Within these cavities, we find, _1st_, The coat of crystals with which
this cavity is always lined; and this is general to all substances
concreting, in similar circumstances, from a state of fusion; for when
thus at liberty they naturally crystallise. _2dly_, We have frequently
a subsequent crystallization, resting on the first, and more or less
immersed in it. _3dly_, There is also sometimes a third crystallization,
superincumbent on the second, in like manner as the second was on the
first. I shall mention some particulars.
I have one specimen, in which the primary crystals are siliceous, the
secondary thin foliaceous crystals of deep red but transparent iron-ore,
forming elegant figures, that have the form of roses. The tertiary
crystallization is a frosting of small siliceous crystals upon the edges
of the foliaceous crystals.
In other specimens, there is first a lining of colourless siliceous
crystals, then another lining of amethystine crystals, and sometimes
within that, fuliginous crystals. Upon these fuliginous and amethystine
crystals are many sphericles or hemispheres of red compact iron-ore,
like haematites.
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