MECHANICAL ACTION OF GLACIERS. 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, mewed 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. 



