Vol. 50.] ON THE GEOLOGY OF MONTE CHABERTON. 307 



naturally class them as of the age of the rocks among which they 

 occur ; they therefore regard most of them as pre-Palaeozoic, while 

 the older Italian geologists, such as Gastaldi, and Gervais and Sterry 

 Hunt, claimed them all as of this age. A later and smaller part of 

 the series has been generally considered Tertiary, but Prof. Sacco's 

 detailed work gives reason for showing that this later series may be 

 Cretaceous. Those who regard these rocks as intrusive are less 

 pronounced in their judgment as to the age, and though generally 

 accepting the division into an ancient and recent series, they have 

 even sometimes called this into question (Cole and Gregory, op. cit.). 

 The first point to be settled was whether the mass of serpentine 

 cut through by the road at Clavieres occurred in the calc-schists, 

 in the dolomite, or between the two (as in Prof. Bonney's section 1 ). 

 The slopes at Clavieres are too much covered by talus, and too 

 near the Italian forts, to tempt one in that direction, so we 

 struck round the Bois de Chaberton, hoping to find the same bed 

 exposed in the Grand Vallon. (See Map, p. 309.) We soon came 

 upon the serpentine, and determined two points about it : — 



(1) That some tufas in the base of the Triassic limestones 

 contain many fragments of the serpentine. Both the field evidence 

 and examination of thin sections showed that the rock containing 

 these is a true tufa and not a fault-breccia. This settles the pre- 

 Triassic age of the serpentine ; 



(2) That the serpentine is intrusive into the calc-schists, as it 

 here occurs in them, as it cuts across the strike of the schists, and 

 as there is fairly well-marked contact-alteration on each side of the 

 serpentine. 



Elsewhere, however, on the mountain the Triassic dolomites are 

 cut through by some sheets of schistose ' pietre verdi ' ; these may be 

 seen in two places in the valley leading northward from the pass of 

 Mont Genevre to the Col des Trois Freres-Mineurs. The first is by 

 a crag above and to the west of Gr. Baisses ; the relations of the 

 ' greenstone ' to the basal quartzite of the Triassic series here is 

 unquestionably that of an intrusive igneous rock. The second is 

 beside the path leading to the Col de Chaberton, a little to the 

 north of the 2145-metre point ; here the evidence of intrusion is not 

 so plain. Two other similar masses of the 'pietre verdi' occur on 

 the northern arete of Chaberton between the summit and the Col de 

 Chaberton. This is indisputable evidence that some of the igneous 

 intrusions are post-Triassic. 



It is advisable, therefore, to examine these rocks somewhat 

 closely, in order to see whether it is possible to gain from them 

 any guidance in determining the age of the intrusive rocks of the 

 ' pietre verdi ' series elsewhere in the Cottians. At Chaberton 

 there are two main types : the serpentine which we now know to 

 be pre-Triassic, but later than the ' schistes lustres,' has been 

 previously described 2 ; but the later igneous rocks are so crushed 

 as to be at present unrecognizable. 



• x ' Two Traverses, etc.,' Quart. Journ. Geol. Soc. vol, xlv. (1889) p. 80. 

 2 Cole & Gregory, ibid. vol. xlvi. (1890) p. 306. 



