CHARACTERISTICS AND DISTRIBUTION OF LAKES 581 
layers, which must tend to prevent convection currents and therefore 
to greatly prolong the cooling of the intermediate layer. Salt water 
is admitted intermittently from the neighbouring fjord, and forms the 
bottom layer, the thin surface layer being formed by fresh water from 
a small stream flowing in at one end and overflowing at the other, and 
being thus constantly renewed ; the intermediate layer is apparently 
formed by very slow mixing of the bottom and surface layers. This 
little lake was used as an oyster nursery with great success, the high 
temperature developing the spat with remarkable rapidity, and with 
a degree of regularity from year to year surpassing anything attained 
in the fjord waters outside (see p. 587). 
That portion of Russia formerly covered by the ice-sheet is known Northern 
as the Lake Region, and includes Finland and the Russian governments 
of Olonets, Novgorod, St Petersburg, and Pskov. So numerous are 
the lakes in this part that they form a labyrinth of sheets of water 
and marshes, communicating with each other by rivers interrupted 
by rapids, the land not covered by water consisting of isthmuses and 
peninsulas. The government of Novgorod alone contains 3200 
separate lakes, and that of Olonets 2000. The Russian portion of 
the Lake Region includes 15,500 square miles of water-surface. 
Lake Enare. — The large lake Enare, in Russian Lapland, has an 
area of about 550 square miles, and drains through the Pasvigelf into 
the Varanger Fjord. Its elevation is 394 feet above sea-level, and 
the maximum depth is 30 feet. 
Of the total area of the rocky table-land of Finland, one-tenth is 
covered by water. 
Lake Saima. — The largest lake in Finland is Lake Saima, with 
an area of 680 square miles. It lies at an altitude of 256 feet, and 
has a maximum depth of 187 feet. It is connected by the Saima 
Canal with the Gulf of Finland, and drains by the Vuokosen, which 
forms the Imatra Rapids — the grandest rapids in Europe, — to Lake 
Ladoga. 
The River Neva (see fig. 70) flows into the Gulf of Finland, and River Neva, 
carries off* the drainage of the two largest lakes in Europe, Lakes 
Ladoga and Onega. 
Lake Ladoga is the largest sheet of fresh water in Europe ; 
it is three times the size of Lake Vener, and thirty times that of 
the Lake of Geneva. Its length is 128 miles, its maximum breadth 
about 80 miles, and its area, including the islands, is 7015 square 
miles. Its maximum depth is 732 feet, its mean depth 300 feet, 
and it is estimated to contain 43,228,000 million cubic feet of 
water. In former times it formed one basin with the Gulf of 
