Dr. W. R. Jones—Topaz and Cassiterite in Malaya. 255 
between the Sangone and the Dora, and reappears on the left of the 
latter at Torre del Colle; andathird zone, bifurcating from the second 
at the Moncumiridge, crosses the Dora, and, running north-east, forms 
the Musiné spur. Within the Avigliana horseshoe, the pietre verdi 
thus attain a visible thickness of 500 metres in Monte Pietraborga, of 
700 metres in the Ciabergia ridge, and of 1,200 metres and more on 
the mountain-side left of the Dora, whence the series continues north 
for about 25 kilometres along the eastern spurs to Lanzo. It is in 
this belt that the peridotitic masses, lherzolite and serpentine, with 
associated euphodite, are more especially predominant. In places, 
e.g. in the Musiné spur, the lherzolite is so decomposed that it is 
quarried for the extraction of magnesite.! All the pietre verdi masses 
of the Rocciacorba and Avigliana area dip at more or less steep angles, 
in some places are almost vertical, and throughout are much contorted. 
The three groups of the Dora Riparia area, viz. of Rocciavré, 
Rocciamelone (in the triangle Susa, Chianoc, and summit) and 
Avigliana, including Rocciacorba, cover each about 50, in the 
aggregate 150 square kilometres, or about 60 square miles, equal to 
the superficial area of the Monte Viso group. The conclusions as 
to the age and origin of the pietre verdi groups of Southern and 
Western Piémont considered in the present paper, will be stated in 
connexion with the areas of Northern Piémont to be dealt with in the 
sequel. 
1V.—TuHeE Ortern or Toraz anp CassIreRIrE at Gunone Baxkav, 
Mauaya. 
By WILLIAM R. JonzES, D.Sc. (Lond.), F.G.S. 
NOPAZ is commonly supposed to have been formed by the action 
of fluorine-bearing vapours on felspar, but evidence has recently 
been advanced with the object of showing that some important veins 
‘intrusive in the porphyritic granite of Gunong Bakau, a mountain 
4,426 feet high, situated in the centre of the Main Granite Range of 
the Malay Peninsula, were formed of a rock in which ‘‘ the topaz 
and cassiterite are not alteration products of previously formed 
minerals”. The author of this theory gives the rock the 
descriptive name of ‘ quartz-topaz’, and adds as his reasons for not 
calling it ‘greisen’ the fact that in places it contains very little 
mica and that, unlike the majority of greisens, it is not an alteration 
product. 
The writer is very familiar with the area in which these rocks 
occur, and in a report® written in 1913 called attention, for the 
1 The quarried exposure of lherzolite with euphodite and magnesite is on 
the south-east slope of Monte Musiné, above Casellette (505 m.). About 
8 kilom. east of this point, midway between it and Turin, lies, at Pianezza, in 
the morainiec area, Roc Gastaldi, an erratic monster-block of euphodite 
measuring 30 by 12 by 14 metres in length, width, and height = 5,000 cubic 
metres or over 10,000 tons. It is surmounted by a small chapel. 
2 J.B. Scrivenor, ‘‘ The Topaz-bearing Rocks of Gunong Bakau’’: Q.J.G.S., 
vol. lxx, p. 363, 1914. 
3 W.R. Jones, Preliminary Report of Mining in the Main Granite Range, 
Federated Malay States, 1913. 
