Yol. 62.] TARNS OP THE CANTON TICINO. 189 



assumed by some authors to account for the formation of corrie- 

 lakes in general ; but it is difficult to realize the mechanics of the 

 process, unless the ice found in these corries behaves rather differ- 

 ently from that of a glacier, and moves as a solid mass and not 

 differentially as in a glacier. The lakes under notice are, however, 

 comparatively unimportant ; but, if any part of their basin is truly 

 rockbound, they are difficult to account for, as they occur entirely 

 in one rock-formation. 



Rock-Basins. 



II (c) . — The chief lakes of the Yal Piora are shown to lie along the 

 junction of soluble calcareous rocks with gneiss or schists. Detailed 

 soundings prove that the axes of greatest depth of these lakes 

 coincide very closely with these lines of junction. Although the 

 district must have been below the snow-line in Glacial times, there 

 is very little sign of glacial action visible at the present day, and it 

 is difficult to understand how ice can have taken any real part in 

 the formation of the lakes. 



Chemical evidence derived from analyses of the rauchwacke and 

 from the composition of the water of the lakes, points strongly to 

 solution having played a conspicuous part in the formation of these 

 lakes. This is confirmed by the fact that, in the case of Lago Tom, 

 the surplus-water does not overflow the containing barrier of 

 rauchwacke at the exit-end, but, percolating into the heart of the 

 limestone, reappears as a spring which rises exactly at the junction 

 of the rauchwacke with the underlying schist. 



Other evidence, such as the high percentage of dolomite-grains 

 in the fine deposit forming the floor of the lake near the exit, 

 evidently representing the less soluble portion of the rauchwacke, 

 points in the same direction. In the case of Lago Ritom, it is shown 

 that the whole of the western end of the lake is occupied by rauch- 

 wacke, although not so shown on the Swiss Geological Survey-map, 

 and that the lake may be considered to occupy the site of a 

 lenticular thickening of the calcareous beds similar to those which 

 occur west and east of it along the same general strike. The 

 marked increase in the amount of dissolved salts in the bottom- 

 water of this lake, coupled with an equally-marked rise in tempera- 

 ture, are facts which point strongly to underground solution in the 

 form of subaqueous springs ; while the high percentage of sulphuric 

 acid can only be explained on the supposition that beds of gypsum 

 occur under this basin, similar to those found along the outcrop on 

 the west and east. 



Lago Tremorgio appears, from its characteristic pothole shape, to 

 be also due to solution. It is situated on calcareous mica-schist, 

 and the eyes of calcite in the schist are dissolved out from the rocks 

 bordering the edge of the lake. It is also possible that the fold of 

 dolomite, visible in the Campolungo Pass above the lake, may 

 continue downwards to the lake-floor ; and, if this should be the case, 

 it would amply account for the basin by solution. The fall of the 



Q. J. G. S. No. 246. p 



