Professor T. G. Bonney — Lake-basins in the AIj^s. 19 



Putting aside for the moment some questions which arise from 

 the physiography of its borders, I pass on now to a fourth like 

 situated on the northern flanlc of the Lago Eitom, at a considerably 

 higher level. This flank, as I said, rises in cliffs and steep grass 

 slopes. A path up the latter, near the side of a cascading stream, brings 

 us into a small upland valley, and we presently reach a lake at its 

 head called the Lago Tom (6,637 feet). Like the Lago Cadagno, 

 it occupies a kind of cirque,^ and lies in the strike of the same 

 rocks, for the enclosing crags consist of similar amphibolitic and 

 granatiferous gneiss, and at the lower end is rauchwacke, which 

 can be traced from the southern side of the basin of the Lago 

 Cadagno across the intermediate spur. -But the Lago Tom is not 

 only on a rock-basin but also on a very remarkable one. The lower 

 end is dammed by a mass of rauchwacke, in shape something like 

 a rude causeway. Its top, as a rule, is not less than 12 to 15 feet 

 above the water, but in one place it is cut by a dry gully two 

 to three yards wide. Still, the bottom of this cannot be less than 

 six feet above the level of the water. Rather to the east of this 

 gap a curving channel or inlet fi'om the lake pierces into the barrier 

 for some distance. Its sides are cliffs, which at its head are three 

 or four yards high. Just on the other side of the barrier, a fairly 

 copious stream breaks out in a shallow glen which continues the 

 line of the dry gully already mentioned. This unquestionably 

 drains the lake, but where it starts is not easily ascertained.^ There 

 is no distinct flow up the inlet already mentioned,^ and yet the 

 stream, within a few feet of its issuing from the rock (a bank of 

 old snow concealed the actual outlet), is a yard or more wide and 

 a few inches deep, running with a brisk current. I siippose, there- 

 fore, that the rocky bed of this inlet is traversed by a number of 

 small fissures,^ through which the water percolates, to be collected 

 and carried off by the stream. The rauchwacke (the usual cream- 

 coloured, rather soft, and broken-looking limestone) is exposed in so 

 many parts of the barrier that the existence of a drift-blocked 

 channel seems to me an impossibility, and the Lago Tom must occupy 

 a true rock-basin. 



Three out of these four lakes are situated within cirques or 

 very precipitous corries, and if we suppose the main outlines of 

 these to be anterior to the Glacial Age, 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. The origin 

 of the largest lake (Ritom) is less easily explained, unless we 

 suppose that in all other respects the physical features of the 

 neighbourhood remain practically as they were when it began to 



^ Its area is given as 1,000 square metres, and it is said to be shallow. But 

 I should think it would not be less than some 20 feet deep, and might be more. 



^ The only sign of disturbance in the lake itself, several yards away from the 

 shore, was clearly an upward flow, i.e. was produced by a strong spring in the bed 

 of the lake. 



3 I twice examined the surface of the water ; on the second occasion (a very still 

 day) I thought I detected a slight movement in some scum on the water. 



* Eauchwacke often has a shattered, almost rubbly aspect. 



