ORES OF TIN AND TUNGSTEN 75 
which completely decomposed the silicates, removing the 
aluminium as fluoride, leaving the silica as grains of quartz, 
and etching the quartz-phenocrysts of the porphyry. The 
residual quartz sank as the alumina was removed in solution, 
and the sand acquired its bedded structure. The White 
Face consisted of quartz-porphyry, which was altered into 
concretionary and radial growths of topaz during the in- 
troduction of the cassiterite; its residue, owing to the 
absence of pyrites, is a white clay, which includes prosopite 
(CaF,, 2A1(F, OH),) due to the decomposition of topaz. The 
pseudo-sedimentary nature of the residual deposits of Mt. 
Bischoff has been fully described by H. Herman (Proc. 
Austral. 1.M.E., 1914, p. 301). 
Lodes of quartz with coarse cassiterite occur at Mt. 
Bischoff, but have been of secondary importance. 
GERMANY, MaLavsia, AND NiGErRIA—The German tin- 
field at Zinnwald includes a * stockwork ” or impregnation 
of thin veins along the upper part of a granite intrusion ; its 
Cassiterite is associated with topaz and tourmaline, and 
is doubtless due to pneumatolytic solutions which spread 
through the crust of granite instead of forming a deep lode 
as at Dolcoath or large ore-body as at Mt. Bischoff. 
In recent years the largest supplies of tin have come 
from the Malay States, which with Siam, Southern Burma, 
and the islands of Billiton and Banka, yielded in 1925 
60 per cent. of the world’s supply. The tin-fields consist 
of granite, in places injected into schist and limestone. The 
tin is associated with tourmaline and was introduced by 
Pneumatolysis, It occurs in thin veins, which are often 
crowded as a stockwork. The tin, however, rarely pays to 
work except where it has been concentrated by river action 
into alluvial placers, some of which are below sea-level, 
or by settlement in situ from decomposed and partially 
removed country rock. Some of the deposits are pseudo- 
bedded like the Brown Face of Mt. Bischoff. In some places, 
3s at Kinta, the tin has been left in situ by the decay of 
altered schist and solution of limestone (W. R. Jones, 0.%.G.S., 
Lexi, 1917, p. 177). One lode, the Lahat Pipe, in limestone, 
Was worked to the depth of 314 feet. Some pegmatite veins 
contain nodules of cassiterite (Cameron, Ming. Mag. XXX, 
1923, p. 276).