When on the top of a hill, whose summit is flat, and of a pretty large extent, we meet with hard stone directly under the stratum of vegetable earth, we must remark, that what appears to be the summit, is not so in fact, but only the continuation of some higher hill, whose upper strata are soft stone and the lower hard; and it is the prolongation of these last strata that we meet with again at the top of the first hill.
On the summit of mountains which are not surmounted by any considerable height it is generally only soft stone, and we must dig very deep to meet with hard. Banks of marble are never found but between these beds of hard stone, which are diversely coloured by the metallic earths which the rain introduces into the strata by filtration, and possibly in every country where there is stone, marble would be found if dug for to a sufficient depth; Quoto enim loco non suum marmor invenitur? says Pliny. In fact it is a much more common stone than it is thought to be, and differs from other stones only by the fineness of its grain, which renders it more compact and susceptible of a brilliant polish; and from which quality it took its denomination from the ancients.
The perpendicular fissures and joints of quarries are often filled and incrusted with concretions, which are sometimes as transparent as crystal, of a regular figure, sometimes opaque: the water flows through the perpendicular clefts, and penetrates even the compact texture of the stone; the stones which are porous, imbibe so great a quantity of water, that the frost splits and divides them. The rain by filtrating through the beds of marle, stone, and marble, load themselves with every matter they can take up or dissolve. These waters at first run along the perpendicular clefts, afterwards penetrate the beds of stone, and deposit between the horizontal joints, as well as in the perpendicular clefts, the matters they have brought with them, and form these different congelations according to the nature of the matters they have deposited; for example, when the water filters through marle, chalk, or soft stone, the matters which they deposit are a very pure and fine marle, which generally enters in the perpendicular cleft of the rocks under the form of a porous, soft substance, commonly very white and light, which naturalists have called Lac lunac, or Medulla Saxi.
When these streams of water, loaded with lapidific matter, flow through the horizontal joints of soft stone or chalk, this matter attaches itself to the surface of the blocks of stone, and forms white, scaly, light, and spongy crust; which some authors have named Mineral Agaric, from its resemblance to Vegetable Agaric: but if the strata are of common hard stone, proper to make good lime, the filter being then more close, the water will issue from it loaded with lapidific matter, more pure and homogeneous, and whose molecules uniting more intimately, will form nearly concretions of the hardness of stone, with a little transparency, and we shall find on the surfaces of the blocks in these quarries, stony incrustations variously disposed, which entirely fill up the horizontal joints.
In grottos and cavities of rocks, which may be looked upon as the basons of perpendicular clefts, the diverted direction of the streams of water, give different forms to the concretion which result therefrom. They in general have the appearance of a cone attached to the top of the vault, although they may more properly be considered as hollow and white cylinders, formed by a concentrical surface; these congelations sometimes descend, by drops, to the bottom, and form pillars, and a thousand other figures, as uncouth and ridiculous as the names which naturalists have been pleased to give them, such as, Stalactites, Stelegmites, Osleocollae, &c.
When these concretic juices issue immediately from marble and hard stone, the lapidific matter conveyed by the water being rather dissolved than loosened, the small constituent parts take a regular figure, and form columns, terminated by triangular points, which are transparent and consist of oblique strata; this is called Spar, or Spall. It is generally transparent and colourless, but when the stone or marble, from whence it issues, contains metallic parts, this spar is as hard as stone; it dissolves, like stone, by acid spirits, and calcines with the same heat; therefore we cannot doubt that it is real stone, and perfectly homogeneous. It might even be said that it is a pure and elementary stone, under its proper and specific form.
Most naturalists nevertheless look on this matter as a direct substance, existing independent of stone; it is the lapidific or crystalline juice which, according to them, not only binds the parts of common stone, but even those of flint. This juice, say they, constantly augments the density of stones by reiterated filtrations, and at length converts them into real flint. When this juice is fixed in spar, it continues to receive still more pure juices, which increase its density and hardness, so that this matter successively becomes glass, then crystal, and at last a perfect diamond.
But if this is true, why, in whole provinces, does this crystalline juice form only stone, and in others nothing but flint? Will they say, that the two soils are not of a like age, and that this juice has not had time to circulate and complete the end of its natural action? This is not probable. Besides, from whence does this juice proceed? If it produces stone and flints, what is it that produces this juice? It is apparent that it has no existence independent of these matters, which of themselves can give to the water that penetrates them a petrifying quality, always relative to their native and specific character; insomuch that when it filtrates through stones it forms spar, and when it issues from flints, crystal: and there are as many different kinds of this juice, as matters from which they proceed. Experience perfectly agrees with this idea. The waters which filtrate through stone quarries, generally form soft and calcinable matters like the stones themselves; on the contrary, those which spring from rock and flint form hard and vitrifiable congelations, which have all the other properties of flint, as the first have all those of stone; so the waters which have penetrated the beds of mineral and metallic substances produce pyrites, marcasites, and grains.
We have observed, that we might divide all matters into two great classes, vitrifiable and calcinable; clay and flint, marle and stone, may be looked upon as the two extremes of each of these classes, the intervals of which are filled with an almost infinite variety of the mixt matters that have always one or other of these substances for their basis.
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.