GEOLOGY. 249 



of the valley. On the retreat of the ice, owing to 

 climatal changes, this hollow, unless previously choked up 

 with sand and stones, will be filled with water, and form a 

 lake. It will be a true rock basin, with ice-worn surfaces 

 around its lip, and over its sides and bottom. 



' And such is the appearance presented by many a lake 

 and tarn in the Highlands of Scotland. One of the largest 

 and noblest of the whole Loch Awe may be taken as 

 an illustrative example.' 



Nor is it only the formation of single lakes which can be thus 

 accounted for ; a continuous succession of lakes in the direc- 

 tion of the movement of the glacier may be thus produced. 



As popular illustrations of the mode of operation I may 

 cite the following : Young boys, and girls too, amuse them- 

 selves making what they call 'ducks and drakes' by 

 throwing flat stones across a placid sheet of water, as 

 nearly parallel to the surface of it as they can, causing 

 them to skim along and above the water, touching and 

 rising again and again, rebounding in ever-diminishing 

 bounds till they sink. The same phenomenon may be 

 seen on a larger scale in the recochetting of a cannon ball 

 fired at a target out at sea ; and the same thing may be 

 seen in the effects of the wind striking the surface of the 

 water in a river, in a lake, or in the sea, for rarely, if ever, 

 does it blow horizontally or parallel to the surface of the 

 water. Where it impinges it raises a little wavelet, and 

 rebounds, but only to strike again a little in advance, again 

 and again to rebound, producing a succession of advancing 

 elevations and depressions. In another volume entitled 

 Forests and Moisture* I have had occasion to refer to another 



* Forests and Moisture ; or, Effects of Forests on Humidity of Climate. In which 

 are given details of phenomena of vegetation on which the meteorological effects of 

 forests affecting the humidity of climate depend, of the effects of forests on the humi- 

 dity of the atmosphere, on the humidity of the ground, on marshes, on the moisture of 

 a wide expanse of country, on the local rainfall, and on rivers, and of the correpOH- 

 dence between the distribution of the rainfall and of forests, the measure of corres- 

 pondence between the rainfall and that of forests, the distribution of the rainfall 

 dependent on geographical position, determined by the contour of a country, the dis- 

 tribution of forests affected by the distribution of the rainfall, and the local effects of 

 forests on the distribution of the rainfall within the forest district. Edinburgh : Oliver 

 and Boyd. London : Simpkin, Marshall, & Co. 1877. 



