Vol. 62.] TABNS OF THE CANTON TICINO. 181 



the original flow, or the relics of an earlier drainage before the 

 present Val Piora came into existence, for it turns off at right 

 angles to the general line of the valley. I think, however, that 

 it flowed over its present exit previous to the formation of the 

 lakes, and that their formation may be connected with the diversion 

 of the drainage into the 4 Foos ' channel. 



As the valley is entirely rock-bound, the diversion cannot be due 

 to the blocking of an old outlet at the western end ; it must, there- 

 fore, be due to the direct encroachment of the Foos on the Piora 

 watershed, through which it has cut. This can only have happened 

 if the Ticino Valley were more rapidly over deepened than the 

 valley into which the original Piora river drained. Obviously, 

 therefore, it did not drain directly into the Ticino; but where else 

 could it drain? There are only two answers to this question. 

 One is, that it drained over the col between Fongio and Pian' Alto, 

 at the western end of Eitom, into the Val Canaria, as suggested 

 by Prof. Bonney ; the other is, that it flowed eastwards, and not 

 westwards, over what is now the Passo del Uomo, and that the 

 drainage to the west of this pass has therefore been reversed. This 

 latter explanation would account, not only for the hanging valley 

 of La Foos, but also for those at the eastern end of Lago Eitom 

 and below Lago Tom. It is probable, however, that there were 

 several stages in this process, which appear to have been as 

 follows : — 



(1) Before the present depression of the Val Piora existed, the general 

 drainage of the district would be very different from that of the present day, 

 and a stream rising on the Camogbe range may perhaps have flowed eastwards 

 into the Medelser Ehein in Pliocene times. About this, however, it is difficult 

 to be certain, as no relic of that period remains. 



(2) The overdeepening of the Ticino Valley, which then appears to have 

 taken place, may have enabled a stream from the Val Canaria to cut back over 

 what is now the Fongio col as far as the head of the present Tom Valley. 

 This diversion, although unconnected with the formation of the lakes, appears 

 to be indicated by the present depression between Fongio and Pian' Alto. 



(3) This overdeepening by the Ticino, postulated above, would enable the 

 Foos, which then rose on the northern slope of the Val Ticino, to cut back 

 its head-waters until it captured the drainage of the present Tom Valley. 



This would naturally result from the circumstance that, whereas the Foos 

 drained directly into the overdeepened Ticino Valley, the stream over the 

 Fongio col drained indirectly into that valley, through the Val Canaria, which, 

 at that time, would not have yet adjusted itself to the Ticino level. Eelics of 

 the drainage as it existed at that time are clearly visible at the present day, 

 not only in the Foos Valley draining northwards and southwards from the 

 col between the Punta Nera and the Taneda to the Ticino, but also in the 

 low watershed (not yet completely destroyed) running parallel with this valley 

 from the Taneda, and now separating Lago Tom from Lago Cadagno. 



(4) This watershed was next gradually cut through by an eastern tributary 

 of La Foos at its weakest point, namely, the point occupied by the rauchwacke 

 along the line of the present Lago Eitom, and this tributary expended farther 

 and farther to the east, to form the present Val Piora. On this supposition, 

 namely that the Foos Valley existed before the Val Piora, it is easy to account 

 for the fact, otherwise difficult of explanation, that the present drainage from 

 the Val Piora turns at right angles where it discharges itself into the Ticiuo 

 Valley. 



(5) An uplift of the range to the west, possibly connected with the melting- 



