250 Dr. Du Riche Preller—Crystalline Rocks of Piémont. 
I1I.—Tae Crrysrattine Rock AR®EAS OF THE Pr&MontESE ALPS. 
By C. 8S. Du RicHE PRELLER, M.A., Ph.D., M.I.H.E., F.G.S., F.R.S.H. 
(Concluded from May Number, p. 205.) : 
Ill. Tax Dora Riranra Grove. (Figs. 3 and 4.) 
|e this group, shown in the sketch-plan Fig. 4, may be included 
the interesting pietre verdi areas (1) of the Rocciavré ridge on the 
right, (2) of Monte Rocciamelone on the left side of the Dora Riparia 
Valley, and (3) of the Rocciacorba ridge and the Avigliana belt of 
spurs where the Dora and the Sangone Rivers emerge from the Alps 
and enter the Po Valley about 20 kilometres west of Turin. — 
1. The Rocciavré Area.—This ridge, about 12 by 4 kilometres in 
length and width, forms the divide between the Dora Riparia and 
Chisone Valleys, north and south respectively, while its eastern end 
lies at the head of the short valley of the Sangone torrent which 
discharges into the Po at Moncalieri south of Turin. Although 
the ridge derives its name from Monte Rocciavré, the latter 
(2,778 m.) is only one and not the highest of a remarkable cluster of 
pietre verdi peaks ranging from 2,600 to 2,900 metres in altitude. 
Of these the most notable are Rocciavré, Cristalliera (2,801 m.), 
Pian Real (2,617 m.), and Rocca Rossa (2,891 m.) at the eastern, 
Gavia (2,841 m.), Rocca Nera (2,852 m.), and Mezzodi (2,777 m.) at 
the northern, and the highest peak Orsiera (2,878 m.) at the western 
end, the elevation thus decreasing from west to east. The whole ridge 
obviously represents a former extensive pietre verdi sheet or cupola 
cut up by erosion and atmospheric denudation into resistant peaks 
which are separated by high co/di or saddles of about 2,500 metres 
altitude. 
The high-level pietre verdi area is accessible either from Perosa in 
the Chisone Valley (700 m.) or from Bussoleno (500 m.) in the Dora 
Valley, on which latter side the flanks of the ridge are deeply cut by 
several torrents in cascade gullies or orrzdi.! On the Chisone or 
southern flanks one of the most remarkable exposures, pointed out by 
Gastaldi as early as 1876,” is that near Colle della Roussa, about 
2,400 metres altitude, where the substratum of minute and tabular 
gneiss with intercalated crystalline limestone, steatite, and graphitic 
rock is overlain horizontally and conformably by a great bank of 
lherzolite more or less altered to serpentine, both compact and 
schistose, upon which rests an equally high bank of euphodite largely 
metamorphosed to amphibolic and epidotic rock with smaragdite, and 
to glaucophanic prasinites. The total thickness of this pietre verdi 
exposure is at least 200 metres. 
Unlike the southern or Chisone flanks, which form part of the 
gneissic-graphitic zone, the northern or Dora Riparia flanks of 
the ridge exhibit from Bussoleno (500 m.) upwards the normal 
1 These cascade gullies, varying from 100 to 300 metres in height, are 
characteristic of the mountain-sides of the Dora Riparia Valley, and are locally 
called orridi both from their weird, forbidding appearance and the enormous 
quantities of detritus and débris brought down through them by the torrents 
when in flood. 
2 Boll. R. Com. geol., 1876, p. 108. 
