cation
with the surface ; in these cases surface tension still has an
important bearing on the formation, but osmosis is probably not a
factor. Frequently, included material is so abundant as to cause
opacity ; in such cases the variety of Chalcedony is known as Jasper.
From
the point of view of genesis most crystals of Dioptase probably come in
this class, being the result of the decomposition of copper ores by
downward percolating water and subsequent deposition of the copper as a
hydrous silicate at a lower level.
Turquois is another mineral whose origin is similar, and in some cases Gypsum falls under this head.
In
Class II., Hypogene minerals, are many of the gems and precious stones.
A consideration of the origin of many of these minerals carries us into
geologically debatable ground. Many have been the hypotheses and
theories advanced to explain the formation of such compounds in
Nature's laboratory. As far back as 1672 Robert Boyle (op. cit.) stoutly
maintained that the gems were deposited from a fluid—in most cases he
seems to have had a watery solution in mind; one of his passages, on
account of its quaintness and its apt expression of the point in
question, may be quoted: " But unless a Concreting stone, or other like
Body be either surrounded with, or in good part conÂtiguous to a Fluid,
'tis not easie to conceive how it should acquire a Curious Angular and
determinate shape. For Concrescent Bodies, as I may so speak, if they
have not room enough in an Ambient Fluid for the most congruous ranging
of their parts, cannot cast themselves into fine and Begular shapes,
such as I shall presently show that divers Gems seem to affect; but the
Matter they consist of must
