Vol. 50.] GXEISSES IN THE COTXIAN SEQUENCE. 24-7 



The gneiss a t the quarries below Meitre, when examined micro- 

 scopically, is seen to consist in the main of a water-clear quartz- 

 felspar mosaic, which forms large white ' eyes,' around which curve 

 lines of long blade-like crystals of white mica. The quartz shows 

 well-marked undulatorv extinction : it, is often pegmatitically asso- 

 ciated with the felspar. Apatite and zircons both occur in the 

 mosaic. The felspar is sometimes idioinorphic, and crystals of it 

 are seen completely enclosed in quartz. 



The formula for the rock is therefore : 



T<$>ay F_ a 1 m a,q, 



and the rock is a muscovite-gneiss. 



The dyke in the Gerrardo Valley agrees with this in the main. 

 The principal differences are the small proportion of orthoclase, 

 many of the crystals of which show a zonal regrowth. Moreover 

 the micas are in thicker crystals, and there are a good many eroded 

 crystals of kyanite ; a few grains of the last-named mineral occur in 

 the Bussoleno gneiss. 



The formula is therefore : 



and the rock may be regarded as an aplite. 



We were unfortunate in not finding in this district any very clear 

 sections showing the junction of the main mass of the gneiss and 

 the schists ; but around Fornielli there appears to be a gradual 

 passage from the normal gneiss to one rendered coarsely ' augen ' by 

 numerous inclusions of quartz-nodules. At the upper end of Fornielli, 

 in a chestnut grove on the south side of the main road, there is 

 an exposure of white gneiss with similar quartz-nodules and impreg- 

 nated with calcareous matter. Close to this the mica-schists are 

 seen in situ ; they are crowded with garnets near the junction, 

 while the adjoining schists are strongly contorted. 



Though it is not possible to prove the actual connexion of the 

 three gneisses, it seems highly probable that they are part of the 

 same massif; the section on the following page (fig. 5) shows their 

 field-relations upon this view. The three rocks agree very closely 

 in composition, as is shown by their formulae, and in the freshness of 

 their minerals in contrast to those of the rotten, decomposed rocks 

 in which they ar-^ intercalated ; the only differences between them 

 are just those which might be expected between an intruded mass 

 and its apophyses. In this case the Susa and Fornielli exposures 

 represent the massif exposed by the erosion of the Dora Valley. 



2. Pale. — Between the gneiss-dyke in the Gerrardo Valley and the 

 next exposure seen at the Col de Vento, the rocks along the line 

 traversed were wholly of the gneissoid mica-schist ; these occur 

 along a line from Pinetti, Ccrvclli, Conde, Travers a Mont, Pois, 

 round the flanks of Punta Ros.sa and south along the Rio di Gravis 

 to Mustione and the Col de Vento. South and west of this line 



