Dr. Du Riche Preller—Ophiolithic Rocks, W. Liguria. 449 
The Voltri group is composed almost entirely of Mesozoic, dark- 
grey, often talcose calc-schists, and infolded, intercalated, and over- 
lying pietre verdi. As such it belongs geologically to the Alpsrather 
than to the Apennines, and the anomaly of its position consists in its 
being wedged between the Savona crystalline massif at one end and 
- the Eocene formation at the other. Up to 1882 it was variously 
assigned to the Archean, Paleozoic, and, more often, to the Tertiary 
formations, and it was not: tiil Issel and Mazzuoli’s investigations of 
1883 and 1884! that a clean-cut line of division was shown to exist 
north of Sestri in the Chiaravagna Valley, and thence to Isoverde 
between the older Triassic Voltri group on the western and the 
more recent Eocene ophiolithic and sedimentary series on the eastern 
side of that line. 
The determining factor in this division was Issel and Mazzuoli’s 
discovery of several isolated deposits of Triassic dolomitic limestone 
immediately west of the line between Sestri and Isoverde, viz. on 
Mte. Gazo, Mte. Torbi, and Mte. 8. Carlo, and also north of the crest 
of the Apennines near Voltaggio, while more recently Rovereto ? 
elucidated those of Cogoleto and Arenzano on the coast, and Franchi’ 
also adduced valuable evidence of the Mesozoic age of the Voltri 
group. ‘These dark-grey and bluish, cavernous, sparsely fossiliferous 
(gyroporelle) limestones are, like their equivalents in the Maritime 
Alps and the Savona region, as also like the grezzoni of the Apuan 
Alps,‘ indubitably Middle Triassic, underlying normally the Upper 
Triassic cale-schist and pietre verdi horizon. The three deposits 
between Sestri and Isoverde present the abnormal phenomenon that 
they rest on serpentine and amphibolite of that horizon instead of 
vice versa.° This reversed sequence is, however, purely local and is 
due to an inverted fold along the greatly disturbed, faulted, and tilted 
contact of the Triassic and Eocene formations, whose marked uncon- 
formity is e.g. well exposed in the Chiaravagna Valley on the eastern 
flank of Mte. Gazo, as also in the Rio Recreusi ravine on the 
eastern flank of Mte. S. Carlo near Isoverde. Along the coast near 
1 L. Mazzuoli & A, Issel, ‘‘ Sovraposizione nella Riviera di Ponente d’una 
zona ofiolitica eocenica ad una zona ofiolitica paleozoica’’: Boll. Soc. ital., 
1883, p. 44 et seq. ‘‘ Zona di coincidenza delle formazioni ofiolitiche eoceniche 
e triassiche nella Liguria occidentale ’’: Boll. R. Com. geol., 1884, p. 2 et seq. 
2 G. Rovereto, ‘‘ Questioni dei calceschisti studiata in Liguria ’’: Boll. Soc. 
geol. ital., 1909, p. 408 et seq. ‘‘ Schisti e serpentine antichi in Liguria ’’: 
Atti Soc. Ligure Sc. nat., vol. ii, 4, 1891. 
3 §. Franchi, ‘* Massiccio cristallino ligure’’: Boll. R. Com. geol., 1893, 
p. 43 et seq. ‘‘Relazione campagna, 1911’’: ibid., 1912, p. 41. ‘‘ Posizione 
della zona a Helminthoidea labyrinthica nell’ Hocene superiore’’: ibid., 1915, 
p. 297. Franchi claimed the Voltri group as belonging to the Mesozoic calc- 
schist and pietre verdi horizon of the Maritime and Cottian Alps in ‘‘ Zona 
Pietre Verdi, Ellero e Bormida ’’, ibid., 1906, p. 89 et seq. 
4 Both the Ligurian and the Apuan dolomitic limestone, independently 
analysed, contains 58 per cent carbonate of lime and 38 per cent carbonate of 
magnesia. 
° This superposition Mazzuoli and Issel regarded at the time as normal. 
They therefore assigned the calc-schists and pietre verdi to the Lower Trias, 
and figured these as such in their map of Liguria of 1887. The formation is 
now, like that of the Piémontese Alps, recognized as Upper Triassic. 
DECADE VI.—VOL. III.—NO. X. 29 
