10 DK. J. W. GREGORY ON THE * SCHISTES [Feb. 1 896, 



' scbistes lustres.' A specimen collected on the western arete of 

 Mont Jovet, just below the summit, agrees in its microscopic struc- 

 ture with the schists of the Cesana district. The microscope shows 

 that it consists of alternate layers of a black, indeterminate, and a 

 clear crystalline material. The rock is intensely crumpled, and the 

 black bands often thin out along lines of shear. A quarter-inch 

 objective resolves the black material into a very fine calcareous 

 and argillaceous dust, and the white clear layers into a mosaic of 

 quartz in which occur tiny crystals of white mica. There is no 

 sign of calcite in the rock, nor does it effervesce with acid. The 

 rock, therefore, agrees exactly with one of the finer and non- 

 calcareous varieties of the ' schistes lustres ' of the Cottians. 1 It 

 appears to have been originally a fine-grained mudstone. 



It may be suggested that possibly the ' schistes lustres ' and the 

 limestones are not part of the same series. But that they are so is 

 very clearly shown just below the summit of Mont Jovet. A few 

 yards along the ridge to the east a layer of limestone strikes right 

 across the ridge. Some rocky ribs run down the northern face at 

 this point, and in one of these, about 30 yards east of the summit, 

 the junction can be clearly seen. It could not be traced far, as 

 there was too much snow on this face of the mountain at the time 

 of my visit ; but the exposures were sufficient to demonstrate the 

 interstratification of the limestones and the schists. 



In spite, therefore, of the unusually extensive development in the 

 schist series of this massive non-foliated limestone, I feel bound to 

 admit that Lory and Zaccagna were right in identifying the central 

 rocks of Mont Jovet as ' schistes lustres.' There is nothing at all 

 improbable in the outcrop of these schists in Mont Jovet, if they be 

 pre-Carbom'ferous in age. This series is extensively developed 15 

 miles to the ea.tand 20 miles to the south, and its extension thence 

 westward Ixlow the Carboniferous and Trias rocks, as suggested in 

 Zaccagna's section, 2 is not in any way improbable. 



Another feature which allies the rocks of Mont Jovet to the 

 crystalline schist series, is the occurrence in them of basic igneous 

 rocks of the ' pietre verdi ' type. These have been recorded 

 on the mountain by Pavre, as well as by Zaccagna and Bertrand. 

 A mass of serpentine occurs in the schists in the upper part of 

 the Vallon des Trasses : it appears almost identical with that 

 described by Bonney from the pass of Mont Genevre, which has been 

 proved to be pre-Triassic. 3 I should be loth to use the characters 

 of intrusive igneous rocks as a proof of age, but the coincidence in 

 this case is significant. 



1 It may be compared with Prof. Bonney 's description of the microscopic 

 structure of the ' schistes lustres ' near Cesana, Quart. Journ. Geol. Soc. vol. xlv. 

 (1889) p. 103. 



2 Zaccagna, ' Riassunto Osserv. Geol. Alpi Graie,' Boll. B. Com. geol. Ital. 

 vol. xxiii. (1892) pi. v. sect. 2. 



3 T. G. Bonney, 'Two Traverses, etc.,' Quart. Journ. Geol. Soc. vol. xlv. 

 (1889) p. 80 ; Cole and Gregory, ' Variolitic Bocks of Mont Genevre,' ibid. 

 vol. xlvi. (1890) p. 305 ; Davies and Gregory, ' Geol. Monte Chaberton,' ibid. 

 vol. 1. (1894) p. 307. 



