226 SCIENCE PROGRESS. 



(iv.) Lakes lying in hollows of erosion. — The forma- 

 tion of rock-basins by erosion is a question which has given 

 rise to a great deal of controversy, and the origin of rock- 

 basins by erosion is still to a large extent enveloped in 

 mystery. There are many lakes, apparently rock-basins, 

 which lie in hollows so diminutive that no one would invoke 

 the action of differential movement to account for their 

 formation, and should their existence as true rock-basins be 

 established they must be admitted to have been produced 

 by erosion, though it by no means follows that the agent 

 of erosion must necessarily have been ice. In our own 

 country Mr. Watts has claimed the lower lakelet in the 

 hollow of Cwm Glas in Snowdon as "certainly confined in 

 a rock-basin, as rock occurs at its actual outlet and at every 

 point where any former outlet might have been possible"( 1 2). 

 M. Delabecque, in the paper on the lakes of the Vosges 

 district to which I have already referred, describes a lake- 

 let of some size (L. Retournemer) which the geological 

 map indicates as being barred by moraine, but states that to 

 him it appears to be entirely situate within rock in situ, and 

 that it must be grouped amongst the rock-basins of which 

 the origin is still but litde explained. The work of M. 

 Delabecque is evidently marked by much care, and he has 

 paid such special attention to lakes with particular reference 

 to their existence in rock-basins that he is hardly likely to 

 have overlooked any drift-filled depression to which this 

 lakelet may be due. Mr. H. P. Gushing (13) describes 

 diminutive lakes on the tops of low hills bordering the Muir 

 Glacier of Alaska. Several are situated on nunataks near 

 the end of the glacier and occupy small depressions or 

 basins on the tops of ridges ; they are only a few yards in 

 diameter and of no great depth. " Some of them clearly 

 occupy rock-basins, rock in place being readily traced all 

 round them . . . other lakes have a portion of their 

 shores formed by glacial debris. . . . That the glacier 

 has done little more than to remove the loosened rock and 

 polish the resulting surface is shown in a vast number of 

 localities. . . . All these basins which I saw lie in 

 small valleys on the mountain tops, whose presence seemed 

 to depend on the fissure systems and on the varying depths 



