The peaks or points of mountains were formerly covered with sand and earth, which the rain gradually washes along with it into the vallies, and has left only the rocks and stone, which forms the nucleus of the mountain. This being left bare will have been still worn by the injuries of the air, the frost will have loosened the large and small parts, which of course have rolled to the bottom. The rocks, at the base of the summit, being left bare, and no longer supported by the earth which surrounded them, will have given way a little, and by dividing one from the other formed small intervals. This separation of the lower rocks could not be made without communicating a greater motion to the upper. By this means the nucleus of the mountain would be divided into an infinity of perpendicular clefts, from the summit to the base of the lower rocks; the rain will have penetrated into all these clefts, and loosened, in the inside of the mountain, all the mineral parts and other matters that it could carry away or dissolve; they will have formed pyrites and other combustible matters, and when by length of time these matters were accumulated in great quantities, they fermented, and by inflaming produced explosions and other effects of volcanos; perhaps likewise, within the mountains, there were masses of these mineral matters already formed before the rain could penetrate therein; in that case, as soon as holes and clefts were made, which gave passages to the water and air, these matters inflamed and formed a volcano. None of these motions could be made in plains, since all is at rest and nothing can be displaced. It is not therefore surprising that volcanos are found only in high mountains. When coal-mines are opened, which are generally met with in argile earth, at a great depth, it sometimes happens that the mineral substances have taken fire: there are even mines of coal in Scotland, Flanders, &c. which have burnt for a number of years. The admission of the air suffices to produce this effect; but these fires produce only slight explosions, and do not form volcanos, because all being solid and full in these places, fire cannot be excited like that of volcanos, in which there are cavities and void places where the air penetrates, which must necessarily extend the conflagration and augment the action of the fire, so as to produce the terrible effects we have just described. ARTICLE XVII. OF NEW ISLANDS, CAVERNS, PERPENDICULAR CLEFTS, &C. &C.