Flint, notwithstanding its hardness and density, has, like common marble and hard stone, its exudations, from whence stalactites of different kinds result, whose varieties of transparency, colours and configuration, are according to the nature of the flint which produces them, and the different metallic or heterogeneous matters which it contains. Rock crystal, all precious stones, white or coloured, and even diamonds, may be regarded as stalactites of this kind. Flints in small pieces, whose strata are generally concentric, are also stalactites, or parasitical stones; from flints of large dimensions, and most fine opaque stones, are only species of flint. Matters of a vitrifiable kind, as we have observed, do not produce so great a variety of concretions as those of the calcinable class; and these concretions, produced by flints, are almost all hard and precious stones; whereas those of the calcareous are only soft matters of no value. Perpendicular clefts are found in rocks of flint, as well as in those of marble and hard stone; they are sometimes even larger there, which proves that matter is still dryer than stone: hills, whether of calcinable or vitrifiable matters, are supported by clay or vitrifiable sand; these are the common and general matters of which the globe is composed, and which I look on as the lightest parts, or the scoria of vitrified matter, with which it is internally filled; thus all mountains or plains have argilaceous earth or sand for their common foundation. For example, we see that in the pits at Amsterdam and Marly la Ville, vitrifiable sand was below every other stratum. In most naked rocks it is observable that the sides of the perpendicular clefts, whether broad or narrow, correspond as exactly as those of a piece of slit wood. In the large quarries in Arabia, which are almost composed of granate, these perpendicular separations are very frequent; and although some are twenty or thirty yards wide, yet the ridges exactly correspond and leave a deep cavity between them.[AK] It is very common to find in perpendicular clefts shells broken in half, and each piece remaining fastened to the stone on the opposite side; which proves these shells were placed in the solid stratum, and before the cleft was made.[AL] [AK] See Shaw's Travels, vol. II. p. 83. [AL] See Woodward, page 198.