Vol. 50.] GNEISSES IN THE COTT1AN SEQUENCE. 2(31 



the derivation of the schists from the gneiss, or of any erosion of 

 the surface of the latter. 



In regard to the possible alternative theory of the gneiss being 

 faulted up through the schist, the evidence is also strong, though it 

 is again all negative. The absence of any slickensides or fault-rock 

 along the junctions, the sharpness of the junction where the gneiss 

 adjoins rocks of dissimilar composition and its indefinite nature at 

 places where the gneiss and the schists are of similar chemical 

 composition, and, finally, the great irregularity and complexity of 

 the faults and thrust-planes that would be necessary to account for the 

 field relations of the two, all combine to dismiss this hypothesis. 



The positive evidence, however, as to the relations of the two 

 series is fairly complete. It may be divided into four groups. 



1. Contact-Phenomena, (a) In the Gneiss. Zaccagna has re- 

 marked on the fact that the ' central gneiss ' passes into a true 

 granite when at some distance from the junction, 1 a fact which he 

 noted in the valleys of the Peliice and the Chisone ; Baretti has 

 described the same in the Paradiso massif.' 2 We may, therefore, to 

 some extent regard the foliation as a marginal structure, such as that 

 which is not uncommon around unquestionably intrusive granites." 



When the rock is examined close to the junction, it can be 

 gradually traced from a coarse to a fine gneiss. This may be seen 

 around the Paradiso massif, as below Casa Ortiera, at Fornielli, near 

 Bussoleno, on the banks of the Chisone, 1^ kilometre above Perosa, 

 and is especially well shown at the junction in the Po Valley below 

 Crissolo ; at this locality the fine gneiss passes into a compact aplite. 

 In other places the margin of the gneiss has incorporated sufficient of 

 the neighbouring schists to considerably affect its general character ; 

 in extreme cases the gneiss has been altered into a greenish talcose 

 gneiss, as near Chialamberto and Perosa. 



(b) In the Schists. The alterations here vary greatly, according to 

 the nature of the rocks with which the gneiss is in contact. When the 

 rock is a quartzite it is converted into a quartz, as at Casa Ortiera ; 

 amphibolites and epidiorites are converted into glaucophane and 

 hornblende-schists, while the mica-schists are rendered gneissoid at 

 the contact and richly garnetiferous farther from the actual junction. 

 Sillimanite, kyanite, cordierite, and other minerals are also developed, 

 while a band of an intensely altered, decomposed rock often occurs 

 along the actual contact -line ; this is especially well shown south 

 of Chialamberto and in the Po Valley opposite Ostana. 



2. Included Fragments. Around the margin of the Paradiso 

 massif, as along the Vonzo Valley, in the gneiss of the Koc del 



1 Zaccagna, 'Sulla Geologia delle Alpi occidental^ ' Boll B>. Com. geol. Ital. 

 toI. xviii. (1887) p. 379, and elsewhere. 



2 Baretti, ' Gran Paradiso,' Atti E,. Accad. Lincei, ser. 3, Mem. vol. i. pt. i. 

 (1877) pp. 208. 210, 214. 



3 See Rosenbusch, ' Mikroskop. Fhysiogr. d. Massigen Gesteine,' 2nd ed. 

 (1887) vol. ii. p. 41. 



