M. de Dolomieu sees no corrosion of quartz, or solution of that substance, upon the surface of the earth; from this, then, he concludes, that siliceous substance is not dissolved in that situation of things. On the other hand, he finds siliceous bodies variously concreted among the solid strata of the earth; and, from this he concludes, that siliceous substance has been both dissolved by water in the strata, and also there again concreted and crystallised in having been separated from the water. This is certainly what we all perceive; but we do not all allow ourselves to draw such inconclusive inferences from our premises. Notwithstanding the greatest accuracy of our observations, quartz may be dissolvable in a minute degree by water, upon the surface of this earth; and, all the appearances of siliceous bodies, in the mineral regions, where we cannot immediately see the operation, may be better explained by fusion than by aqueous solution. But, from his chemical experiments, our author has conjectured that there may be a phlogistic substance, by means of which the siliceous earth is dissolved when in darkness; and that this solvent loses its power, if exposed to the light of day. I have one observation to oppose to this ingenious theory. Under deep black mosses, through which no ray of light can penetrate, every condition for dissolving siliceous bodies should be found, according to the supposition in question; neither will sufficient time be found wanting, in those deep mosses, upon the summits of our mountains; yet, examine the matter of fact? not the smallest solution is to be perceived in the siliceous parts of the stones which are found under those mosses, but every particle of iron is dissolved, so that the surface of every stone is white, and nothing but the siliceous earth of the feld-spar, and perhaps the argillaceous, is left. Here we have in this author an instructive example: No person, in my opinion, has made such enlightened or scientific experiments, or such judicious observations with regard to the nature of siliceous substance, as a compound thing; no person reasons more distinctly in general, or sees more clearly the importance of his principles; yet, with regard to mineral concretions, how often has he been drawn thus inadvertently into improper generalization! I appeal to the analogy which, in this treatise, he has formed, between the stalactical concretions upon the surface of the earth, and the mineral concretions of siliceous substance. As an example of the great lights, and penetrating genius, of this assiduous studier of nature, I refer to the judicious observations which he has made upon the subject of aluminous earth, in this dissertation.