The substances of the first class can never acquire the nature and properties of the other. Stone will always be as remote from the nature of flint, as potters earth is from marle; no known agent will ever be capable of making them quit the combinations peculiar to their nature: the country which produces stone and marble will remain to do so as certainly as those wherein there is only flint and granate will never have either stone or marble. If we observe the order and distribution of matters in a hill composed of vitrifiable matters, we shall commonly find, under the first bed of vegetable earth, a bed of clay, a vitrifiable matter, analogous to flint, and which, as I have observed, is only a decomposed vitrifiable sand: this bed of argilaceous earth or sand answers to a bed of gravel met with in hills composed of calcinable matters: beneath which we meet with some beds of free-stone scarcely ever more than six inches thick, and divided into small pieces by perpendicular clefts. Under these beds are many others of the same matters, and also beds of vitrifiable sand, the free-stone becomes harder and its blocks encrease in size in proportion as we descend; underneath these we find a very hard matter which I have called live rock, or flint in large masses, which is so hard as to resist the file, graver, and acid spirits, more than vitrifiable sand, and even powdered glass, on which aqua-fortis seems to have some effect. If struck by another hard body it emits sparks, and exhales a very penetrating smell of sulphur. This massy flint, as I have termed it, is generally found with beds of clay, earth, coals, and vitrifiable sand, answers to the strata of hard stone and marbles, which serve as a base to hills composed of calcinable matters. Water, by flowing through perpendicular clefts, and by penetrating the strata of these vitrifiable sands, clays, and earths, becomes impregnated with the fine and most homogeneous parts of these matters, and forms many different concretions, such as talcs, amianthus's, and various other substances produced by distillations through vitrifiable matters.