68 
THE PRINCIPAL FLOOR. 
The crystallised specimens of the Cornish mineral Ten - 
nantite — a sulphide of copper, iron, and arsenic, of somewhat 
rare occurrence — are followed by those of Fahlerz, or grey 
copper ore, a species which, although uncommon in this country, 
occurs near Liskeard in fine crystals, which exhibit well the 
characteristic tetrahedral forms which have gained for this 
mineral the name of Tetrahedrite. The series of copper ores is 
brought to a conclusion by the specimens of Endellionite or 
bournonite, an antimonial sulphide of copper and lead, of which 
some magnificent crystals of well-defined rhombic form, and 
remarkably high lustre, are exhibited from Liskeard in East 
Cornwall. 
Tin. 
Case 8 . — From the earliest recorded times Britain has been 
famous for its tin. The Cassiterides, or “ Tin islands ” of the 
historian, have been thought by some to be the Scilly Islands ; 
but in all probability the name was given by the early navigators 
to the western part of England where we find evidences of 
mine-workings of the highest antiquity. 
Diodorus Siculus describes the trade with Cornwall, “ Bolerion,” 
for tin, and mentions the place of shipment, — the Ictis, an 
island adjoining to Britain. He says, “ It is something peculiar 
that happens to the islands in these parts, lying between Europe 
and Britain ; for at full-tide, the intervening passage being 
overflowed, they appear islands ; but when the sea returns a 
space is left dry, and they are seen as peninsulas.” The late Sir 
G. C. Lewis supposed the Isle of Wight to be the Ictis, but it 
does not fulfill the conditions of the geographer : whereas St. 
Michael’s Mount and Looe Island in all respects agree with the 
description. 
Tin is almost exclusively obtained from Gassiterite or tin-stone, 
a peroxide of tin containing nearly 80 per cent, of metal. In 
the granite and clay-slate of Cornwall and Devon this mineral 
occurs in veins, and is readily separated from its gangue, or 
matrix, and from most of its accompanying minerals, by taking 
advantage of the great density of the ore. With the specimens 
of tin-stone will be found several pseudomorphous crystals, in 
which the original felspar of a porphyritic granite has been 
removed, and oxide of tin has taken its place, preserving still 
the true felspar form. 
During the lapse of ages the tin-bearing rocks have been 
worn down by the combined influence of air and water, and the 
contents of the mineral lodes have been carried down to the 
lower grounds, and deposited as alluvial detritus. Specimens of 
such stream tin are exhibited from several Cornish localities, and 
with them are some examples of the fibrous varieties of tin-stone, 
known from their peculiar structure as “ wood-tin,” and toad’ s- 
eye tin.” 
Case 9. — On the first shelf of this Case are placed several 
samples of $ tannine, known also as tin pyrites and bell-metal 
