2~)6 DR. J. W. GREGORY ON THE WALTJEXSIAN [May 1 894, 



•crushing of the schists indicate a region of greater disturbance than 

 in the Cima del Liste area. 



Boulders of the central augen-gneiss occur on the northern face 

 •of the Punta Cornour, and this, combined with other facts subse- 

 quently noted around Bobbio Pellice (5 kilometres south), led me to 

 think that this is probably an outcrop of the gneiss to the south. 

 Ill-health, however, prevented an examination of the ground in this 

 direction. 



4. Angrogna Valley. — We had thus crossed from Perrero to the 

 highest point between the German asca and the Pellice valleys, 

 keeping along the line coloured by Zaccagna and Mattirolo as 

 ' central gneiss,' without seeing the slightest tracts of this rock, 

 except for the single boulder on the Punta Cornour. Limestones, 

 dolomites, grits, andsome quartz-porphyry rendered slightly schistose, 

 were the main rocks met with. The north-and-south traverse was 

 therefore discontinued, and we then descended the Angrogna Yalley 

 towards the east in the hope of finding on its flanks some further 

 exposures of gneiss, which it was possible that Zaccagna might have 

 accidentally represented too far west. 



Nevertheless, from Passo Roussa as far east as Gaisset, the only 

 rocks seen were schists with interstratified beds, of which some 

 are apparently intrusive amphibolites and others clastic green rocks. 

 At Gaisset, however, a mass of fine-grained and locally foliated 

 gabbro crosses the valley and forms a massive hill on the north side 

 of that chalet ; before crossing the stream near Pra del Torno, a 

 dyke of compact dark-green porphyrite crops out across the path. 

 Below Pra del Torno the normal mica-schists recur, and they are at 

 first garnetiferous. Farther from the gabbro massif the schists are 

 finer and less crystalline, and opposite the bridge at the angle of a 

 sharp bend of the river, nearly 1 kilometre S.8.W. of Pra del Torno, 

 the schists are comparatively unaltered and anthracitic. 



A little farther round the curve a sharp crag, liocca Roccaglie, 

 overhangs the path to the north, and this is cut through by dykes 

 of white aplite ; a much finer exposure occurs in some rocks which 

 here interrupt the course of the stream. 



The main rock is a member of the 'pietre-verdi' series; the 

 commonest variety is a rock which is probably an altered andesite, 

 cut through by a fine-grained gabbro dyke. Both rocks have 

 been subsequently invaded by a complex of intrusive aplite- veins (see 

 fig. 9, p. 257). The aplite or granulite-veins vary in width from 

 a few inches to 4 feet ; and the coarseness of the rock varies with 

 the width of the dykes. Microscopic examination shows that the 

 rock consists in the main of quartz-felspar mosaic, with some larger 

 eroded crystals of orthoclase and a foliation determined by bands of 

 white mica. Its formula, excluding accessory minerals and some 

 foreign material absorbed from the surrounding rock, is : 



l'tf>ay ma aq 



which is that of the typical gneiss of the district. 



