190 PROF. E. J. GAKWOOD ON THE [May I906, 



surface of the lake several feet below the exit in winter, and 'the 

 continuous flow of springs which issue all the year round from the 

 hillside below the lake, point also to solution as the probable origin 

 of this lake. It does not appear to be possible to attribute this 

 lake to glacial excavation, as it does not form part of a valley- 

 system which could have been overdeepened by ice to produce the 

 present basin. 



II (d). — There remain four important rock-basins which are 

 more difficult to explain. They resemble the foregoing group in the 

 fact that they all lie along the junction of two different rock-types, 

 so that, whatever the weathering agent may have been which 

 finally produced the basin, the occurrence of the lakes in their 

 present position seems certainly here again to have been determined 

 by structural lines of weakness. Lago di Lucendro lies in the track 

 of an extension of the Lucendro Glacier, and Lago della Sella also 

 occupies a valley at the head of which a glacier-system still 

 exists. It is possible, therefore, that differential weathering in 

 pre-Glacial or inter-Glacial times, along the junction between the 

 gneiss and the biotite-schist, may have caused an unequal removal 

 of material by ice in Glacial times. This excavation does not appear 

 to have been so much in the nature of digging, as of the removal of 

 less resistant, possibly more weathered, material. This is shown 

 by the marked convexity of the outlines of the gneiss even under 

 water, by the presence of the rocky headland at the exit of Lago 

 di Lucendro, and also by the occurrence of the islands of solid 

 rock running through Lago della Sella. The two remaining lakes 

 of this group, namely Lago Scuro and Lago Taneda, which occur 

 within a stone's throw of the main watershed and occupy basins 

 140 feet deep, are certainly very puzzling : for, although they lie 

 along the junction of schist and gneiss, the theory of solution does 

 not appear to be applicable here. Had these lakes lain in glaciated 

 valleys, they would have undoubtedly been considered by many 

 geologists as typical ice-excavated basins. Their occurrence, how- 

 ever, a few metres below the watershed, as welt as the presence of 

 submerged reefs running round the southern end of Lago Scuro 

 seems to preclude the possibility of excavation by ice. 



The lakes, then, of the Canton Ticino that have so far been 

 examined, with the possible exception of the rock-pools of the 

 St. Gotthard Hospice, do not seem to be due to ice-erosion in the 

 generally-understood sense, if we adopt the most recent definition 

 by Prof. James Geikie for this class of lake, as contained in the 

 following statement : x 



' Rock-basins of glacial origin differ from all others in the fact that they 

 are totally independent of geological structure and the character of the rocks 

 themselves.' 



1 ' Structural & Field-Geology ' 1905, p. 416. 



