But there is another kind of earthquake very different in its effects, and perhaps equally so in its cause; such are felt at great distances, and shake a long course of ground, without any new volcano, or eruption in the old ones appearing. We have instances of earthquakes being felt at the same time in England, France, Germany, and even in Hungary; these earthquakes always extend more in length than breadth; they shake a zone of ground with greater or less violence in different places, and are almost always accompanied with a rumbling noise like that of a coach rolling over the stones with rapidity.
With respect to the causes of this kind of earthquake, it must be remembered that the explosion of all inflammable matters produces, like gunpowder, a great quantity of air; that this air by the heat is in a state of very great rarefaction, and that by its state of compression in the bowels of the earth, it must produce very violent effects. Let us suppose, that at a depth of one or two hundred fathoms, pyrites and other sulphurous matters are collected in great quantities, and that by the fermentation produced by the filtration of the water, or other causes, they inflame; what must happen? First these matters are not placed in horizontal layers, as are the ancient strata, which have been formed by the sediment of the waters; on the contrary, they are formed in perpendicular fissures, in caverns, and in other places where the water can penetrate. Inflaming, they produce a quantity of air, the spring of which being compressed in a small space, like that of a cavern, will not only shake the ground directly above, but will seek out for passages by which it may escape. The roads which offer themselves are caverns and trenches, formed by subterraneous rivulets: into these the rarefied air will precipitate with violence, form in them a strong wind, the noise of which will be heard at the surface, accompanied with shocks of the earth, &c. this subterraneous wind, produced by the fire, will extend as far as the subterraneous cavities, and cause an agitation more or less violent as it is distant from the vent, and finds the passages of a larger or lesser extent: this motion being made longitudinal, the shock will be the same, and the earthquake be felt through a long zone of ground. This air will not produce any eruption, or volcano, because it will find sufficient space to expand, or rather because it will have found vents, and issue forth in form of wind and vapour. Even should it not be allowed that there exist internal passages, by which the air and vapours can pass, it may be conceived that in the place where the first explosion is made, the ground being lifted up to a considerable height, that the most adjoining to this spot must divide and split in an horizontal manner by the force of its motion; and by this means passages communicating one with the other may be opened to great distances; and this explanation agrees with every phenomena. It is not at the same moment or hour that an earthquake is felt in two distant places. Neither fire nor eruption attend those earthquakes which are heard at a distance, and the noise always marks the progressive motion of this subterraneous wind. This theory is confirmed by two other facts; it is well known that mines exhale unhealthy air and suffocating vapours, independent of the wind produced by the current of water: it is also known that there are holes, abysses and deep lakes in the earth, which produce winds, as the lake Boleslaw, in Bohemia, which we have already spoken of.
All this being considered, I do not see how it can be imagined that earthquakes produce mountains, since the cause itself of these earthquakes are mineral and sulphurous matters, which are generally found only in perpendicular clefts of mountains and other cavities of the earth; the greatest number of which have been produced by the operation of water; since this matter by inflaming produces only a momentary explosion and a violent wind which follows the subterraneous roads of the water: since the duration of the earthquakes at the surface of the earth is so short that their cause can only be explosion and not a durable fire: and in short, since these earthquakes, which extend to a considerable distance, very far from raising chains of mountains, do not produce the smallest hills throughout their whole extent.
Earthquakes are, in fact, most frequent in places near volcanos, as in Sicily and Naples, but it is known, by observations, that the most violent earthquakes happen in the time of the greatest eruptions of volcanos; that they are very limited, and cannot produce a chain of mountains.
It has been sometimes observed, that the matters thrown out of Mount Ætna, after laying for many years and afterwards moistened with the rain, have rekindled and thrown out flames with such violent explosions as even to produce a slight shock.
In a furious eruption of Ætna in 1669, which began the 11th of March, the summit of the mountains sunk considerably;[AB] which proves the fire of this volcano comes rather from the top than from the bottom of the mountain. Borelli is of the same opinion, and says, "That the fire of volcanos does not proceed from the centre, nor from the foot of the mountain, but that it issues from the summit, and flames kindle but at a small depth."[AC]
[AB] See Trans. Phil. Abridged, Vol. II. page 387.
[AC] Borelli, De incendiis Montis Etnae.
Mount Vesuvius in its eruptions, has thrown out great quantities of boiling water. Mr. Ray, who thinks that the volcanean fire proceeds from a great depth, says, that it is the water of the sea which communicates by subterraneous passages with the foot of the mountain; he gives, as a proof of it, the dryness of the summit of Vesuvius, and the agitation of the sea at the time of these eruptions, which sometimes retreats from the coasts, and leaves the Bay of Naples almost dry. But, if these facts are true, they do not prove, in a solid manner, that the volcanean fire proceeds from a great depth; for the water which is thrown out is certainly rain water, which penetrates through the fissure, and collects in the cavities of the mountains. Rills and rivulets flow from those containing volcanos as well as other lofty mountains, and as they are hollow, and have been more shaken, it is not astonishing that the water collects in their caverns in their internal part, and that these waters are thrown out in the time of eruptions with other matters. With respect to the motion of the sea, it proceeds solely from the shock communicated to the waters by the explosion, which causes them to advance or retreat according to different circumstances.
The matters which volcanos generally throw out, come forth in the form of a torrent of melted minerals, which inundates all the environs of those mountains; these rivers of liquified matters extend to considerable distances, and in cooling form horizontal or inclined strata, which for position are like the strata formed by the sediment left by the waters: but it is very easy to distinguish the one from the other. First, because strata of lava are not throughout of an equal thickness: secondly, because they contain only matters which have evidently been calcined, vitrified, or melted; and thirdly, because they do not extend to any great distance. As there are a great number of volcanos at Peru, and as the foot of most of the mountains of the Cordeliers is covered with matters thrown out by eruptions, it is not astonishing that marine shells are not met with there, as they must have been calcined and destroyed by the fire; but I am persuaded, if we dig in argilaceous earth, which, according to M. Bourguet, is the common earth of the valley of Quito, shells would be found there, as they are in other places, at least where the ground is not covered, like that at the foot of the mountains, with matters thrown out of a volcano.