ge of
nothing but light and heat.

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.

In others, again, the primary crystals are siliceous, and the secondary
calcareous. Of this kind, I have one which has, upon the calcareous
crystals, beautiful transparent siliceous crystals, and iron sphericles
both upon all these crystals, and within them.

_Lastly_, I have an agate formed of various red and white coats, and
beautifully figured. The cavity within the coated part of the pebble is
filled up without vacuity, first, with colourless siliceous crystals;
secondly, with fuliginous crystals; and, lastly, with white or
colourless calcareous spar. But between the spar and crystals there are
many sphericles, seemingly of iron, half sunk into each of these two
different substances.

From these facts, I may now be allowed to draw the following
conclusions:

1_st_, That concretion had proceeded from the surface of the agate body
inwards. This necessarily follows from the nature of those figured
bodies, the figures of the external coats always determining the shape
of those within, and never, contrarily, those within affecting those
without.

2_dly_, That when the agate was formed, the cavity then contained every
thing which now is found within it, and nothing more.

3_dly_, That the contained substances must have been in a fluid state,
in order to t