252 Dr. Du Riche Preller—Crystalline Rocks of Prémont. 
crystalline sequence of mica-schist with gneiss intercalations at the 
lower, and cale-schists with pietre verdi at the higher levels. The 
Bussoleno gneisses, quarried on the right of the Dora down to Villar 
Foechiardo, differ greatly from the primitive, large-grained, glandular, 
eye-gneiss of the Dora—Maira massif which crops out on both sides 
of the valley from Villar Focchiardo to 8. Michele. Those younger, 
intercalated gneisses are of the minute and tabular type, in part with 
porphyroid and eye-structure, more or less tourmaliniferous, often 
rich in albite and poor in mica, or vice versa, while the mica-schist 
is frequently garnetiferous.! Such gneiss intercalations also occur 
higher up the ridge, where they are associated with omphacitic 
eclogite and prasinite. 
The pietre verdi area of the ridge as a whole may be described as 
composed of peridotitic, serpentinous, and euphoditic masses extending 
in the shape of a triangular trough from the Orsiera peak at the 
western to Colle della Roussa at the southern, and to Rocciavré, Pian 
Real, and Rocca Rossa at the eastern end, while euphodites and 
prasinites predominate more especially in the Rocca Nera and 
Mezzodi peaks in the centre of the northern part. The serpentinous 
masses reach their maximum thickness of about 300 metres in the 
Orsiera and of 500 metres in the Pian Real peak. The contact 
between serpentine, euphodite, and prasinite is generally marked 
by serpentinous, chloritic, and actinolitic schist, and in places 
by eclogite with large uralitized omphacite crystals, while the 
chloritic schist, e.g. between Pian Real and Rocca Rossa, contains 
diallagic, viz. smaragdite, crystals of unusually large size.” The 
euphodites present a great many varieties and are, as usual, largely 
altered to their pees and zoisitic derivatives. 
In the total rise of 2,300 metres from the valley floor near Bussoleno 
to the crest-line of the ridge, the garnetiferous mica-schists, with 
tourmaliniferous gneiss intercalations and quartzite, occupy about 
1,200 metres, followed by about 600 metres of cale-schist with 
intercalated prasinitic and serpentinous schist, and lastly by about 
500 metres of pietre verdi forming the cupola of the ridge. 
The ravine of the Sangone east of the ridge to Giaveno, where the 
torrent enters the Po Valley, is deeply and entirely eroded in the 
gneiss and mica-schist of the Dora—Maira massif without pietre verdi, 
but in other directions the Rocciavré group is linked with large pietre 
verdi areas both past and present. Thus, on the north the pietre verdi 
extend across the Dora Valley to Rocciamelone and the Lanzo valleys, 
while west, south-west, and south a large number of small intermittent 
outcrops afford evidence of a former extensive area which lay in the 
great syncline of the calc-schist formation 20 to 25 kilometres in 
1 The gneisses of Bussoleno are among those regarded by Professor Gregory 
as intrusive and Pliocene (‘‘ Waldensian Gneisses ” : Q.J.G.S., 1894, p. 232 
et seq.). His views were traversed in detail by Franchi, si Appunti Monti di 
Bussoleno’’: Boll. R. Com. geol., 1895, p. 177 et seq., and by Novarese, 
‘* Rilevamento Valle Germanasca ’’ : ibid., 1895, p. 277 et seq.; also by 
Stella, ‘‘ Valli Oreo e Soana’’: ibid., 1894, p. 349, footnote. 
2 Franchi mentions such crystals up to 50 ‘centimetres in length. Op. cit., 
1895, p. 3 et seq. 
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