76 
ECONOMIC GEOLOGY 
Many of the detrital tin deposits are worked by hydraulic 
sluicing (cf. p. 53) at costs as low as 3d. per cubic yard 
(W. R. Jones, Tin Fields, 1925, p. 179). 
The tin mines of Nigeria work alluvial deposits derived 
from intrusive granites, The fields are on and near the 
plateau of Bauchi, S. of the famous city of Kano. The tin 
has long been worked; it may have supplied some of the 
metal for the ancient bronzes of Benin. Tin was smelted 
by workers from Kano who cast it into impressions of straw, 
and sold it in 9-inch lengths weighing 66 to the Ib. (G.S. Nig., 
Bull. 4, 1922, pp. 43-5). The age of the Nigerian tin granite 
is regarded as pre-Palzozoic. The field is also exceptional 
by the absence of tourmaline (ibid., p. 41). The ores are 
nevertheless of pneumatolytic origin, for the adjacent granite 
has been intensely silicified and impregnated with topaz and 
often fluor-spar, The Nigerian tin was due to fluoric and 
not to boric acid. 
Borivia—Bolivia is now one of the chief tin-producing 
countries, yielding in recent years about 20 per cent. of the 
total output. The ores have been regarded as different in 
origin from those of other fields, owing to the reported ab- 
sence of tourmaline and topaz. The tin-fields trend along 
the Andes S. of the capital, La Paz. The country is composed 
largely of Silurian and Devonian slates, which have been 
invaded by granite and quartz-porphyry, altered by infiltra- 
tion of silica and crushed into quartz-schists. Vast volcanic 
eruptions in the Upper Cretaceous and Kainozoic discharged 
andesites and rhyolites. 
The tin deposits are of two types. The first are fissure- 
lodes and brecciated zones, in which quartz and cassiterite 
have replaced the country rock. The second type includes 
the famous silver mines of Potosi; the ores are quartz with 
complex sulphides of copper, lead, tin, and antimony, with 
a little cassiterite and tourmaline. The reported absence 
of tourmaline led A. W. Stelzner (Z. 4. g. G., xlix, 1897, 
pp. 116, 120) to describe the Bolivian tin ores as unique, 
and as deposited by ordinary mineral springs. According to 
W. R. Rumbold (Econ. Geol., iv, 1909, pp. 321-64) the ores 
with the granite and quartz-porphyry are normal and always 
contain tourmaline; whereas it is rare with the sulphide 
ores, which he regards as much younger and formed in the 
volcanic period. Miller and Singewald (Min. Dep. S.