262 DR. J. W. GREGORY ON THE WALDENSIAN [May 1 894, 



Pelvo at the end of the Bocciarda Yalley, at Mustione, etc., fragments 

 of the amphibolites of the mica-schist series are included in the 

 gneiss ; the contact-alteration on the margin of the inclusions, and 

 the flow of the gneiss around them, prove that they are fragments of 

 the schists caught up by the molten rock. 



3. Apophyses from the Gneiss. Opposite Ostana in the Po 

 Valley, near Bobbio in the Pellice Valley, and below Pra del Torno in 

 the Angrogna Valley, dykes of aplite, which present all the characters 

 of apophyses from the gneiss, break through the schists ; in the last- 

 case quoted a complete passage can be traced from the fine-grained 

 aplite to a well-foliated gneiss. 



4. Transgressive Junctions. Cases of these are the contact of 

 the gneiss at Bussoleno with the calc-schists and gneissoid mica- 

 schists ; of the gneiss on the south side of the Valle Grande with 

 the ' pietre verdi ' series, quartzites, and mica-schists ; at the 

 upper Col de Vento, where it works across from the serpentine to 

 the calc-schists and dolomite ; the discordance of the strike of the 

 second series around Crissolo, where, according to a manuscript 

 map prepared and kindly lent to us by Dr. Gianotti, the gneiss 

 transgresses from the mica- schists to the calc-schists. 



5. The up-tilting of the schists at their contact with the gneiss 

 at the Roc del Pelvo, and the irregularity of the line of junction of 

 the two rocks — when specimens showing it are examined under 

 the microscope — are further proofs of the intrusive nature of the 

 Waldensian gneisses. 



V. The Contact-Metamorphism. 



The normal phenomena of contact-metamorphism are well shown 

 around the Waldensian gneisses, and the development of new minerals 

 has sometimes taken place on an extensive scale. Among others the 

 following have thus been formed: quartz, white mica, biotite, ortho- 

 clase, microcline, oligoclase, garnets, kyanite, epidote, zoisite, and 

 sphene. The extent to which this has proceeded varies enormously; 

 thus the quartzites of the Valle Grande have been converted into 

 quartz-schists along only a very narrow band ; at the Roc del Pelvo, 

 in the Comba di Bocciarda, new minerals have been developed for 

 merely a short distance from the gneiss, while the alteration around 

 the dykes of the Angrogna Valley only affects the neighbouring rocks 

 over a breadth varying from | to about 2 inches. In other places, as 

 in the Po Valley, the rocks have been intensely altered for 50 feet 

 from the margin. There is, however, nothing in this to justify the 

 charge of capriciousness often made against contact-metamorphism : 

 it may be selective metamorphism, but good reasons can be found to 

 explain the selection. 



The variation in the extent to which this action has proceeded 

 around the Waldensian gneisses seems to depend on two main factors, 



