20 Professor T. G. Bonney — Lalxe-hasins in the Alps. 



be formed. The Val Pioi'a, as I have said, consists of a lower step 

 (the Lago Eitom) and an upper step, with which the Lago Cadagno 

 is connected. These are separated by a rocky slope, precipitous 

 in places, nearly 300 feet high. Supposing a glacier to be descending 

 the Piora valley, we must assume this wall to be already in existence, 

 or it could not acquire any plunging force, and even then the fall 

 seems hardly adequate to produce the erosion of a basin like that 

 of Lake Kitom. Possibl}', however, the ice, just at this part, 

 may have been "jammed"; for the main glacier was probably 

 augmented by another ice-stream, which descended a shallow, but 

 fairly well-marked valley, leading from a gneissic peak lying 

 south-east down to the corresponding corner at the head of the 

 Eitom basin; while the narrow "gate" by which the water is 

 now discharged towards the Val Bedretto would block the mass 

 of ice above it, and this would produce more than usual friction 

 on the bed of the valley now occupied by the lake and its delta. 

 This basin, then, the part which lies below the present contour- 

 line of 6,000 feet (in round numbers), is the utmost that, in my 

 opinion, can possibly be attributed to the erosive action of ice. 

 Of this action, all the other dominant features in the surrounding 

 scenery exhibit nothing more than superficial traces, and they 

 appear to be due to the usual meteoric agencies. 



The broad outlines of the Val Piora must have been determined 

 at an early date. It lies, as I have said, in an infold of schists, 

 belonging to the upper part of the crystalline series, and of 

 some rauchwacke of Triassic age, which, after crossing the lower 

 end of the Val Canaria, reaches the floor of the Val Bedretto at 

 and near Airolo. Eegarding this simply as a fold (it is really 

 a complicated and faulted one), the natural line for the discharge 

 of its drainage would be towards the Val Bedretto, in the direction 

 of Airolo, passing over the col mentioned above as lying to the 

 north of Fongio. The top of this col is probably about 6,800 feet 

 above the sea. Fongio itself is clearly a prolongation of the gneissic 

 range between the Val Piora and the Val Bedretto. Hence, at 

 some very early date, differential movements in the mountain mass 

 must have diverted the drainage of the Val Piora from a western 

 direction to its present outlet on the east side of Fongio, through 

 some chance dip already existing in the range. Owing to the rapid 

 descent to the north a groove would soon be formed, and the 

 direction of discharge finally determined. Since then, as I suppose, 

 all of the lower half of the Val Piora that lies below the contour- 

 line of 6,800 feet (in round numbers), with some of the upper, 

 must have been excavated. Somewhat beneath this level, perhaps 

 at about 6,500 feet, a rather marked increase of steepness is often 

 perceptible in the slopes on the northern side of the lower part of 

 the Piora valley. Besides this, a structure which is conspicuous 

 in the Val Bedretto itself may not be without significance. Looking 

 up that valley from such a point as the top of Fongio, one perceives 

 that the slopes become near a certain part very much steeper, and 

 begin to descend from that level rather abruptly towards the bed of 



