188 PROF. E. J. GARWOOD ON THE [May I906, 



determined near the northern shore. The lake is, therefore, deepest 

 towards its upper end, and shallows gradually to the exit. The 

 northern shore is chiefly composed of loose material and boggy 

 swamp, through which rise here and there hummocks of solid rock. 

 The shape of the lake and the subaqueous contours, as well as the 

 presence of loose material at the upper end, point to damming of a 

 tributary valley by a landslip or moraine, and the reversal of the 

 drainage over a gap in the watershed at its upper end. This view 

 is borne out by the character of the exit, and also by the presence 

 of a deep valley just beyond the accumulation at the northern end 

 of the lake, into which it would apparently drain if the loose 

 material were removed. There is, however, just a possibility that 

 the lake may be to some extent rock-dammed. 



VI. General Conclusions. 



The results arrived at by a detailed investigation of the tarns in 

 the northern portion of the Canton Ticino may, then, be summarized 

 as follows. 



The lakes can, for convenience, be grouped under two heads : — 



(I) Those (a) entirely, or (b) partly dammed by loose material. 



(a) Lago Camoghe. (b) Lago di Murinascio. 



„ Lisera. ,, Lucomagno. 



„ Pettano. „ Orsirora. 



„ d'Elio. „ Orsino. 



St. G-otthard tarns. 



(II) True rock-basins due to (c) solution, (d) differential weathering. 



(c) Lago Ritom. (d) Lago Scuro. 

 „ Tom. .. Taneda. 



„ Cadagno. „ di Lucendro. 



,, Tremorgio. ,, della Sella. 



Those grouped under I (a) require no comment. Those under 

 I (b), in so far as they are rock-basins, may possibly be due to 

 differential weathering, either by frost or by ice. It is, however, 

 difficult to ascertain what depth, if any, we are to consider as 

 rock-basins in each case, but it cannot be a very great depth. 

 Lago di Murinascio and Lago Lucomagno lie upon ledges dipping 

 steeply into the scarp-face at the foot of precipitous cliffs, above 

 which there is no possible gathering-ground for a glacier, unless the 

 ice poured over the cliffs in each case from the north ; but the cliffs 

 are too steep for it to arrive, even then, in anything but a 

 fragmentary state. Consequently, even Prof. Bonney's general 

 concession, that the ice, as it descended from the ranges above, 



' would impinge on the level floor, on which under these circumstances it might 

 have some erosive force,' (Geol. Mag. 1898, p. 19.) 



does not seem applicable here, as the corries would be filled with 

 snow and protected from any bombardment of this nature. It is, 

 of course, always possible that the slight irregular erosion of a few 

 feet might have been produced by the ice collected in the cirque, as 



