Dr. Du Riche Pretler—Crystalline Rocks of Piémont. 203 
The majestic appearance of Monte Viso—Pliny’s Mons Vesulus—is 
largely owing to the surrounding area of calc-schists having been 
considerably lowered by erosion which scooped out a socket-like 
depression round the base of the more resistant pietre verdi mass, and 
thus made the pyramid—the highest point of the Cottian Alps—all the 
more imposing. Close to it rise two similarly shaped but lower 
spurs: Visomut on its eastern side, and Visolotto grafted on its 
western flank, all three being in their upper or pyramidal parts 
composed entirely of pietre verdi. The same applies to Colle delle 
 Trayersette (3,287 m.) at the western, to Monte Granero (3,170 m.) at 
the northern, and to Lobbie di Viso (2,990 m.) at the southern end of 
the group, which thus forms an enormous lenticular mass in the cale- 
schist formation, parallel to the Dora—Maira gneiss massif which 
separates it from the Po plain. 
1. The Pretre Verdi Area of Monte Viso.—The least difficult access 
to the central part of the group for examining the pietre verdi series 
is from Barge (500 m.) to Paesana and thence up the Po ravine to 
Crissolo (1,385 m.) and to the summit of Monte Viso (3,843 m.), the 
four stages being (1) the eastern mica-schist zone to Paesana; (2) the 
Dora—Maira gneiss massif, in the ravine or chiusa of which the Po 
is joined by the torrent Lenta; (3) the western mica-schist, and then 
the predominant cale-schist formation with the quarried crystalline 
limestone beds of Crissolo; and (4) the pietre verdi up to the 
summit. 
The base of the Visomut spur discloses a great bank of serpentine. 
passing to schist, at least 500 metres in thickness, followed to the top 
by alternating banks of gneissiform euphodite and enphoditic and 
amphibolic schist, the former conspicuous by smaragdite, the latter by 
its epidotic veins, both of which minerals are largely prominent 
throughout the whole Monte Viso group. The depression between 
Visomut and Monte Viso, in which are embedded Lago Grande and 
other tarns, is composed of calc-schist, chloritic and serpentinous 
schist. From this point the succession of pietre verdi banks can be 
traced uninterruptedly to the summit of Monte Viso along the path 
leading from the Quintino Sella refuge (about 3,000 m.) up the 
southern flank. From the refuge, which is built on a felspathic 
euphodite bank, to the summit, the flank presents a series of alternating 
banks—as shown in the section, Fig. 2!—of euphodite, epidotic, 
amphibolic, actinolitic, and prasinitic schists from 200 to 300 metres 
in thickness, with smaller intervening banks of serpentinous schist. 
The euphodite, varying from compact to schistose, is largely of 
porphyric texture with greyish violet felspar and diallage altered to 
smaragdite. The amphibolic and prasinitic schists and their varieties 
predominate largely, and, together with euphodite, constitute the 
summit of Monte Viso, as they also do that of the almost perpendicular 
I This section is founded on Zaccagna’s great transverse section west to east 
of 70 kilom. from St. Paul in Dauphiné through the cale-schist formation, 
Monte Viso, and the Dora—Maira massif to Rocca Cavour in the Po Valley 
(Boll. R. Com. geol., 1887, p. 416, tav. ix). Franchi gives a similar section of 
the Monte Viso group at a lower level further south (ibid., 1898, p. 482, 
tay. ix; also Stella, ibid., 1896, p. 288). (For Fig. 2 see June Number.) 
