h islands off (Strabo says, some way
off) the north-west coast of Spain, which contained tin mines, or, as
Strabo says, tin and lead mines--though a passage in Diodorus derives
the name rather from their nearness to the tin districts of north-west
Spain. While geographical knowledge of the west was still scanty and the
secrets of the tin-trade were still successfully guarded by the seamen
of Gades and others who dealt in the metal, the Greeks knew only that
tin came to them by sea from the far west, and the idea of tin-producing
islands easily arose. Later, when the west was better explored, it was
found that tin actually came from two regions, north-west Spain and
Cornwall. Neither of these could be called "small islands" or described
as off the north-west coast of Spain, and so the Cassiterides were not
identified with either by the Greek and Roman geographers. Instead, they
became a third, ill-understood source of tin, conceived of as distinct
from Spain or Britain. Modern writers have perpetuated the error that
the Cassiterides were definite spots, and have made many attempts to
identify them. Small islands off the coast of north-west Spain, the
headlands of that same coast, the Scillies, Cornwall, the British Isles
as a whole, have all in turn been suggested. But none suits the
conditions. Neither the Spanish islands nor the Scillies contain tin, at
least in serious quantities. Neither Britain nor Spain can be called
"small islands off the north-west of Spain." It seems most probable,
therefore, that the name Cassiterides represents the first vague
knowledge of the Greeks that tin was found overseas somewhere in or off
western Europe.

AUTHORITIES.--Herodotus iii. 115; Diodorus v. 21, 22, 38; Strabo ii.
5, iii. 2, 5, v. 11; Pliny, _Nat. Hist_, iv. 119, vii. 197, xxxiv.
156-158, are the chief references in ancient literature. T.R. Holmes,
_Ancient Britain_ (1907), appendix, identifies the Cassiterides with
the British Isles. (F. J. H.)

CASSITERITE (from the Gr. [Greek: kassiteros], tin), the mineralogical
name for tin-stone, the common ore of tin. It consists of tin dioxide,
or stannic oxide (SnO2), and crystallizes in the tetragonal system. The
crystals are usually 4-sided or 8-sided prisms, striated vertically, and
terminated by pyramids (fig. 1). Twins, with characteristic re-entrant
angles, such as figs. 2 and 3, are common. Certain slender prismatic
crystals, with an acute 8-sided pyramid,