Vol. 50.] GNEISSES IN THE COTTIAN SEQUENCE. 243 



Adding the letter <t» to indicate foliation, and the bracket "■" to 

 show simultaneous crystallization, the formula for this rock, ac- 

 cording to M. Michel-Levy's system, 1 would be : 



The rock is therefore a muscovite-granitoid gneiss. 



Near the actual junction of the two rocks occur many flakes of 

 chlorite which have doubtless been formed by the alteration of 

 fragments of the ' pietre verdi ' ; a few corroded and broken 

 garnets, showing optical anomalies due to strain, also occur. The 

 contact-line is irregular, the gneiss having cut into the included 

 block ; this is now so altered that it is not easy to say what was 

 its original constitution. The principal constituents are rounded 

 grains of plagioclase, small, strongly dichroic crystals of green 

 hornblende, and some irregular grains of glaucophane; these are 

 scattered through a mass composed of rounded grains of epidote. 

 A little zoisite and some patches of titauoferrite are also present. 



The green included fragment is, therefore, an altered basic 

 igneous rock and may be called a glaucophane-epidiorite. 



Farther up the valley of the Vonzo the gneiss leaves the 

 river and runs due north, below the chalets of Culet and west of 

 the crag below the Capella della Madonna di Ciavinis. No section 

 showing the junction could be found, but close to it the ' pietre- 

 verdi ' series is very contorted and along it loose blocks of garneti- 

 ferous amphibolite and glaucophane-schist are very abundant ; these 

 both probably indicate contact-alteration. 



The junction of the two series is, however, much better shown 

 on the south side of the Stura Valley, along a line to which we 

 were guided by Dr. Gianotti ; the sections occur on both sides 

 of the steep Vallone Verso, north of the hamlet of Ortiero. In 

 descending the valley for some distance below the main bridle-path 

 one crosses the ordinary ' pietre verdi ' series ; the rocks of this series 

 then become much contorted and garnetiferous, and 40 feet lower 

 down pass into a crushed decomposed rock crowded with large 

 garnets : immediately below this is a fine-grained gneiss-rock which 

 Dr. Gianotti called a ' talcose gneiss ' and accepts as the transition- 

 rock between the gneiss and the schists. The talcose gneiss passes 

 gradually below into the normal augen-gneiss. The actual junction 

 here is clear, but it is not easy to be sure of its exact nature, as 

 there has been a certain amount of slipping and squeezing out of 

 the soft decomposed garnet-rock. 



1 ' Structures et Classification des Koches eruptives,' Paris, 1889, pp. 29-30, 

 37-38. 



r = granitoid. F 5 = apatite. a l = orthoclase. 



a — granitic. F 6 = zircon. q = quartz. 



7 = pegmatoid. m = white mica. k = kyanite. 



e = epidote. 



The brackets are used for extraneous materials caught up by a rock during 

 its intrusion. 



