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.
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.
In some matters the perpendicular clefts are very wide, as in the quarries quoted by Shaw, which perhaps is the reason that they are not so frequently met with. In the quarries of flint and granate, the stone may be cut out in very large pieces without the smallest inconveniency, as the obelisks and pillars seen at Rome, which are upwards of sixty, eighty, an hundred, or one hundred and fifty feet long. It appears that these large pillars were raised from the quarry, and that they are to be had of any required thickness, as well as some species of free-stone. There are other matters where these perpendicular strata are very narrow; as in clay, marl, and chalk, and they are wider in marble and most hard stones. Some are imperceptible from being filled with a matter nearly similar to that of the stone itself, which nevertheless breaks off the continuity of the stone, and are what the workmen call hairs. I have often remarked that in marble and stone these hairs cross the blocks entirely, and differ from particular clefts only because their separation is not complete; these kind of clefts are filled with a transparent matter, which is a true spar. There are a great number of considerable clefts in the quarries of free-stone; this proceeds from these rocks often resting on less solid bases than marble or calcinable stones, which generally rest on clay. There are many places where free-stone is not to be met with in large masses; and in most quarries where it is good it lies in the form of cubes and parallel pipedes placed on each other in a very irregular manner, as in the hills of Fontainbleau, which at a distance appear to be the ruins of ancient buildings. This irregular disposition proceeds from the base of these hills being composed of sand, which permits the rocks to sink one on the other, particularly in places that formerly have been worked, which has occasioned a great number of clefts and intervals between the blocks; and we may observe, in every country where sand and free-stone abound, that there are many pieces of rock and large stones in the middle of plains and vallies; whereas in a country consisting chiefly of marble and hard stone, these scattered pieces, which have rolled from the hills and mountains, are very scarce, which proceeds only from the different solidity of the base on which these stones rest, and from the extent of the banks of marble and calcinable stone, which is more considerable than that of free-stone.