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



249 



there appears to be a great extension of the upper schists with 

 ' pietre verdi,' with gabbros, serpentine, ampliibolites, and vario- 

 lites. The true Col de Vento is on the mica-schists, but imme- 

 diately south of it is a dyke of gneiss intrusive into the 'pietre 

 verdi ' series ; a narrow and not very easy gully leads to a small 

 col which crosses the ridge at a height of 2290 metres. This 

 runs along the junction, the gneiss forming the north wall and the 

 serpentine the south. The serpentine 1 is greatly schistified and 

 contorted, while the gneiss has a somewhat greenish colour, doubt- 

 less due to the absorption of the neighbouring material. At the 

 upper end the gully forks, and in the northern branch the gneiss can 

 be seen in contact with a much altered limestone belonging to the 

 schist series. The gneiss between the two branches of the gully T is 

 much decomposed. Close by the summit of the right-hand branch 

 tbe gneiss crosses the gully and forms the south wall, and the rock 

 is there still fresh. 



Between the top of this gully and the Col de Vento is a band of 

 limestone which is cut into by the gneiss, and the junction is well 

 shown on the south wall of the northern fork of the gully. The 



Fig. 6. — Junction of gneiss and limestone in a gully north of 

 Col de Vento. 



accompanying figure shows the actual junction and the way in 

 which the limestone-bedding ends off against the intrusive gneiss. 

 The limestone is altered at the contact : the microscope shows it to 

 consist of a clear twinned calcite, some small rounded masses of 

 untwinned calcite, and numerous small crystals of dolomite. These 

 are all included in what appears to be a crushed calcareous ground- 

 mass ; in this some authigenous blade-like crystals of white mica 

 are further evidence of alteration. 



In connexion with the Col de Vento gneiss-dyke it is necessary to 

 consider a boulder which occurs beside the path a little below 

 Mustione, of which a sketch is reproduced in fig. 7, p. 250. This 

 shows two fragments of the ' pietre verdi ' series included in gneiss ; 

 both inclusions are distinctly altered on the margin, the larger to a 

 depth of 1 inch, and the smaller to -i inch. The gneiss belongs to the 



1 ' Serpentine ' is here used as a general term for any altered peridotite. 

 This rock was probably a lherzolite, as the microscope shows the presence 

 of both rhombic and monoclinic pyroxenes, and of a little oligoclase ; these are 

 included in a light-green serpentine groundmass containing many needles and 

 prisms of tremolite ; some of the latter are sufficiently large to 3how the horn- 

 blende cleavages. 



Q. J.G.S. No. 198. s 



