LIMNOLOGICAL PROBLEMS 
379 
moss -covered mountain slopes, always saturated with water, and 
rarely frozen, every period of heavy rains will at all times of the year 
send immense masses of water down into the lake-basins. In both 
cases the height of the water undergoes very considerable variations, 
but these are in the former case periodical, in the latter quite irregular 
(Scotland). 
The chemical composition of the water is much less uniform than 
in the arctic regions. On the whole, it must be considered poor in 
lime ; during its passage through layers of moss and peat a consider- 
able portion has absorbed large quantities of humic acids ; the lakes 
are further filled with organic material to a much higher degree than 
the arctic lakes, and this in suspended form is carried out into the 
lakes from the surrounding territory. Owing to the steep course of 
the affluents, this material is very rough ; and, owing to the steep 
rocky sides of the lakes, much of this rough material (branches, leaves, 
hay, fruits) in unpulverised form is carried out even into very great 
depths (100 m. and over). Here the material, owing to the preserving 
action of humic acids, does not decay, but undergoes only a slow 
process of disintegration, the result of which is a remarkable sort of 
liquid brown peat (G. West, 1905, p. 968). Lakes with clay-filled 
water such as often occur in arctic regions are no doubt rare. The 
transparencjij of those lakes in which the water is coloured by humic 
acids (especially the Scottish) is very slight, generally only 5-7 m., 
and the colour of the water is brown (Bachmann, 1907, p. 7). Some 
of the Norwegian lakes are remarkable for their exceedingly great 
transparency, 14-18 m. (Huitfeldt-Kaas, 1906, p. 130) ; brown lakes 
are rare. Huitfeldt-Kaas mentions that the transparency of the 
water in Norway is much more influenced by detritus than by plankton 
(p. 126). The lakes are for the rest subject to the same fate as 
the surrounding territory ; their surface receives only little direct 
sunlight ; the rainfall is everywhere great ; through long periods of 
the year, especially in Scotland, immense clouds and fogs shroud the 
country and persist longest in the valleys where the lake-basins occur. 
The low summer temperature with the usually very humid atmosphere 
do not allow of any appreciable evaporation from the surface, and 
consequently no great concentration of the water takes place in 
summer. 
This zone presents greater variations in temperature than perhaps 
any other. It contains some lakes which, like several of the large 
Scottish lakes, must be classed among the tropical lakes, with water 
at a temperature >4° C. throughout the year, e.g. Loch Katrine, the 
temperature of which hardly sinks below 4° '44 (Pettersson, 1902, 
p. 8 ; Forel, 1901b, p. 35), and Loch Ness, the surface temperature of 
which rarely sinks below 5° C, and which at any rate never freezes ; 
