254 Dr. Du Riche Preller—Crystalline Rocks of Piémont. 
8. The Rocciacorba and Avigliana Area (Fig. 4).—This area is of 
special interest, owing alike to its complex nature and its singular 
configuration, and also because of its vicinity to Turin, whence it is 
easily reached by Rivoli, Giaveno, or Avigliana. As already mentioned, 
the eastern spurs of the Dora Riparia and Sangone Valleys converge 
to a horseshoe or amphitheatre—about 25 kilometres in circumference 
and 10 kilometres in width—in the centre of which lies Avigliana. 
The southern end of the horseshoe, on the right of the Sangone, is 
formed by the Pietraborga spur (926m.) of the Rocciacorba ridge, 
the middle or western part by the Ciabergia (1,178 m.) and Sacra 
S. Michele (962 m.) spurs between the Sangone and Dora Riparia 
Valleys, and the northern bend, on the left of the Dora, by the 
promontory of Torre del Colle (600 m.), and the mountain-side 
comprising, among others, Rocca della Sella (1,508 m.), Monte Curto 
(1,825 m.), and, at the eastern extremity, Monte Musiné (1,150 m.). 
Across the centre of the horseshoe, as a connecting link between the 
Pietraborga and Torre del Colle spurs, stretches the low Moncumi 
(642 m.) and Avigliana ridge, and between this and the Ciabergia or 
western spur lie, in an old morainic depression, the two small lakes 
of Avigliana (452m.). From the Moncumi ridge eastward spread the 
enormous glacial and alluvial deposits of the Dora Riparia, sloping 
down towards Rivoli and Turin.’ On the rugged and precipitous 
flanks of the horseshoe, patches of glacial deposits reach up to 
900 metres altitude or 500 metres above the valley floor, but the 
continuity of the rock formations can be traced all round the crags 
and in the numerous gullies or orrzdz of the different spurs. 
The pietre verdi of this area lie in the horizon of minute gneiss and 
mica-schists, which are, however, in evidence only in outcrops on the 
western margin, at the base of the primitive gneiss, and therefore in 
reversed sequence, as the continuation of the corresponding reversal 
further south, already referred to. The minute gneiss and mica- 
schists with crystalline limestone form banks of considerable thickness 
on the Ciabergia and 8. Michele ridge where they are quarried. 
Together with the pietre verdi on their eastern margin, they are the 
continuation of the gneissic, dioritic, and peridotitic belt of the Roccia- 
corba ridge, which, about 8 by 3 kilometres in length and width and 
about 900 metres in altitude, extends from Monte S. Giorgio, near 
Piossasco, on the south, to the Pietraborga spur above Trana at its 
northern end. The crest-line of this ridge exhibits in succession 
the masses of garnetiferous and graphitic mica-schist and minute 
gneiss, lherzolite, serpentine, and dioritie rocks with associated 
eclogite, and amphibolic and prasinitic schists which constitute the 
series and reach their greatest thickness in Monte Montagnazza 
(892 m.). 
From the Pietraborga spur the pietre verdi radiate N.W., N., and 
N.E. in several more or less defined zones, of which the western 
follows the Ciabergia ridge, crosses the Dora Riparia at 8. Michele, 
and reappears on the left bank at Chiayrié, while the middle zone, 
starting from the same point, forms the Moncumiand Avigliana ridge 
1 Of this ‘‘ Morainic Amphitheatre of Rivoli’”’, Professor F. Sacco has given 
an interesting description in Boll. R. Com. geol., 1887, p. 141 et seq. 
