The
intimate structure of the crystals may be in the form of thin lamellae
crossed, thus possibly accounting for the anomalous double refraction
(Hussak). Crystals usually show a definite point of attachment, and as
a rule form a two-layered coating in mineral veins. In some cases
crystals are found seemingly perfectly developed all over, but these
have very likely had some definite point of attachment from which they
have become detached, afterwards having the deficient parts made up
before the circulation of water bearing the substance in solution
ceased.
In
its mode of occurrence there are many interesting points to notice. By
far its most common situation is in the mineral veins along the line of
faults, and in the fault-breccias of such faults. While not strictly
confined to the neighbourhood of calcareous rocks, there is yet no
doubt that it is more often found in such relation ; its distribution
in such a vein is curiously irregular, it may be abundant at one point
and entirely absent a short distance above or below, or a quarter of a
mile further along the fault. As a rule there is evidence of it having
been deposited, at nearly the same time as many of its associates, by
uprising heated waters bearing it in solution, when these waters
reached such a point that relief of pressure and decrease of
temperature made it no longer possible for the water to carry so much
mineral in solution. These deposits, in most cases at any rate,
occurred after the last great movements along the fault ceased, for the
crystals rarely show any sign of crushing. Much more rarely, deposits
of Fluor Spar occur in volcanic rocks, and here they are probably due
to the slow solution of the disseminated material and its redeposition
in a seggregated condition at
