In South America, the Lake of Titicaca, in Upper Peru, covers a surface of about 

 4000 square miles ; and immense swampy plains and lagoons are common along the 

 course of the rivers. 



Lakes are sometimes considered under the two divisions of fresh and salt water, but 

 there are many occupying intermediate stations with reference to these extremes. We 

 shall follow another arrangement of them, into four classes, which will more fully 

 embrace their physical conditions, and then notice some remarkable phenomena, by 

 which several are distinguished. 



1. There is a class which have no apparent affluents or outlets. They are fed chiefly 

 by subaqueous springs, and occur frequently in hollows, which have the appearance of 

 extinct volcanic craters. They are generally small, but of more stable character than the 

 larger sheets of water formed by rivers. Not receiving any great superficial current, 

 they are not subject to those changes of their depth and outline, which take place in the 

 lakes with affluents, through deposition of the mud and sand brought into them by turbid 

 torrents. Collections of water of a similar kind abound in the great steppes of northern 

 and western Asia. They are called lakes, but are perhaps more properly pools, being 

 formed of accumulated rains and melted snow, which are largely, and in some cases 

 entirely, evaporated by the summer heat, though several have a circumference of from ten 

 to twelve miles, and a depth of six or seven feet. Their waters are commonly saline, and 

 what is most remarkable, and hitherto unexplained, sheets of fresh water are found in 

 their immediate vicinity. The most considerable example of this class is the Lake of 

 Tuzla, wliich lies northward of the great range of Taurus, on the high central plateau of 

 the Lesser Asia. Though narrow, it extends fifty miles in length, and is so extremely 

 salt, that no fish or aquatic animal can live in it. Even the wild fowls are afraid to 

 venture upon its waters, for by so doing their wings become stiff with a thick coating of 

 salt, and any substance thrown into them speedily receives a saline incrustation. Strabo, 

 the geographer of antiquity, a native of the peninsula, was personally acquainted with 

 this lake, and mentions these circumstances, the accuracy of which, modern travel has 

 confirmed. The Sultan Murad IV. made a causeway across it, upon the occasion of 

 marching his army to the attack of Bagdad, for, owing to excessive evaporation during 

 the summer and autumn heats, the lake is extremely shallow. The remains of this 

 causeway are now almost concealed by a saline encasement, and a thick crust of solid salt 

 covers the bed of the lake. 



2. Another class have outlets, but no apparent affluents. These lakes usually occupy 

 a high elevation above the level of the sea, and derive their supplies from subterranean 

 springs. One on Monte Rotondo, in the island of Corsica, is at the height of 9000 feet. 

 They are not inconsiderable in number, and are frequently the sources of important 

 rivers. The course of the great Volga may be traced up to a lake of this kind, but which 

 is only slightly elevated above the sea-level. 



3. A third class receive affluents, but have no outlets. Lakes of this description are 

 exceedingly rare, but they are the most peculiar of all. That of Celano in Italy, the 



