LIMNOLOGICAL PROBLEMS 
383 
innumerable transitional stages occur everywhere between such as 
exhibit as much of the lake character as it is possible to find in the 
region dealt with and the smallest ponds and drying-up pools. One 
of the features to a great extent characterising the landscape of 
the Baltic district is just its great number of small lakes, ponds, 
and pools. 
Characteristic of all the lakes of this zone is their uniform 
appearance. First and foremost, nearly all are lakes of the level 
country ; their height above the sea varies little and is very rarely 
over 200 m. Only in Riesengebirge, Eifel, and Schwartz wald do 
some more elevated lakes occur. The country surrounding the lakes 
exhibits everywhere a similar appearance : never perpetual snow, 
rarely naked rock, but fertile forests, meadows, and arable land, in 
part bogs and heaths. The ground is nearly everywhere loose and 
easily worked, and consists mainly of humus, clay, and sand, often 
mixed with considerable quantities of lime. The lakes are generally 
of small size, rarely over 3000 hectares, and all shallow ; the depth rarely 
exceeds 30-40 m., and attains at most about 70 m. The shape of the 
lake-basins is often circular ; steeply sloping sides are rare, and there 
is generally a very broad littoral region, on the windward side 
covered by sand or gravel, on the lee side by detritus, peat formations, 
etc. (Klinge, 1890, p. 264). The bottom mostly consists of soft lake- 
bottom deposits, the so-called lake "gytjes,""' very rich in organic 
material and very often highly charged with lime ; the quantity of 
lime is often so great that the bottom layer ma}^ be directly used 
as marl to improve the arable soil. 
Great variations in the height of the water do not generally occur. 
Neither sudden thaws nor violent torrents will produce appreciable 
variations in the height of the water, as the loose ground everywhere 
absorbs the moisture and the slope is so slight. Owing to the low 
banks, even a slight sinking of the surface of the water is very visible, 
and involves a very great restriction in the area of the lake in 
autumn. The decrease in the height of the water naturally results 
in an increasing concentration. This becomes much more evident 
as, on account of the slight declivity of the land, the water is on the 
whole slowly renewed, and especially so in sunnner. 
With regard to temperature also the lakes of this zone present 
great similarities. Polar lakes do not occur, tropical lakes hardly 
ever — they are at any rate rare. The great majority are frozen during 
a shorter or longer period of the year, yet only exceptionally for 
more than three months. It happens in certain years that the lakes, 
on the whole, do not freeze at all. A very high summer temperature 
is common to all the lakes ; it probably always exceeds 18° C, not 
rarely it is 24-26°, and may even be higher. Owing to the slight 
