rZ BELT— -ON PRODUCTION OF LAKES BY ICE ACTION, 



formed by glaciers leaving terminal moraines in their retreat, and 

 the scooping out of long deep channels is easily understood; but 

 the production of deep rock-basins is not so easily explained, and 

 their glacial origin has been disputed by eminent geologists. 

 We owe the theory of the production of rock basins by ice action 

 to Professor Ramsay, who in 1859 showed that there was an 

 intimate connection between mountain lakes and the evidences of 

 glacial action, and argued that the rock basins had been ground or 

 scooped out by ice, either in soft rocks surrounded by harder, or 

 more generally, in places where a greater height of ice had accu- 

 mulated and exerted a greater grinding pressure on the rocks 

 beneath. In 1862 he extended his theory to account for the pro- 

 duction of the great lakes of Switzerland, and even those of North 

 America, contending that there is such a gradation of size from the 

 least to the greatest that we cannot apply the theory to the one and 

 not to the other. 



The Lake of Geneva is 984 feet deep, the Lake of Zug 12T9 

 feet, and the Lake of Brienz more than 2000 feet, and its bottom 

 about 200 feet below the level of the sea. In Italy even these 

 depths are exceeded, and we have the Lake of Como 1929 feet 

 deep, and the Lake of Maggiore 2625 feet, and its bottom 1940 feet 

 lower than the sea level. With regard to these great depths it has 

 been urged by Sir Charles Lyell and others, that though the pas- 

 sage of prodigious masses of ice for ages over the surface would 

 doubtless produce depressions where the hardness of the rocks 

 beneath was not uniform, yet a depth would soon be reached where 

 the movement of the ice in the basins would be arrested, and the 

 discharge of the glaciers would be over and not through the ice- 

 filled hollows. In a glacier as in a river, the lower strata move 

 much more slowly than those at the surface, being impeded by the 

 friction on the bed of what we may call the ice river — and as in the 

 Lake of Maggiore the ice, on Professor Ramsay's theory, would 

 have in its exit to ascend a slope of five degrees from its deepest 

 part. It is contended that in such a case it would be simply dam- 

 med up, the glacier passing over it. It is true that in Australia 

 there are deep hollows in the courses of the streams, in which water 

 is stored up during the dry season, but this is a peculiarity of inter- 

 mittent rivers and dependent upon the intermittent action. Again, 



