RELATION TO MESOZOIC ROCKS IN THE LEPONTINE ALPS. 201 



hood of the Lago di Eitom. To a considerable extent it is diagram- 

 matic, as it is designed only to indicate the relation of the principal 

 tj^pes. The questions, then, for which we must seek an answer are 

 these : — (1) Are we dealing with a simple fold ? (2) What is the 

 order of succession in the series ? Omitting for the moment the 

 rauchwacke from our consideration (as we shall presently see we 

 are justified in doing) it is evident that these schists in the Val Piora 

 do not form a simple fold. The Dark-mica schists with the Black- 

 garnet schists are exposed on the Alpine pastures between the Lago 

 di Cadogra and the Lago di Eitom. They form the clifis overlook- 

 ing the Ritom Boden, whence they sweep upwards in a westerly 

 direction, and may be traced up the flanks of the Plan Alto and on 

 the summit of Pougio, two mountains on either side of a pass west 

 of the Lago di Eitom, leading into the Val Canaria. Beneath these 

 (apparently) we find above the S.E. end of the Lago di Eitom some 

 kyanite schists, and at the ]S".E. end the well-known schist with 

 fine staurolites *, which overlies a white quartzite. The latter rocks 

 I did not succeed in tracing very far, and could not find at the top of 

 the above-named pass. The Calc-mica schists appear to rise from 

 beneath this quartzite, and are found in great force on the hill-side 

 leading to the pass. Commonly they are concealed beneath turf 

 or debris, but a fair section is obtained in a gully near the jN^.W. 

 corner of the lake. The neighbourhood of the col affords important 

 evidence (fig. 4, p. 208). North of it rises the Pian Alto, a craggy 

 mass of the Dark-mica schist. Just west of it, and a few dozen 

 feet lower down, some rauchwacke crops out from the turf. The 

 former rock occurs on the rounded summit of Fongio, on the S. and 

 S.W. sides of which is the Black-garnet schist t. At the base of 

 the actual summit and near the edge of the crags descending to 

 the Val Piora we find schists of the Calc-mica group with some 

 white marble, much crushed, and a little further to the south 

 gneiss, also much crushed, crops out. It is therefore clear that 

 the greater part of the Calc-mica-schist group is cut out bv a 

 fault. 



If we follow the " Piora schists " eastwards to Olivone and 

 westwards to Binn (and over all this distance they can be traced 

 practically without a break), we find that the characteristic Black- 

 garnet schist is repeatedly picked up and that this rock generally 

 occurs very near to the gneiss or to the Tremola schists — that is, to 

 some member of what I have termed the Lepontine series, which is 

 unquestionably the more ancient. I think therefore that the appa- 

 rent order of succession in the Val Piora is the reverse of tlie true 

 one, and that it exhibits only a part of an overfold the remainder of 

 Avhich is cut out l)v thrust-faults (fig. 1). It is possible that the 

 Durk-mica schist also may be divided from the Tremola scliists by a 



* Dr. von Fritscb gives 20 metres as its maximum thickness. I liave never 

 seen so much as this exposed. 



t The whole of Fongio soutli of the raucliwaeke is coloured iu Von Fritsdi'a 

 map as gneiss. This is one of the few errors which I have noticed in this ex- 

 cellent map. 



p2 



