MECHANICAL ACTION OF GLACTERS. 149 
Great Britain, may be found the first suggestion, and illus- 
tration, and proof of this fact. There ‘ he has shown that 
the innumerable rock-enclosed basins of the Northern 
Hemisphere do not lie in gaping fissures, produced by 
underground disturbance, nor in areas of special subsid- 
ence, nor in synclinal folds of the strata, but that they are 
true hollows of erosion.’ 
I cite the statement of Professor Geikie, in his Scenery 
of Scotland, viewed in connection with its Physical Geography ; and 
to this work I am indebted for the following illustrations :— 
* Lakes, at least those which mottle the surface of Scot- 
land, may be grouped into three classes: 1st, those which 
lie in original hollows of the superficial drifts ; 2d, those 
which have been formed by a bar of drift across a valley 
or depression ; 3d, those which lie in a basin-shaped cavity 
of solid rock.’ 
Lakes of each of these kinds may be seen in Scandin- 
avia and Finland. It is in regard to the formation of the 
last description of lakes that there is any difficulty-—the 
formation of a cup-like hollow in solid rocks, sometimes 
along the line of a valley, sometimes on a plateau, some- 
times on a hill top, or on a watershed. 
There are in many rivers deep holes. At the Cape of 
Good Hope one hears constantly of See-Koo vleys, or hip- 
popotamus holes, and occasionally, even in the rocky bed 
of a river, we find cylindrical cavities called pot-holes,, In 
the bottom of such are generally found a few well-rounded 
pebbles and boulders. The cavities are due to the circular 
movements of these or other stones and boulders, which, 
caught by an eddy, have been kept whirling there, and by 
friction abrading the rock they have gradually formed, 
these holes working downward into the solid rock. And 
often on the sea-shore may be seen cavities lined with 
sea weed and filled with sea-water, each a natural aqua- 
rium. Some of these are formed, as are the pot-holes, by 
boulders lying in their bottom which have been kept 
whirling round in the eddies of a vexed tideway instead 
of a rapid brook or river. 
