376 THE FRESH- WATER LOCHS OF SCOTLAND 
very slowly ; the material deposited is mainly sand and clay with a 
slight admixture of organic material. Porsild (1902, p. 207 ^) remarks 
that the bottom material was never finely pulverised mud, but 
generally large, well-preserved particles. The deposited bottom 
material was odourless, and thus probably destitute of bacteria, all 
processes of decomposition going on very slowly. 
The transparency and colour of the water vary a good deal : in the 
clay -filled lakes the water is grey and the transparency very slight ; 
in lakes not directly fed by rivers from the inland ice the water may 
be exceedingly clear and the transparency great (Vanhoffen, 1897, 
p. 169). With regard to the chemical composition of the lake- water 
we do not know anything, but we may advance as an hypothesis that 
whilst farther south the chemical nature of the lake- water, the quantity 
and quality of decomposed and suspended organic and inorganic 
constituents, are dependent upon the heterogeneous nature of the 
surrounding country of the lake territory and vary from lake to lake, 
this is hardly so much the case in the arctic zone, where the differences 
in the nature of the surrounding country are not so great ; further, 
we may suppose that the lake- water will prove to be exceedingly poor 
in lime everywhere in the arctic regions. 
Our knowledge of the temperature of the lakes is also very in- 
complete. We know only that the arctic lakes are open but few 
months of the year. Many of the lakes examined by Ekman (1904, 
p. 10) in Sarek were never quite free from ice. Three small lakes 
were covered with ice of a thickness of 2 m. even on the 27th July 
1903, and are supposed to thaw only in very warm summers. The 
lakes examined by Greely on Grinnell Land at 82° N. lat. were free 
from ice only during one and a half months, from the middle of July 
to September. A great many high arctic lakes are thus no doubt of 
ForeFs type of polar lakes, the surface temperature of which never 
exceeds 4° C. and the bottom temperature of which is ^ 4° C. 
They have always " inverse stratification,"" the water resting in layers 
almost throughout the year, the colder above the warmer ; in summer 
they have a very short period of circulation (Forel, vol. ii. p. 303). So 
far as I know, the temperature of such lakes is only known from theo- 
retical considerations. The only lake of whose temperature we have 
some knowledge, and, as far as I know, most like ForePs type 
of polar lakes, is the large deep Torne Triisk in Swedish Lapland. 
According to Ekman (1904, p. 8), it was still almost homothermous 
on the 25th July 1900, four weeks after it had thawed, with a 
temperature of 3*1° C. at the surface and 3*3" C. from 70 to 85 m. 
Even in July the lake had thus not yet attained the temperature of 
1 The fnll reference to the literature cited will be found in the bibliography at 
the end of the paper. 
