2i)2 DR, J. W. GREGORY ON THE WALDENSIAN [May 1894, 



The microscopic structure of the rock has been described by Prof. 

 Bonney (op. supra cit. p. 104) ; the only addition that I would desire 

 to make to this is in regard to the presence of the large number 

 of dark segregations. These are often at least 4 inches wide and 

 12 to lfi inches long; the longest diameter of the fragments is 

 always parallel to the foliation. 



On the opposite side of the river the gneiss is much better seen 

 in some larger quarries, while on the eastern margin of the mass a 

 certain amount of evidence as to the junction is obtainable. The 

 gneiss forms an irregular bank occupying the axis of an anticlinal ; 

 the schists on the east side dip 10° W.S.W., while in the cliff 

 below the ruined castle of Brandonegna they dip 10°N.W. At 

 the foot of the cliff the gneiss is fairly coarse, but when traced 

 towards the junction it becomes finer, and it is not easy in the field 

 to fix on the exact line of separation between the gneiss and the 

 gneissoid mica-schists which overlie it. The microscope, however, 

 leaves no doubt of the distinctions between the igneous and the 

 clastic rocks. 



The igneous gneiss near the junction is finer-grained than that 

 seen in the quarr}- on the opposite side of the river ; it differs from 

 the gneisses that have been previously described from Bussoleno 

 and the Gerrai do Valley by the presence of a good deal of biotite in 

 addition to the white mica. Epidote in small rounded grains is also 

 abundant. This and the micas are included in a groundmass of a 

 water-clear quartz-felspar mosaic, -which was the last constituent to 

 solidify. The formula may thus be written : 



T4>ay F a 6 («)M«icfj «,</ ; 



and so the rock is a typical Waldensian gneiss with some foreign 

 inclusions. 



An examination of one of the most gneissoid of the adjoining 

 mica-schists shows several marked differences. The materials in 

 this belong to two very different sets. There are some quartz and 

 zoisite aggregates which doubtless represent broken-down plagio- 

 clase ; they are included in an indeterminate granular material, 

 which contains many small authigenous flakes of white mica. These 

 represent the original constituents of the mica-schists, now com- 

 pletely metamorphosed by the intrusion of the second group of con- 

 stituents. There are thin bands of fresh granulite, composed of 

 quartz, orthoclase, and some white mica. 



The formula for this rock is : 



T&ay (zq ;r)F. ( .ma l a^q. 



It is probably due to the injection of the schists by gneissic 

 materials, similar to those cases around the protogine intrusion of 

 Mont Blanc which have been described by M. Miohel-Levy. 



The brown schists recur to the west of this narrow band of gneiss, 

 and before attaining Meano a cliff is passed in which they are 



