Dr. Du Riche Preller—Orystalline Rocks of Prémont. 258 
width,! and connected the Rocciavré group with the similar groups 
of Oulx, Chaberton, Genévre, Maurin, Chabriére, and Monte Viso. 
2. The Rocciamelone Area.—This imposing mountain, the ancient 
Mons Romuleus, with its peaked summit rises straight from the Dora 
Riparia on the right of the valley at Susa (503 m.) to an altitude of 
3,537 metres in a horizontal distance of only 7 kilometres. The most 
accessible ascent is from Susa or, further down the valley, from 
Bussoleno. The massif lies in the calc-schist horizon, which also 
includes the series of similar high peaks immediately north of it, as far 
as the Levanna and Gran Paradiso gneiss massif. From the summit 
and the sanctuary of Madonna della Neve down the southern flank the 
cale-schist alternates at first with micaceous schist, lenticular masses 
of bluish and white crystalline limestone, serpentine with ophicalce or 
‘green marble, and amphibolites and prasinites, as far as the spur of 
Tre Cresti. From this point in an oblique direction towards Bussoleno 
and Chianoc, the alternations of calc- and micaceous schist with 
pietre verdi become more frequent, and nearer the valley floor are 
replaced by minute and tabular gneiss, intercalated masses of 
erystalline limestones, and the fossiliferous calcareous beds of Chianoc 
and Foresto. 
On this southern flank, shown in the section Fig. 3, the pietre verdi 
exhibit remarkable aggregations of amphibolic and prasinitic rocks 
with both massive and schistose serpentine, which latter becomes so 
predominant as to eclipse the calc-schist altogether. Especially is 
this the case in and above the Rio Muletta gorge, which descends 
to Bussoleno from the Croce di Ferro ridge, and exhibits an almost 
perpendicular cliff of serpentine no less than 500 metres in height, 
overlain by another 500 metres of amphibolites and prasinites. The 
serpentine cliff rests on crystalline limestone which then alternates 
with calc- and serpentine-schist down to the minute and tabular 
gneiss near Chianoc, the latter belonging to the same horizon as the 
Bussoleno gneisses on the opposite side of the valley. Lower down 
the valley, as already mentioned, the typical primitive, glandular 
gneiss with large elements and greyish-green mica appears on both 
sides, extending on the left to St. Didero, Borgone, and Condové, and 
on the right to St. Antonino, Vayes, and the chiusa or defile of 
S. Michele. In this section of about 12 kilometres the bed of the 
Dora Riparia is therefore eroded entirely in primitive gneiss, which 
here forms the northern extremity of the Dora—Maira gneiss massif. 
The crest of the great ridge of peaks and crags which runs on the 
left of the Dora from Rocciamelone eastward for about 36 kilometres 
to the spur of Monte Musiné, north-east of Avigliana, is almost 
entirely composed of pietre verdi with only a few narrow outcrops of 
eale-schist in the intervening eroded saddles or colli. As this ridge 
forms more properly part of the area of the Lanzo valleys, I shall 
refer to if again in connexion with that region. 
1 The cale-schist formation extends north-west, and entirely encircles the 
mica-schist and minute gneiss massif of Rocca d’Ambin (3,377 m.), about 
20 by 10 kilometres in length and width, which lies between the upper Dora 
Riparia Valley on the Italian and the Are Valley on the French side, and at 
its northern extremity is crossed by the Mont Cenis road from Lanslebourg 
fo Susa. ; 
