[AJ] Histoire de l'Academie des Sciences, anno 1715, p. 4.
We have a remarkable example of these sinkings near Folkstone, in the county of Kent; the hills in its environs sunk gradually by an insensible motion, and without any earthquake. These hills internally are rocks and chalk, and by their sinking they have thrown into the sea rocks and earth which were adjacent to it. The relation of this fact may be seen in the Abridgment of the Philosophical Transactions, vol. VI. page 250.
In 1618, the town of Pleurs, in Valtelino, was buried under the rocks, at the bottom of which it was situated. In 1678, there was a great inundation in Gascony, caused by the sinking of some pieces of one of the Pyrennees, which forced the water to spring forth that was contained in the subterraneous caverns of those mountains. In 1680, there happened a still greater in Ireland, by the sinking of a mountain into caverns filled with water. We may easily conceive the cause of these effects. It is well known there are subterraneous waters in an infinity of places; these waters carry off by degrees the sand and earth over which they pass, consequently may in time destroy the bed of earth on which the mountain rests; and this bed of earth being more deficient on one side than on the other, the mountain of course must be overthrown; but if this base is worn every where alike, the mountain will sink and not be overthrown.
Having remarked on the sinkings and other changes on the earth, occasioned by what may be called the accidents of nature, we ought not to pass over the perpendicular clefts found throughout the strata of the earth: these clefts are perceptible not only in rocks and quarries of marble and stone, but also in clays and earths of every kind, which have never been removed. I call them perpendicular clefts, because, like the horizontal strata, they are oblique, by accident only. Woodward and Ray speak of these clefts, but in a confused manner; and they do not term them perpendicular clefts, because they thought they might be indifferently oblique or perpendicular. No author has explained the origin of them, although it is apparent that they have been produced, as we observed in a preceding article, by the dryness of the matters which compose horizontal beds. In whatsoever manner this drying happens, it must have produced perpendicular clefts; for the matters which compose the strata could not have diminished in size without splitting in a perpendicular direction to these strata. I comprehend under this name of perpendicular clefts all natural separations of rocks, as well as those which may have been occasioned by any convulsive accident. When some considerable motion happens to masses of rocks, these clefts are sometimes found obliquely placed, but this is because the mass is of itself oblique, and with a little attention it is always easy to discover that these clefts are in general perpendicular to the horizontal strata, particularly in quarries of marble, lime, stones, and all large chains of rocks.
Mountains internally are principally composed of stone and rocks in parallel beds: between the horizontal beds small strata of a softer matter than stone is found, and the perpendicular clefts are filled with sand, crystals, minerals, metals, &c. these last matters are of a more modern formation than the horizontal beds in which we find sea-shells. The rains have by degrees loosened the sand and the earth on the upper parts of mountains, and have left the stone and rocks entirely naked, in which we readily distinguish the horizontal strata and perpendicular clefts: in plains, on the contrary, the rain-water and flood having brought a considerable quantity of earth, sand, gravel, and other such matters, have formed a bed of tufa, soft and dissoluble stone, sand, gravel, and earth, mixed with vegetables. These beds contain no marine shells, or at least only fragments, which have been detached from mountains, with gravel and earth. We must carefully distinguish these new beds from the old, where almost always a great number of entire shells are found placed in their natural situation.
If we observe the order and internal disposition of matters in a mountain, composed, for example, of common stones, or calcinable lapidific matters, we generally find a bed of gravel under the vegetable earth, of the nature and colour of the stone which predominates in this ground; and under the gravel we meet with stone. When the mountain is divided by some trench, or deep cut, we easily distinguish all the strata of which it is composed. Each horizontal stratum is separated by a kind of joint, which is likewise horizontal, and their thickness generally increase in proportion as they lower from the summit of the mountain, and are all divided vertically by perpendicular clefts. In common, the first stratum which is met with under the gravel, and even the second, are only thinner than the beds which form the base of the mountain, but are so divided by perpendicular clefts, that pieces of any length are not to be seen: they perfectly resemble the cracks of ground which is very dry, but go not very far, gradually disappearing in proportion as they descend, and towards the bottom there are no great number but where they divide the strata in a more regular manner. These beds of stone are often many leagues in extent, without any interruption; we almost always meet with the same kind of stone in the opposite mountains, whether divided by a small neck or a valley; and the beds of stone disappear only in places where the mountain sinks and becomes level with some large plain. Sometimes, between the first stratum of vegetable earth and that of gravel, marl is found, which communicates its colour and other qualities to the other two: then the perpendicular clefts of the quarries which are beneath are filled with this marl, where it acquires an hardness in appearance equal to that of stone, but by exposing it to the air it crumbles, softens and becomes ductile.
In most quarries the beds of stone formed on the summit of a mountain are soft, and those near the base are hard; the first is commonly white, of so fine a grain as scarcely to be perceived; it becomes more grained and harder in proportion as it descends, and the lowest stone is not only harder than that of the upper, but it is also closer, more compact and heavier its grain is fine and glossy, and often brittle, and breaks as clear as flint.
The interior part of a mountain is therefore composed of different beds of stone, the upper of which are of soft stone and the lower of hard, and much broader at the bottom than at the top; which indeed almost necessarily follows, for, as they become so much the harder as they descend, it may be fairly supposed that the currents and other motions of the water which have hollowed the vallies and given a shape to the turnings of a mountain, will have laterally worked on the matters of which the mountain is composed, and have worn them away in proportion as they were hard or soft. Now the upper strata being the softest, it will naturally have suffered the greatest diminution. This is one of the causes to which the inclination of mountains may be attributed, and this inclination will be still less steep in proportion as the earth and gravel have been washed away by the rain; and for these reasons it is, that hills and mountains composed of calcinable matters, have an inclination much less than those composed of live rock and flint in large masses; the last in general are of considerable heights and nearly perpendicular, because, in these masses of vitrifiable matters, the upper beds, as well as the lower, are of great hardness, and have alike resisted the action of the waters.