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with the ocean, and some are still open ; but some also have 
at certain geological periods been occupied by fresh water 
almost exclusively. It is clearly impossible to understand 
their physical geography without studying carefully the geology 
of their coasts. 
Let us pass on now to consider the various kinds of 
lake basins, or rather the lake basins that appear to exist 
under different circumstances. The first group, being marine 
basins, or those that still contain water more or less salt, and 
whose level is similar to or below that of the sea, must pro- 
bably have had direct communication with the sea at one time, 
and were therefore parts of the oceanic basin. Most of these are 
broad and open, but some, as the Red Sea and the Dead Sea, and 
some lakes or sea channels in mountain districts, are elongated, 
narrow, and tortuous, and often very deep. They are known 
by various special names — as fjords in Norway, friths in Scot- 
land. These are well worthy of study. They almost always 
occupy crevices in hard rock, and sometimes, though not 
always, they certainly appear to be connected with faults or 
axes of disturbance. Such are some of the phenomena of that 
class of lake basins whose relation to the general depression of 
parts of the earth and elevation of other parts is most manifest. 
There is another class of lakes and lake basins whose origin 
is apparently quite different, but not less clear. In large open 
flat spaces on wide plains, and on steppes, we occasionally find 
pools. These are generally shallow, and owe their water con- 
tents to occasional rains that fall in the neighbourhood, or to 
floods that come down periodically. Of this kind, are the 
great lakes of Africa, recently discovered and described by 
Dr. Livingstone, Burton, Speke, Grrant, and others. Such is 
also the Lake Tchad, in Central Africa (see the form of this 
lake in the plate) . Such again is Lake Torrens, in South Aus- 
tralia, whose waters according to the earlier describers, were 
almost illimitable, but which would suddenly shrink almost to 
nothing. In Europe, the Lakes Ladoga and Onega, between 
Finland and Russia, are remarkable examples (see plate), and 
the innumerable lakes of South Sweden, and the interior of 
Finland, are of the same nature. The Flatten See in Austria, 
and some of the lakes in Ireland (as Lough Neagh), partake of 
this character. In all, there is a total absence of any other 
physical peculiarity in the district around, than the existence 
of wide flat plains, or low undulating ground. They occupy 
depressions in the plains, and the nature of the rock is of 
small importance. Almost all rocks, in fact, have surfaces more 
or less irregular, when they have been worn by exposure to 
similar causes, and it is very easy to understand that such 
depressions are occupied by the excess of rain-water or river- 
