Vein
Agates are similar in composition to the true Agates, but they are
found in more or less elongated fissures in the rocks instead of
inclosed spaces; hence they are more prone to contain sufficient
foreign matter to render them opaque.
The
two chief varieties are the Ribbon Agate, in which the bands are
arranged nearly parallel, and the Brecciated Agate ; the latter is of
interest in indicating movement in the fault, in which it has been
formed, after partial filling of the space with silicious material, and
then a uniting of the broken fragments into a solid mass by a further
deposition of silica. The former kind is found in Saxony at Schlottwitz
and Halsbach, and the latter at Altendorf, besides in many less
important localities in other parts.
The varieties 7, Onyx; 8, Sardonyx; and 9, Jasper Agate, have been dealt with above.
10. Silicious Sinter is a cellular form of Quartz which has been deposited by uprising heated waters.
11.
Flint, formed by the deposition of compact silica by downward
percolating water, often around organic remains, does not concern us in
the present relation.
12. Hornstone is a compact brittle form of silica, with a splintery fracture, slightly translucent.
13.
Lydian Stone, or Touchstone, is a shale which has been altered by heat
in the vicinity of masses of intrusive rock ; it sometimes shows rather
pretty banding of greens and browns, but is very rarely cut and
polished.
14.
Jasper includes the opaque, compact varieties of silica, coloured by
various impurities. It is of various colours, according to the
impurities present; thus disseminated
