188 ON THE CONCRETIONARV AND coals, in consequence of the successive deposition of a material differing in texture or colour. If the agate contains a central portion of calcareous spar, it is obviously only a variation of the former case. It is thus also easy to explain, why the agate sometimes contains an interior covering of siliceous crystals, from changes that have taken place in the quality of the solution ; these presenting their usual geometric forms, or else being confused, accordingly as the cavity is filled or not. It cannot be objected that siliceous earth is insoluble in water ; because its solubility is proved by numerous facts, and by none more decidedly than the existence of vegetable remains in chalcedony. And that the solid substances in question can transmit water, is certain; since it exists in rocks, and finds a passage through many much more solid than the amygdaloidal bases, as is proved by the daily formation of calca reous stalactites. I have also proved, that the agates are sufficiently porous to transmit oil, and also sul phuric acid; that property being the basis of the process used for staining them black. There is there fore no difficulty in understanding, how the rocks should admit the mineral solutions into their cavities, and how the first crust of agate should permit the deposition, not only of successive ones of the same nature, but, from changes in the nature of the solution, of calcareous spar also. One source for the amygdaloidal nodules is thus established, but it does not follow that this is the sole one. The minerals which these cavities contain are numerous and various, and we have no proof that some of them can be formed by aqueous deposition ; while it is certain that they are sometimes produced from fusion, as they are found constituting imbedded