« PreviousContinue »
which it seems to have travelled at the rate of more than 20 miles per minute.
An historical account of the earthquakes which have happened in New England says, that of 5 considerable ones 3 are known to have come from the same point of the compass, viz. the north-west: it is uncertain from what point the other 2 came, but it is supposed that they came from the same with the former. The velocity of these has been much less than that of the Lisbon earthquakes : this appears from the interval between the preceding noise and the shock, as well as from the wave-like motion before-mentioned. All the greater earthquakes that have been felt at Jamaica seem, by the accounts given of them, to have come from the sea, and, passing by Port-Royal, to have gone northwards. The velocity of these also was far short of the velocity of the Lisbon earthquakes. The earthquake of London, on the 8th of March 1750, was supposed to move from east to west. The same thing happened in a slight shock felt there in the last century, as the person who told Mr. M. this, had an opportunity of observing; for being by accident in a scalemaker's shop at the time when it happened, he found that all the scales vibrated from east to west. All the shocks that have been lately felt at Brigue, in Valais, have likewise come from the same point of the compass, viz. the south.
5thly, The Lisbon great earthquake has been succeeded by several local ones since, the extent of which has been much less.
Such were the earthquakes in Switzerland; those on the borders of France and Germany; those in Barbary, &c.
SECT. II. However well these facts may agree with the supposition before laid down, that subterraneous fires are the cause of earthquakes, one doubt may perhaps remain: viz. how it is possible that fires should subsist which have no communication with the outward air? In answer to this might be alleged the example of green plants, which take fire by fermentation, when laid together in heaps; where the admission of the outward air is so far from being necessary, that it will effectually prevent their doing so. But to pass by this, we have many instances more immediately to the purpose. It can hardly be supposed that the fires of the generality of volcanos receive any supply of fresh air (for this must be effectually prevented by that vapour which is continually rushing out at all their vents,) and yet, they subsist, and frequently even increase, for many ages. Now these are fires of the very same kind with those he supposes to be the cause of earthquakes. Other facts, still more expressly to the purpose, are
In the earthquake of Nov. 1, 1755, we are told that both smoke and light flames were seen on the coast of Portugal, near Colares; and that on occasion of some of the succeeding shocks, a slight smell of sulphur was perceived
to accompany a" fog, which came from the sea, from the same quarter whence the smoke appeared." In an account of an earthquake in New England, it is said that at Newbury, 40 miles from Boston, the earth opened, and threw up several cart-loads of sand and ashes; and that the sand was also slightly impregnated with sulphur, emitting a blue flame when laid on burning coals. One of the relaters of the earthquake in Jamaica in 1692, has these words: "In Port-Royal, and in many places all over the island, much suphureous combustible matter has been found (supposed to have been thrown out on the opening of the earth,) which on the first touch of fire would flame and burn like a candle. "St. Christopher's was heretofore much troubled with earthquakes, which, on the eruption there of a great mountain of combustible matter, which still continues, wholly ceased, and have never been felt there since."
Again, we are told that on the 20th Nov. 1720, a burning island was raised out of the sea, near Tercera, one of the Azores, at which place several houses. were shaken down by an earthquake, which attended the eruption of it. This island was about 3 leagues in diameter, and nearly round; whence it is manifest that the quantity of pumice stones and melted matter, which must have been requisite to form it, was amazingly great: in all probability it must have far exceeded all that has been thrown out of Etna and Vesuvius together within the last 2000 years. This may satisfy us that the fire which occasioned all this must have subsisted for many years, and this without any communication with the external air. It is worth observing that several instances of this kind have happened among the Azores. There are besides many marks of subterraneous fires about these islands, several places sending up smoke or flames. These islands are also subject to violent and frequent earthquakes. We have more instances to the same purpose, near the island of Santerini in the Archipelago, where there have been several little islands raised out of the sea by a submarine volcano. Of the eruption of one of these in the year 1708, with all the circumstances that attended it, there is a very good account in the Phil. Trans. No 314, 317, 332. It was raised in a place where the sea had been formerly 100 fathoms deep, and was attended with earthquakes before it showed itself above water, as well as after. It is reported, that the island of Santerini itself was originally raised out of the sea in the same manner; but, be that as it will, we have certain accounts of new islands raised there, or additions made to the old ones, from time to time, for above 1900 years backwards, and there have always been earthquakes at the time of these eruptions.
Another example of the same kind happened at Manila, one of the Philippine islands, in the year 1750. This also was attended with violent earthquakes, to which that island, as well as the rest of the Philippines, is very much subject. Add to these the many instances of vast quantities of pumice stones which have
been sometimes found floating on the sea, at so great a distance from the shore, as well as from any known volcano, that there can be little doubt of their being thrown up by fires subsisting under the bottom of the ocean.
From these instances, we may with great probability conclude, that the fires of volcanos produce earthquakes. He does not however suppose that the earthquakes, which are frequently felt in the neighbourhood of volcanos, are owing to the fires of those volcanos themselves; for volcanos, giving passage to the vapours that are there formed, should rather prevent them, as in the instance at St. Christopher's, before mentioned. We also meet with frequent instances confirming the same thing among the Andes. Antonio d'Ulloa (speaking of what happens among these mountains) says, 'Experience shows us, that on the fresh breaking out of any volcano it occasions so violent a shock to the earth, that all the villages which are near it are overthrown and destroyed, as it happened in the case of the mountain Carguayraso. This shock, which we may without the least impropriety call an earthquake, is seldom found to accompany the eruptions, after an opening is once made; or if some small trembling is perceived, it is very inconsiderable; so that after the volcano has once found a vent the shocks cease, notwithstanding the matter of it continues to be on fire.' The greater earthquakes therefore seem rather to be occasioned by other fires, that lie deeper in the same tract of country; and the eruptions of volcanos, which happen at the same time with earthquakes, may with more probability be ascribed to those earthquakes than the earthquakes to the eruptions, whenever at least the earthquakes are of any considerable extent..
Sect. 3. It may be asked perhaps why we should suppose that several subterraneous fires exist in the neighbourhood of volcanos? In evidence of this, we have frequent instances of new volcanos breaking out in the neighbourhood of old ones: Carguayraso, just mentioned, may supply us with one example to this purpose; and in the night of the 28th of October 1746, in which Lima and Callao were destroyed, no less than four new ones burst forth in the adjacent mountains. To the same purpose we may allege the instances of many volcanos. lying together in the same tract of country: as for example, the many places, not so few as 40, among the Azores, which either do now, or have formerly sent forth smoke and flames; the many volcanos also among the Andes, already mentioned: thus Etna, Strombolo, and Vesuvius, and Solfatara too, are all in the same neighbourhood: and Mons. Condamine says he has traced lavas, exactly like those of Vesuvius, all the way from Florence to Naples. In Iceland also we have, besides Hecla, not only several other volcanos, but also a great number of places that send up sulphureous vapours. But the examples of this kind are so frequent that there are few instances to be produced of single volcanos, without evident marks either that there have been others formerly in
their neighbourhood, or that there are at present subterraneous fires near them.
This frequency of subterraneous fires, in the neighbourhood of volcanos, will appear still more probable, if we consider the internal structure of the earth; and as it will be necessary also, in order to understand what follows, to know a little more of this matter than what falls under common observation, he gives the reader some account of it.
The earth (as far as one can judge from the appearances) is not composed of heaps of matter casually thrown together, but of regular and uniform strata. These strata, though they frequently do not exceed a few feet, or perhaps a few inches in thickness, yet often extend in length and breadth for many miles, and this without varying their thickness considerably. The same stratum also preserves a uniform character throughout, though the strata immediately next to cach other are very often totally different. Thus, for instance, we shall have perhaps a stratum of potters' clay; above that a stratum of coal; then another stratum of some other kind of clay; next a sharp grit sand stone; then clay again; next perhaps sand stone again; and coal again above that; and it frequently happens that none of these exceed a few yards in thickness. There are however many instances in which the same kind of matter is extended to the depth of some hundreds of yards; but in all these, a very few only excepted, the whole of each is not one continued mass, but is again subdivided into a great number of thin laminæ, that seldom are more than 1, 2, or 3 feet thick, and frequently not so much. Beside the horizontal division of the earth into strata, these strata are again divided and shattered by many perpendicular fissures, which are in some places few and narrow, but oftentimes many, and of considerable width. There are also many instances where a particular stratum shall have almost no fissures at all, though the strata both above and below it are considerably broken: this happens frequently in clay, probably on account of the softness of it, which may have made it yield to the pressure of the superincumbent matter, and fill up those fissures which it originally had; for we sometimes meet with instances in mines, where the correspondent fissures in an upper and lower stratum are interrupted in an intermediate stratum composed of clay, or some such soft matter. Though these fissures do sometimes correspond to one another in the upper and lower strata, yet this is not generally the case, at least not to any great distance: those clefts however in which the larger veins of the ores of metals are found are an exception to this observation; for they sometimes pass through many strata, and those of different kinds, to unknown depths. From this constitution of the earth, viz. the want of correspondence in the fissures of the upper and lower strata, as well as on account of those strata which are little or not at all shattered, it will come to pass that the earth cannot easily
be separated in a direction perpendicular to the horizon, if we take any considerable portion of it together; but in the horizontal direction, as there is little or no adhesion between one stratum and another, it may be separated without difficulty. Those fissures which are at some depth below the surface of the earth are generally found full of water; but all those that are below the level of the sea must always be so, either from the oozing of the sea, or rather of the land waters between the strata.
The strata of the earth are frequently very much bent, being raised in some places, and depressed in others, and this sometimes with a very quick ascent or descent; but as these ascents and descents in a great measure compensate one another, if we take a large extent of country together, we may consider the whole set of strata as lying nearly horizontally. What is very remarkable however in their situation is, that from most, if not all large tracts of high and mountainous countries, the strata lie in a situation more inclined to the horizon than the country itself, the mountainous countries being generally, if not always, formed out of the lower strata of earth. This situation of the strata may be not unaptly represented in the following manner. Let a number of leaves of paper, of several different sorts or colours, be pasted upon one another; then bending them up together into a ridge in the middle, conceive them to be reduced again to a level surface, by a plane so passing through them as to cut off all the part that had been raised; let the middle now be again raised a little, and this will be a good general representation of most, if not all large tracts of mountainous countries, together with the parts adjacent, throughout the whole world.
From this formation of the earth, it will follow that we ought to meet with the same kinds of earths, stones, and minerals, appearing at the surface, in long narrow slips, and lying parallel to the greatest rise of any long ridges of mountains; and so in fact we find them. The Andes in South America, as it has been said before, have a chain of volcanos that extend in length above 5000 miles: these volcanos are probably all derived from the same stratum. Parallel to the Andes is the Sierra, another long ridge of mountains, that run between the Andes and the sea; and these two ridges of mountains run within sight of each other, and almost equally, for above 1000 leagues together,' being each, at a medium, about 20 leagues wide. The gold and silver mines wrought by the Spaniards are found in a tract of country parallel to the direction of these, and extending through a great part of their length.
The same thing is found to obtain in North America also. The great lakes, which give rise to the river St. Laurence, are kept up by a long ridge of mountains that run nearly parallel to the eastern coast. In descending from these towards the sea, the same sets of strata, and in the same order, are generally met with throughout the greatest part of their length. In Great Britain too we have