Dr. Du Riche Preller—Ophiolithic Rocks, W. Liguria. 447 
1V,—Tue Opniotiraic Groups oF THE LicguURIAN APENNINES. 
I. Western Liguria. 
By C.S. Du RicHE PRELLER, M.A., Ph.D., M.I.H.E., F.G.8., F.R.S.EH. 
‘W\HE groups of ophiolithic, that is, crystalline ferro-magnesian 
rocks in the comprehensive sense, to be dealt with in this and 
the subsequent paper are those west and east of Genoa.’ Of these 
the principal western or Voltri group was already referred to in the 
preceding paper” as being, on its western margin, contiguous to the 
erystalline massif of Savona, and as constituting with the latter 
the geological contact-zone of the Alps and the Apennines. The 
anomaly of its position and petrological character is enhanced by 
the abrupt, clean-cut division of its eastern margin from the adjacent, 
more recent, and smaller ophiolithic group of Sestri and Isoverde, 
which is coeval with the groups of Eastern Liguria. 
Tue Groups or Wrsrern Licuria. (Fig. 1.) 
The border of the large and important area known as the Voltri 
group runs on the south, from its contact with the Savona massif 
near Varazze, along the Riviera to Cogoleto, Arenzano, Voltri, Pra, 
Pegli, and Sestri Ponente; then north up the Chiaravagna Valley, to 
Isoverde in the Iso Valley, to the Bocchetta Pass on the crest of the 
Apennines, and thence to Voltaggio in the Lemmo Valley. From 
here it extends, as the northern margin, across the Corrente, Stura, 
Orba, and Erro Valleys to near Ponzone, south of Acqui, whence it 
turns south to the Giovo Pass and Corona, and along the contact line 
of the Savona massif down to the coast at Varazze. The divide of 
the Apennines runs more or less parallel to the coast from the Giovo 
Pass (522 m.), to Mte. Ermetta (1,262 m.), Mte. Reisa (1,184 m.), the 
Turchino Pass (552 m.), Mte. Penello (996 m.), and the Bocchetta 
Pass (777 m.), so that only the southern part or about one-third of 
the area hes in Liguria proper, while the northern and larger part 
belongs to the Po watershed; in its entirety the complex, covering 
no less than 36 by 22 km. or 800 square kilometres, is by far the 
largest in the Apennines. It exceeds even that of the Lanzo Valleys 
in the Piemontese Alps; like the latterit has, owing to its prevalently 
peridotitic and serpentinous rocks, resisted denudation and hence 
preserved its compactness, continuity, and also its often austere 
aspect in aremarkable degree. The three principal roads along which, 
besides the coast-road, the ophiolithic and sedimentary rocks of the 
area may be studied, are those crossing the Apennines south to north 
from Varazze by the Giovo Pass to Sassello and Acqui on the west, 
from Genoa by the Bocchetta Pass to Voltaggio on the east, and from 
Voltri by the Turchino Pass to Campoligure, Rossiglione, and Ovada 
in the centre, to which last-named runs in great part parallel the 
railway from Genoa to Ovada. 
! The term ophiolithic, though in its original and narrow sense it refers only 
to serpentines, applies to eruptive basic rocks or pietre verdi comprehensively, 
irrespective of age, like, e.g., the ‘ roches ofitiques’ of the Pyrenees. 
2 GEOL. MAG., September, 1916, p. 400 et seq. 
