CHARACTERISTICS AND DISTRIBUTION OF LAKES 583 
water (in the south) had a temperature of 55° '6 Fahr. (13°1 C), 
and the coldest bottom water (in the north) was 39° Fahr. (3°"9 C). 
These temperatures seemed relatively low for the time of year, but 
still lower figures were recorded in 1899, especially in the north. 
In the latter year the vertical distribution of temperature was found 
to be inverse at all the deep-water stations, the difference from the 
distribution in 1897 being attributable to the unusually low tempera- 
ture which had prevailed throughout North-Western Russia during 
the spring and early summer of 1899. Although Ladoga certainly 
belongs to the category of temperate lakes, according to ForeFs classi- 
fication, it would appear to come very near the border-line separating 
temperate from polar lakes, in which the vertical distribution is 
always inverse. The maximum temperature gradient occurred at 
a much lower level in 1899 than in 1897, because of the generallv 
higher temperature of the water in 1897. The lake is covered with 
a sheet of ice annually from December till April, and near Valaam 
Island masses of ice are sometimes piled up to a height of 75 feet, 
presenting from a distance the appearance of hills of weathered schist. 
Notwithstanding the freezing of the lake, its animal life is very 
abundant, including not only fishes but a species of seal, which may 
be seen in winter at the edge of the ice-cracks. 
Lake Onega lies 236 feet above sea-level, and has an area of 
3763 square miles. The length of the lake is 145 miles, the greatest 
depth 740 feet,^ and the volume of water is estimated at 21, 000^000 
million cubic feet. The River Svir connects it with Lake Ladoga, 
and a series of lakes and rivers affords communication with the 
White Sea. Its northern shores form numerous bays running to the 
north-west, and the water-system is prolonged towards Lapland by 
chains of small lakes and rivers following the same direction, separ- 
ated by lines of hills between 800 and 1000 feet high. The River 
Vitegra brings it into connection with the Volga system on one 
side and with the Mezen on the other. 
Three small lakes ^ lying to the south of Lake Onega, and 
communicating with that lake by the Megra River, belong to the 
class of intermittent lakes, and are connected with the " Karst ^ 
1 Halbfass gives the maximum depth, as 124 metres (407 feet), and the altitude 
as 174 metres (571 feet). 
2 See Geogr. Journ., vol. xxxi. p. 441, 1908. 
^ In Austria- Hungary a region along the east side of the Adriatic Sea, known 
as the Karst, is a tract of land underlain by white limestone nearly free from soil. 
Atmospheric agencies have eroded its surface so that sink-holes abound, and 
numerous short gullies, ravines, and valleys in the limestone terminate abruptly, 
discharging their waters into caves or subterranean tunnels, from which the 
streams may not emerge till they reach the coast. Topography similar to that of 
this region, and developed in the same way, is known as Karst topography. 
