Vol. 50.] GNEISSES IN THE COTXIA.N SEQUENCE. 245 



clumps of tremolite. A few grains of olivine still remain. The rock 

 was therefore originally a saxonite, though the rhombic pyroxene was 

 probably hypersthene rather than enstatite. The mass occurs 

 intruded in some chlorite-schists belonging to the ' pietre-verdi ' 

 series ; its sudden western termination may be due either to its 

 having been cut off by the gneiss or to the original poridotic segre- 

 gation not having extended far in that direction. 



We were glad to find that Signor Ing. Sommariva, of the Vonzo 

 mine, had arrived at the same conclusions — as to the intrusive nature 

 of the gneiss — as we had ; the mine lies near the contact of the two 

 rocks in a synclinal, which he believed to have been formed at the 

 time of the gneissic intrusion. 



(b) The Waldensian Gneisses. 



1. Bussoleno — Susa.— The town of Bussoleno is situated on both 

 banks of the Dora Riparia in the midst of a wide tract of moraine 

 and alluvium, which here separates the Grand' Uja and Punta 

 Lunel continuation of the Northern Cottians from the northern 

 end of the eastern range. The mountains on both sides of the 

 valley are mainly formed of mica-schists : but an extension of the 

 calc-mica-schists of Cesana runs along the base of the south side 

 of the valley as far east as the gorge of the Giordani, south-west of 

 Bussoleno. Prom this point a low bank of gneiss forms the foot of 

 the hill-slopes for some distance towards the east, and is extensively 

 quarried below the villages of Combe, Fornielli, and Meitre. A 

 similar rock, no doubt an extension of the same mass, occupies a 

 corresponding position on the north side of the valley extending 

 from Foresto di Susa to Grange. These are the northern exposures 

 of the Waldensian gneiss, and Zaccagna has figured them as 

 spreading over a wide extent of ground south and south-east of 

 Bussoleno, 1 and thence extending in one unbroken line as far south 

 as Venasca. It was therefore natural to strike from Bussoleno 

 towards the south-west, to the line marked by Zaccagna as the 

 junction of the gneiss and the schists. 



One soon found, however, that it was not possible to apply that 

 geologist's map too literally, and that a large extent of his ' central 

 gneiss ' is really occupied by mica-schists, calc-schists, and even 

 ' pietre verdi.' Thus the foot of the hill-slopes at Capella Santa 

 Parnella, about 1 kilometre south-west of Bussoleno, is formed 

 of calc-mica-schists, in places garnetiferous ; the foliation of the 

 schists here dips 45° to the north-east, and strikes north-west 

 and south-east. Higher up the slopes the calc-schist is succeeded 



1 There is a slight discrepancy here between the lettering and colouring of 

 Zaccagna's map. According to the former, the gneiss extends as far west as 

 Chiomonte, and it forms the whole of the basins of the Giordani and of the 

 lower part of the Scaglione ; the colouring, however, marks the western ter- 

 mination of the gneiss about 1 kilometre to the east side of the Scaglione, the 

 course of which is thus wholly on the schists; the colouring is no doubt the 

 more correct. 



