378 THE FRESH-WATER LOCBS OF SCOTLAND 
water to its organisms everywhere in the arctic zone from lake to lake 
as well as in all localities within the same lake. 
The North European Lakes 
The uniform character of the arctic lakes does not characterise the 
lakes of the northern temperate zone. The country surrounding 
the lakes is most varied : perpetual snow, naked rock, but much 
oftener beds of moss and peat which creep round the mountain crests 
and clefts like a mantle, in Scotland about | m. thick, and through 
which the water oozes on its way down to the water-basins ; wide 
bogs, forests with humic acid ground, and in part, but to rather a 
slight extent, arable land. 
The height of the surface of the lakes above the level of the sea is 
extremely variable. The zone contains numerous mountain lakes, 
especially in Norway and North Sweden, elevated into completely 
arctic conditions, and many, e.g. several Scottish lakes, very near the 
level of the sea. The shape of the lake-basins is often long and 
narrow ; a great many may no doubt be considered as exceedingly 
large pre-glacial river-beds, formed by erosion (see Ahlenius, 1900, 
p. 28 ; 1905, p. 17) ; their depth is often very considerable. More than 
half of the twenty-seven European lakes whose depth exceeds 200 m. 
lie in Norway and Scotland, and the four deepest lakes of Europe are 
in Norway (Hornindalvatn, 486 m.) and Scotland (Loch Morar, 329 m.) 
(see Holmsen, 1898-9, p. 1 ; Helland, 1872, p. 538 ; John Murray, 
1904c, p. 67, and Halbfass, 1903-4, p. 221). Most of the lakes are of 
medium size or small ; still, the zone includes several large lakes — the 
great Swedish and Finnish-Russian lakes. A great many, especially the 
Scottish and many Norwegian lakes, have exceedingly precipitous sides 
with depths of more than 100 m. near land. The shores are generally 
covered with rubble-stones, dislodged and rounded by the waves ; in 
front of the river mouths we often find large, well-marked delta 
formations, in sharp contrast to the firm rocks. Still, a littoral region 
is probably in many cases, especially in the deeper lakes, fairly 
sharply delimited from a pelagic region, differing from the latter by 
greater variations in temperature. Only in very few cases is the bottom 
naked rock ; the primary lake bottom is probably always covered by 
secondary deposits. In Scottish lakes it can be stated that lime is 
absent except where the rocks are limestone, and most probably also 
in the great majority of Scandinavian lakes, especially those north of 
the large Swedish lakes. In so far as the surroundings mainly consist 
of snow and the ground is frozen in winter, the height of the water 
will undergo regular periodical variations, being highest in early 
summer and decreasing later ; where the surrounding country is 
