348 Dr. Du Riche Preller—Crystalline Rocks, N. Piémont. 
ITV.—Tue Crysratitine Rock-arEeas oF THE PI£MONTESE ALPS. 
II. Norrarrn Prémont. 
By C. 8. Du RICHE PRELLER, M.A., Ph.D., M.I.E.E., F.G.S., F.R.S.E. 
(Concluded from the July Number, p. 313.) 
Ill. Tue Lanzo, Ivrea, anp Vat Susta Area. (Figs. 1! and 4.) 
\HIS, perhaps the most complex crystalline area of the Piémontese 
Alps, is also eminently su? generis, for it forms a unit of rocks 
outside, and different from the great cale-schist horizon with pietre verdi 
of secondary composition. As previously stated, itis the continuation 
of the mica-schist and minute gneiss belt which skirts the Po Valley 
from the Rocciacorba group to Avigliana, Monte Musiné, and Lanzo, 
and, extending from Lanzo to Ivrea, Biella, and Val Sesia, reaches 
beyond the latter into Lombardy to Lakes Orta and Maggiore. 
Within the Piémontese Alps, viz. from Lanzo to Val Sesia, it is 
bounded on the north by the calc-schist and pietre verdi fringe 
which skirts the Monte Rosa gneiss massif, and on the south by the 
Pliocene and Pleistocene deposits of the Po Valley, its superficial 
area being roughly 90 by 15 kilometres or 1,350 square kilometres, 
equal to over 500 square miles. The lower hills of the southern 
margin vary from 600 to 900 metres in altitude, which in the centre 
of the area increases to 1,000 and at the northern margin to 2,500 
metres. In its general direction south-west to north-east, the area 
is intersected more or less at right angles by the following affluents 
of the Po: Stura di Lanzo, Malone, Orco, Chiusella—Dora Baltea, 
Elvo—Cervo, and Sessera—Sesia. The valley floors vary at the lower 
limit of the crystalline area from 300 to 500, and in their upper 
parts from 600 to 1,000 metres in altitude. 
As distinguished from the crystalline schist and pietre verdi zone 
of the Maritime, Cottian, and Grajan Alps, Gastaldi labelled the 
Lanzo—Val Sesia area the ‘‘ external crystalline zone’”’.? Gerlach,*as 
early as 1869, designated it as the Ivrea dioritic zone, from the 
conspicuous outcrops of diorite within and outside the great morainic 
amphitheatre of the Dora Baltea Valley around Ivrea, which town 
is itself built on diorite.4 Within recent years it has come to be 
called the dioritic-kinzigitic zone, chiefly in consequence of Franchi’s 
and Novarese’s surveys,° and in order to give due prominence to the 
[The text-illustration (Sketch-map) given in the previous part (p. 306) is 
largely referred to here in the second part of this paper.—ED. GEOL. MAG. | 
* Gastaldi, ‘‘ Studi geol. Alpi Occid.’?: Boll. R. Com. geol., 1871, p. 3 
et seq. 
* Gerlach, Die Pennischen Alpen, 1869. 
* The lakelets S. Michele, Campagna, Sirio, Pistone, and Nero immediately 
north of Ivrea, near Montalto, within the moraine wall of Serra d’Andrate, all 
lie in the diorite zone. 
5 C. F. Parona, ‘‘ Valsesia e Lago d’Orta’’: Atti Ist. Lomb., vol. xxix, - 
Milano, 1886. V. Novarese, ‘‘ Trattato Weinschenk Zona d’Ivrea’’: Boll. R. 
Com. geol., 1905, p. 181 et seq.; ‘‘ Zona d’Ivrea’’: Boll. Soc. geol. ital., 
1906, p. 176 et seq. 8S. Franchi, ‘‘ Zona dioritica-kinzigitica Ivrea-Verbano”’ : 
Boll. R. Com. geol., 1906, p. 270 et seq.; ‘‘ Anfibolo secondario del grupp 
glaucofane in una diorite di Val Sesia’’: ibid., 1904, p. 242. 
