from
the Deccan in India and the first cataract of the Nile; though fairly
common in many other places, the quality is not so good.
Bloodstone
is the same as the above, but with small red spots. Heliotrope, again,
is similar with rather larger spots of red ; it was the Prasius of
Pliny. The best qualities have bright red well-defined spots on a
uniform translucent ground of dark green. Such specimens are found in
India, west of Cambay; also at Creag nan Stardean in Bum, one of the
Inner Hebrides.
6.
Agate.—Most Agates consist entirely, or almost entirely, of
crypto-crystalline silica ; but from this normal type many departures
are to be recognised, and thus as the term Agate is now used it has
reference rather to the origin of the stone than its special variety;
thus true Agates are all formed in the steam cavities of eruptive
rocks, usually lavas; it is a curious fact that Agates are practically
only found in rocks whose percentage of silica is about 45 or 50, and
by no means all rocks having this amount of silica contain Agates. The
other variety of Agate known as Vein Agate is rather different in
origin and will be referred to later.
It
will thus be seen that the Agates have their shape determined by that
of the cavity in which they are formed, that they are of newer
formation than the rocks in which they occur, and that they are
inorganic in origin. They consist essentially of innumerable tine bands
of silica in one form or another, of which bands the outer are the
first formed normally ; the bands are of extraordinary thinness, and
Sir D. Brewster, who carefully measured them, stated they varied from 1/17220 to 1/55760 of an inch, thus Agate in a
