relation- to mesozoic rocks in the lepontine alps. 199 



2. The Schists of the Yal Piora. 



The Yal Piora is a well-marked and fairly wide upland gien run- 

 ning roughly from east to west ; at its lower end is the Lago di 

 Ritom, a sheet of water about 2000 metres long by 500 wide. 

 Formerly it must have extended about 880 metres further east, the 

 upper end of the rock-basin in which it lies being now a level and 

 rather marshy meadow. The water from the western end escapes 

 through a narrow gap in the mountains and leaps down (at first in 

 fine cascades) to the Yal Bedretto. The lake is 1829 metres above 

 the sea, and the range rises rapidly on either side of the gap, on the 

 west to about 2200 metres, on the east to a long ridge, which varies 

 from about 2000 to 2800 metres. On the northern side of the Yal 

 Piora the mountains rise steeply to the watershed parting the drain- 

 age systems of the lihine and Hhone from that of the Ticino. As is 

 shown on Dr. von Eritsch's map, the southern range consists of 

 gneiss, the northern of a group of gneisses or schists. Of these the 

 members which crop out nearest to the Yal Piora are commonly 

 characterized by the conspicuous presence of red garnets and green 

 actinolite ; they are lithologically identical and practically continu- 

 ous with the well-known rocks displayed on the lower part of the 

 southern slope of the St. Gothard Pass, for which (merely to avoid 

 circumlocution) I have proposed the name of the Tremola schists. 

 The Yal Piora itself is occupied by the ranch wacko and its associated 

 rocks *, and by the group of schists with which we have now to 

 deal ; this apparent trough, sometimes narrowing, sometimes widen- 

 ing, extends eastwards along the upper part of the southern slope of 

 the Lukmanier Pass, and westward across the Yal Canaria to 

 Airolo in the Yal Bedretto, and in botli directions far beyond these 

 limits. 



At the first glance the map suggests that the last-named schists 

 overlie the rauchwackc and occu]3y a trough between the two 

 masses of gneiss. But a further study of it, and still more an exa- 

 mination of the terrain, indicates the existence of the gravest diffi- 

 culties in this hypothesis. For the moment, I will place the rauch- 

 wack() on one side and confine myself to the mineral characters and 

 relations of the schists : that these form one group of rocks of con- 

 siderable thickness, but very closely related, cannot be doubted. 

 They vary much in mineral character, but for the present minor 

 details may be passed over in order to concentrate our attention on the 

 more salient features. AYe will refer to them again, for the sake of 

 brevity, as the Piora schists. They exhibit two fairly well-defined 

 subgroups or types — one, the thickest and most persistent, being 

 strong schists consisting of alternating bands (of variable thiclvocss) 

 of a brownish or yellowish (juartzosc rock and a dark, sometimes 

 almost black, micaceous schist. Some members of the latter are 

 characterized by the presence of numerous garnets, about the size 

 of a pea, but sometimes rather larger, generally black. So far as 



^ For brevity and distinctness I will refer to this group, -whicb consists of a 

 peculiar limestone with dolomite, anhydrite, or gypsum, as the " Rauehwacke.'* 



Q.J. G.S. No. 182. i> 



