174 PKOF. E. J. GARWOOD ON THE [May I906, 



Lago Camoghe, lying on the western slope of the peak of that 

 name, is another example of a scree-dammed tarn. Its formation 

 may, however, have been influenced by the occurrence of a bed of 

 rauchwacke, which occupies the pass between Pian' Alto and the 

 Cima di Camoghe. This rock, although not so shown in Dr. K. von 

 Fritsch's map, appears to strike rouud the south-western end of 

 this lake, and reappears at the head of the gully at a point where 

 the spring issues a little below the Alpe di Lago. 



The Origin of the Piora Lakes. 



So far as I am aware, only one writer on this district has expressed 

 any opinion regarding the origin of these lakes, namely Prof. Bonney, 

 in his paper in the ' Geological Magazine ' for 1898, already quoted. 1 

 In discussing the possibility of their formation by ice-erosion, he 

 does so, evidently, with a desire to give all due weight to a 

 theory which is perhaps that most generally held in regard to the 

 formation of lakes of this class ; but he is obviously not quite 

 satisfied, himself, with the application of the theory to these par- 

 ticular lakes. I will, in the first place, point out the serious 

 difficulties in the way of accepting this theory, and afterwards 

 suggest what seems to me a more probable mode of origin. 



Let us first take Lago Ritom. In the case of this lake, there 

 are only three directions in which ice can reasonably be assumed to 

 have travelled, namely : (1) from the depression between Pian' 

 Alto and Pongio at the western end of the lake ; (2) from the 

 district on the north, now drained by Lago Tom ; (3) from the Yal 

 Piora on the east, including possibly a tributary from the cirque 

 now occupied by Lago Cadagno (see Pis. 711 & XI). 



In the first case, there would be no sufficient gathering-ground 

 inside the Ritom drainage-area, and ice advancing from this quarter 

 must have originated outside the district and overtopped the 

 western watershed ; but we have no evidence of an accumulation 

 of ice in the district to this depth, and had the ice in the Yal Ticino 

 reached to this height, it would have invaded the Ritom basin much 

 more readily from the east through the present exit of the lake, which 

 is nearly 1000 feet lower. 



The second case postulates a glacier descending from Lago Tom. 

 This could not possibly have excavated the upper end of Lago 

 Ritom, and consequently this supposition may be at once dismissed. 



There remains only the third alternative, namely that of a glacier 

 descending the Yal Piora, augmented possibly by ice from the Lago- 

 Cadagno basin. There can be no doubt, I think, that this district 

 lay above the snow-line during the Pleistocene Period, and that 

 snow and ice accumulated here and occupied the Yal Piora. My 

 difficulty lies in understanding how this ice could acquire any 

 velocity in moving westward along the basin now occupied by Lago 

 Ritom, and why it should have turned off at right angles and 

 discharged itself to the south-east, unless these lines of drainage 

 had been previously determined. 



1 But see also A. Delebecque, Comptes Rendus Acad. Sci. Paris, vol. cxxxix 

 (1904) pp. 936 et seqq. 



