The enormous ravages produced by earthquakes have made some naturalists think that mountains and other inequalities of the surface of the globe were only the effects of subterraneous fires, and that all the irregularities must be attributed to the violent shocks which they have produced. This, for example, is the opinion of Mr. Ray; he imagines that all mountains have been formed by earthquakes, or the explosion of volcanos, as Monti di Cinere, the new island near Santorini, &c. but he has not considered that the slight elevations formed by the eruption of a volcano, or by the action of an earthquake, are not internally composed of horizontal strata, as all other mountains are; for by digging in the Monti di Cinere we meet with calcined stones, cinders, burnt earths, metallic dross, pumice-stones, &c. all mixed and confounded like a heap. Besides, if earthquakes and subterraneous fires had produced the great mountains of the earth, as the Cordeliers, Mount Taurus, the Alps, &c. the prodigious force necessary to raise these enormous masses might, at the same time, have destroyed a great part of the surface of the globe; and earthquakes, requisite to produce such effects, must have been of inconceivable violence, since the most famous of which history makes mention have not had sufficient power to form a single mountain; for example, in the time of Valentian I. an earthquake happened, which was felt throughout all the known world;[AA] and yet not a mountain was thrown up by it.
[AA] As Ammianus Marcellinus relates, lib. 26. cap. 14.
It is nevertheless certain, that although we might be able to find an earthquake sufficiently powerful to throw up the highest mountains, it would not be sufficient to disorder the rest of the globe.
For supposing that the chain of the highest mountains which cross South America from the Magellanic lands to New Grenada, and the Gulph of Darien, had been produced by an earthquake, and then let us see by calculations the effect of this explosion. This chain of mountains is near 1700 leagues in length, and commonly 40 in breadth, comprehending the Sieras, which are not so lofty as the Andes. The surface therefore is 68,000 square leagues; I suppose the thickness of the matter displaced by the earthquake to be about one league, that is, the height of these mountains taken from the top to the caverns, which according to this hypothesis must support them, is one league, then I say, the power of an explosion must have raised a quantity of earth equal to 68,000 cubical leagues to a league in height. Now the action being equal to the re-action, this explosion must have communicated the same motion to the rest of the globe. The whole globe consists of 12,310,523,801 cubical leagues, from which subtracting 68,000, there remains 12,310,455,801 cubical leagues, the quantity of which motion will be equal to that of 68,000 cubical leagues raised one league; from whence we perceive that the force which will have been great enough to elevate 68,000 cubical leagues would not have displaced the whole globe a single inch.
There would therefore be no absolute impossibility in the supposition that mountains have been raised by earthquakes, if their internal composition as well as their external form were not evident proofs of their being the work of the sea. Their internal parts are composed of regular and parallel strata, intermingled with shells, and their external consists of a figure whose angles are every where correspondent: is it credible then that this uniform composition and regular form should have been produced by irregular shocks and sudden explosions?
But as this opinion has prevailed among some philosophers, and as the nature and effects of earthquakes are not well understood, it may possibly be pertinent to hazard a few ideas with a view of explaining those intricate subjects.
The earth has undergone great changes on its surface; we find at considerable depths, holes, caverns, subterraneous rivulets, and void places, which sometimes communicate by chinks, &c. There are two kinds of caverns; the first are those produced by the action of subterraneous fires and volcanos; the action of this fire uplifts, burns, and throws out to a distance the matters that are above, and at the same time divides and deranges those which are on the sides, and thus produces caverns, grottos, and irregular holes, but which however is only effected in the environs of volcanos; and these kinds of caverns are more rare than those produced by water. We have already observed that the different strata which compose the terrestrial globe are all interrupted by perpendicular fissures: the waters which fall on the surface descend through them; collect when stopped by clay, and form springs and rivulets; by their natural propensities they seek out cavities or small vacancies, and always incline to open a passage till they find a vent, carrying along with them sand, gravel, and other matters they can divide and dissolve; by degrees, in the internal part of the earth they form small trenches; and at last issue forth in the form of springs, either at the surface of the earth, or bottom of the sea; the matters which they carry along with them, leave caverns whose extent may be very considerable, the origin of which is quite different from those produced by volcanos or earthquakes.
There are two kinds of earthquakes, the one caused by the action of subterraneous fires, and the explosion of volcanos which are only felt at small distances at the time of eruptions: when the matters which form subterraneous fires ferment, heat, and inflame, the fire makes an effort on every side to get out, and if it does not find a natural vent, it raises the earth above and forces itself a passage by throwing it out; such is the beginning of a volcano whose effects and continuation are in proportion to the quantity of inflammable matters they contain. If the quantity of matters is not considerable, an earthquake may ensue, without a volcano being formed. The air rarefied by the subterraneous fire may also escape through small vents, and in this case there will be only a shock without any eruption or volcano. But when the inflamed matter is in great quantities and confined by solid and compressed bodies, then a commotion and volcano is certain to ensue; but all these commotions form only the first kind of earthquakes, and can only shake a small space of ground. A violent eruption of Ætna will cause, for example, an earthquake throughout the whole island of Sicily; but it will never extend to the distance of three or four hundred leagues. When any new mouth bursts out in Vesuvius, there are earthquakes at Naples, and in the neighbourhood of the volcano; but these earthquakes never shake the Alps, nor extend into France, or other countries remote from Vesuvius. Therefore earthquakes produced by volcanos, are limited to a small space, are properly but the effects of the re-action of the fire; and they shake the earth, as the explosion of a powder magazine produces a shock perceptible at many leagues distance.