Dr. Du Riche Preller—Orystalline Rocks, N. Piémont. 305 
I. Tue Lanzo Vatieys anp Gran Parapiso Groups. 
1. Zhe Lanzo Valleys (Figs. 1 and 3).—The Stura di Lanzo 
emerges from the Alps and enters the Po Plain close to the town of 
Lanzo (540 m.) about 380 kilometres north-west of Turin, and 
discharges into the Po a short distance below that city. The point 
of exit lies immediately west of Lanzo, where the Stura cuts through 
a lherzolite wall 400 metres in height. This defile, or chiusa, of 
St. Ignazio, 3 kilometres in length, thus forms the gateway and 
drainage exit of what is indisputably the largest, most concentrated, 
and most diversified pietre verdi area in the Alps, covering, as it 
does, no less than 600 square kilometres or about 240 square miles. 
Immediately west of the St. Ignazio defile the Stura divides into 
two branches—the southern or Stura d’Usseglio, and the northern or 
Stura Val Grande—from which latter branches off, a few kilometres 
higher up, at Ceres, the western arm or Stura di Balme. The more 
or less parallel courses of the three torrents from their sources to 
their confluence near St. Ignazio are from 20 to 30 kilometres in 
length. The Usseglio Stura rises in Rocciamelone (3,557 m.), the 
middle or Balme Stura descends from Uja Bessanese (3,622 m.) and 
Uja Ciamarella (3,670 m.), and the Val Grande Stura has its source 
in La Levanna (3,555 m.). Of these four mountains, forming 
a erest-line of barely 15 kilometres south to north along the Franco- 
Italian frontier, the first three constitute a formidable calc-schist 
massif, while the treble-peaked Levanna forms part of the Gran 
Paradiso gneiss massif. The Usseglio and Balme Sturas have eroded 
‘their beds chiefly in pietre verdi, while the Val Grande branch lies, 
in its upper part, exclusively in the gneiss formation, and traverses’ 
pietre verdi masses only in its lower course. The floors of the three 
valleys are from 1,000 to 2,000 metres below the crest-lines of the 
dividing ridges with steep, often almost vertical mountain sides.’ 
As is seen from the sketch-plan, Fig. 3, the drainage area of the 
Lanzo valleys forms approximately a rectangle about 30 by 
20 kilometres in length and width, enclosed by four ridges whose 
extreme points are Rocciamelone and Levanna on the west and 
Monte Arpone, Lanzo, and Cima Angiolino on the east.2, Within 
these ridges, the Lanzo valleys with their three parallel divides form 
a trough or syncline of pietre verdi in two unequal parts: the larger, 
western part, about two-thirds of the whole, which lies in the calc- 
schist, and the smaller, eastern part; about one-third of the whole, 
which lies in the mica-schist formation. These two juxtaposed 
divisions or old centres of eruption of different age are separated by 
a narrow strip of calc-schist running south to north from Monte 
‘ 
1 The three Stura valleys, as also the Orco Valley (Gran Paradiso), are 
easily accessible by roads leading to Usseglio, Balme, Forno, and Ceresole at 
the upper ends, 1,265, 1,458, 1,226, and 1,613 m. altitude respectively. The 
Usseglio and Balme Valleys are also known as the Viu and Ala Valleys 
respectively. 
2 The short valley of the Tesso, immediately north of Lanzo, lies, strictly 
speaking, outside the drainage area of the St. Ignazio defile, but the torrent, 
rising in Cima Angiolino, belongs to the Stura watershed and forms part 
of the Lanzo valleys. 
DECADE VI.—VOL. III.—NO. VII. 20 
