598 THE FRESH-WATER LOCHS OF SCOTLAND 
45 miles in length, by nearly 10 miles in breadth, and has an area of 
225 square miles. The greatest depth (about midway between 
Lausanne and Evian) is 1015 feet, and as the surface lies 1200 feet 
above sea-level, the bottom is less than 200 feet above sea-level. More- 
over, the lake-floor is covered by deposits to an unknown depth, so 
that originally it was probably much below sea-level. The material 
brought down by the river has not only raised the bottom of the 
lake, but has diminished its area by filling it up in part ; formerly it 
extended at least to Bex, north of St Maurice. The mean depth of 
the lake is 506 feet, and the volume of water contained in it is 
estimated at about 3,175,000 million cubic feet. This lake has been 
studied systematically for many years, and forms the subject of a 
classic memoir by Forel.^ Most of the promontories round the lake 
are river-cones, which are specially marked between Vevey and 
Villeneuve, and at the mouth of the River Drance, near Thonon, 
there is a typical delta. The eastern end of the lake is known as the 
Haut Lac, the centre as the Grand Lac, and the narrower western 
end as the Petit Lac. The Haut Lac is a transverse river-valley 
cut out by the Rhone, and subsequently, owing to a change of level, 
partly filled up again ; the Petit Lac is the river-valley of the Arve. 
These two rivers met the River Drance opposite Morges, and the 
combined stream ran north to the Lake of Neuchatel. The elevation 
of the land then dammed back the water, giving rise to the Lake of 
Geneva, and lastly the cutting of the gorge at Fort de TEcluse gave 
the lake its present exit to the west, and gradually lowered its level. 
Iceland. Lake Thingvallavatn, in the south-west of Iceland, is the largest 
and best-known lake. It covers an area of 40 square miles, and 
has a depth of 364 feet. It derives its water chiefly from the ice-fleld 
of Lang-Jokull, though one small stream, the Oxara, runs through 
it. The lake is said to be due to earth-movements, as its south- 
western shore is part of a long fault scarp. 
Lake Thonsvatn, the second largest lake on the island, with an 
area of 38 square miles, occupies a basin formed by subsidence in an 
area of volcanic tuffs. 
Lake Hoitarvatn is filled with fragments of ice from two glaciers 
which extend into the water.^ 
Asia. All the large northern Asiatic rivers take their origin in the 
marshes and lakes scattered over the surface of the plateaus which 
occupy the centre of Asia.^ It seems probable that the water which 
^ Le Leman : Monographie limnologicpie, 3 vols., Lausanne, 1892-1904. 
'■^ Bisiker, Across Iceland, London, 1902. 
3 See p. 526. 
