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.
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