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 their crystallizing. _Lastly_, That as this fluid state had not been the effect of solution in a menstruum, it must have been fluidity from heat and fusion.