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.