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 volcanic
 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 deeputed
 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 economically
 valuable Bolivian tin ores were formed under pneumatolytic
 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 confact
 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 reconstituted
 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 denudaton.
 As the most concentrated deposition of tin would
have been at the upper edge of the granite, most of the ore