Vol. 62.] TAENS OF THE CANTON T1CINO. 169 



followed for a few yards past the Hotel to a point where the path 

 to Pian' Alto turns up to the left. Here the gneiss dips under 

 the end of the lake, and is not again seen; north of this no outcrop 

 of rock in place is met with, until we reach a conspicuous gully 

 which ends in a well-marked delta, the broad depression that 

 intervenes being obscured by fallen talus. If, however, we turn 

 up the path to Pian' Alto, we find that the gneiss is immediately 

 overlain by a bed of pale pink rauchwacke, closely resembling 

 in texture the rock which crops out at the south-eastern end of the 

 lake. 



The junction between the gneiss and the rauchwacke runs 

 almost along the track leading to the little knoll behind the Hotel, 

 marked 1930 m. in the Swiss Government map (see PI. VII) ; in 

 fact, the track has taken this line up the dip-slope of the gneiss, 

 on account of the depression formed by the unequal weathering at 

 the junction of these two rocks. Prom here the outcrop of the 

 calcareous rock can be traced northwards, across the scree-covered 

 depression, to. a conspicuous bluff occupied by a small plantation of 

 nine fir-trees. Here the rock is rather lighter in colour and much 

 honeycombed, and closely resembles the rock surrounding the exit- 

 end of Lake Tom. Masses of this rock are found in the gully- 

 section farther north; and it crops out again farther on, by the 

 side of the path running round the north-western edge of the 

 lake (see PI. XIII, fig. 1). 



If we now follow the upper portion of the outcrop, south from the 

 tree-covered bluff, we find a conspicuous band of the same rock, 40 to 

 50 feet thick, running for some distance round the eastern flank of 

 Pongio, behind the 1930-metre hill, and dipping at some 6° to 10° 

 northwards in the neighbourhood of the 1980-metre contour-line. 

 After continuing for some distance in this direction, it ceases 

 abruptly. It is found, however, a few yards below, on the path 

 running round the hill into the Yal Canaria. An analysis of this 

 band is given below (p. 177). 



The rock is everywhere micaceous, and in places contains large 

 tufts of actinolite, which appear to be arranged roughly along 

 certain planes. A good section, showing nests of actinolite- crystals, 

 is exposed in the little quarry at the back of the Hotel. 



It will be seen, then, that from the edge of the gneiss the western 

 end of Lake Ritom is composed of calcareous rocks, up to a height of 

 1980 metres. The abrupt disappearance of this mass at either end 

 of the outcrop is very suggestive ; and we may, I think, regard the 

 area occupied by Lake Eitom as a large lenticle of calcareous rock, 

 possibly including a bed of gypsum, enclosed between the gneiss on 

 the south and the crystalline schists on the north. 



An interesting feature is brought out by a comparison of the 

 northern and southern shore-lines of the lake. The former, dis- 

 regarding the scree and delta-material, is seen to run in a straight 

 line along the length of the lake ; the latter, on the other hand, is 

 broken by numerous bays and rocky headlands (see PI. YIII). 



The surface of the southern mass of gneiss, as shown above, can 



