74 
ECONOMIC GEOLOGY 
begun in 1873, but was delayed by the heavy rainfall, which 
ultimately helped by providing cheap water-power. The 
country (Fig. 26) consists of slates and quartzite of Cambrian 
or Ordovician age; in Devonian times they were folded and 
intruded by masses of granite and ring-dykes of quartz-por- 
phyry. The mine is situated among a group of altered and 
faulted dykes, a funnel-shaped mass of horizontally bedded 
brown sand containing in places from 10 to 15 per cent. of 
cassiterite. As this bedded material occurs in a hollow sur- 
rounded by quartz-porphyry Mt. Bischoff was regarded as a 
volcano, with the crater filled by the sands of the ‘ Brown 
Face.” Some concretions of carbonate of iron were regarded 
as water-worn pebbles. An adjacent white sand and clay 
known as the * White Face,” was rich in tin derived from 
aki of 
B.F. 
An 
w 
Fro, 26.—SECTION ACROSS THE Mount BiscHOFF MINE, TASMANIA. 
S, slate; P, quartz-porphyry ; Q, quartz-vein; W.F., the White Face; 
B.F.. the Brown Face. 
broken veins of cassiterite. Examination of the mine in 
1904 led me to the conclusion (Science Progress, 1906, 
pp. 126, 127) that these materials were not alluvial, but were 
due to the settling of quartz grains left as the country was 
decomposed by pneumatolytic solutions. The Brown Face 
is a gossan due to the weathering of a mass of tin-bearing 
pyrites in porphyry and slate. The slate was injected by 
quartz-porphyry dykes and both rocks were charged with 
pyrites and cassiterite introduced by boric and fluoric acids, 
1 The geology of the mountain was described by Kayser, dust. Assoc. 
Adv. Sei. iv, 1892, pp. 352-8. The topaz was recognized by von Groddeck 
(Z. d. g. G., xxxvi, 1884, p. 643). A later account of the mine has been 
given by J. G. Weston-Dunn (Econ. Geol, xvii, 1922, pp. 154-93). 
Pseudo-bedded tin deposits in the Malay Peninsular have also been de- 
scribed bv Scrivenor and Jones.