ORES OF TIN AND TUNGSTEN 77 
Amer., 1919, pp. 94-100), on the other hand, consider that 
the ores belong to one period which was later than the vol- 
canic eruptions, and so of modern date and moderate depth. 
W. M. Davy (Econ. Geol., xv, 1920, pp. 463-06) accepts the 
late Kainozoic origin of both types, but claims that the 
granite and quartz-porphyry ores were formed under deep- 
uted conditions, and the tin-silver sulphide ores at moderate 
epths. 
The sulphide deposits are famous for their richness in 
silver and yield but little tin; their stannite has been even 
less productive than that of New South Wales. The economi- 
cally valuable Bolivian tin ores were formed under pneu- 
matolytic conditions like those of other important tin-fields. 
. TIN-FIELDS IN GENERAL OF PNEUMATOLYTIC ORIGIN— 
Sulphide tin ores occur at Campiglia in Tuscany at the con- 
fact of augite-porphyry with Jurassic limestone. Tin in 
small quantities occurs in pegmatites in South Carolina and 
the Black Hills of South Dakota, and in stringers in granite 
ear its contact with limestone in Alaska (Fay, Tr. Amer. 
LM.E. xxxviii, 1908, pp. 664-82); and also in quartz 
lodes containing lumps 6f coarse cassiterite in the Mt. 
Cudgewa tin-field in the Mitta-mitta Valley, Victoria (Gregory, 
Bull. G.S. Vict., 1007, No. 22, p. 107), which, though yielding 
beautiful museum specimens, were too scattered in barren 
uartz to be mined profitably. } 
The essential feature of the chief tin lodes is their formation 
under deep-seated conditions within or beside intrusive 
massifs of Upper Palzozoic granites, by the action of boric 
and fluoric acids with superheated steam. The granite 
attacked by these pneumatolytic agents has been recon- 
stituted as bands of pegmatite; the felspars have been de- 
Stroyed, leaving a rock composed of quartz and mica, which 
1 known as greissen and has been described as the parent 
rock of tin. 
The rise of the tin-bearing solutions through the granite 
Mass explains why tin deposits beside small granite intrusions 
are often richer, as in Burma, than beside large intrusions. 
A small outcrop (Fig. 27) may be the tip of a projection 
from a large granite mass, while a broad outcrop shows that 
the upper part of the granite has been removed by denuda- 
ton. As the most concentrated deposition of tin would 
have been at the upper edge of the granite, most of the ore