LAVA. are assigned as the bases of the more common lavas are found intermixed with micaceous ones, with gneiss, granite, &c. and they generally rest on masses of granite. Hence lavas must consist of all these matters, and the fire must act upon them all whenever it meets with them. Our author has constantly observed, that volcanoes situated at the greatest distance from the centre of the chain or group of mountains on which they are established, produce lavas of a more homogeneous composition, and less varied, and which contain most iron and argillaceous earth. Those, on the contrary, placed nearer the centre, are more diversified in their products; containing substances of an infinite variety of different kinds. The seat of the fire, however, he observes, does not long continue among the granites, the inflammation being either extinguished, or returning to the centre of the schistus rocks in its neighbourhood.
From this knowledge of the materials of which lavas are composed, we acquire also a considerable knowledge of the matters that are found in greatest quantity in the bowels of the earth. The excavations made by mines, &c. on the surface of the earth, are mere scratches in comparison of the depths of volcanic fires; and as he considers the mountains themselves as the productions of those fires, it thence follows, that by attentively examining the materials of which they are composed, we may thence determine what kind of substances are most common at these great depths in the earth.
Thus our author thinks it probable, that schoerls and porphyries, though rare on the surface, are very common in the internal parts of the earth. As an instance of the truth of his observations, our author informs us, that he was convinced, from no other circumstance but merely inspecting the lavas of Mount Aetna, that in some parts of the island of Sicily, there existed granites, porphyries, with schistus and argillaceous horn-stones. In this opinion he persisted, notwithstanding the generally opposite sentiments of the inhabitants themselves. He searched in vain three-fourths of the island; and at last found that all the mountains, forming the point of Sicily called Pelorus, contain rocks of the kind above mentioned. He then saw that the base of these mountains was produced under Mount Aetna on one side, and under the Lipari islands on the other. "We must, therefore, (says he) believe, that these mountains have furnished the materials on which the volcanoes have, for thousands of years, exerted their power."
By travelling among those elevations called the Neptunian Mountains, or Mons Pelorus, he was enabled to discover the reason why the products of Aetna and the Lipari islands differ from one another. This, he says, is the unequal distribution of the granite and schistus rocks among them. The islands rest almost immediately on the granite, or are separated from it by a very thin stratum of argillaceous rock which contains porphyry; but the Sicilian volcano is situated on the prolongation of the schistus rock, which it must pierce before it reaches the granite; and accordingly very little of its lava seems to have granite for its basis. If the seat of the fire was still more distant from the centre of the mountains, their lavas would be more homogeneous; because the schist, which succeeds to the
horn-stone, is less various, and hardly includes any bodies foreign to its own substance. Thus the lavas, in the extinguished volcanoes of the Val di Noto, which lie 15 leagues to the south-east of Aetna, contain neither granite nor porphyry; but have for their bases simple rocks, with particles of chrysolite and some schoerls.
To the granites which extend to Metazzo, opposite to Lipari, he ascribes the formation of pumice; as they contain an immense quantity of scaly and micaceous rocks, black and white, with fossil granites or gneiss, the basis of which is a very fusible feldt-spar; and these he supposes to be the proper materials of the pumice, having found pieces of them almost untouched in pumice-stones. There are beds of almost pure feldt-spar; to the semivitrification of which he ascribes an opaque enamel like lava mentioned in other parts of his works. Few porphyries, however, he acknowledges, are to be met with among the Neptunian mountains, though these stones abound in the lavas of Aetna. "They are not distant (says he) from the granites; and those I have found have neither the hardness nor perfection of those pieces which I gathered in the gullies, and which had been apparently washed out of the anterior parts of the mountain by water. But though the porphyries I saw here bear no proportion to those in the products of Aetna, I was sufficiently convinced of their existence, and their analogy with those of volcanoes, by discovering that the centre of these mountains contains a great number of them. Porphyries, in general, are very rare on the surface of the earth. Nature generally conceals them from us by burying them under calcareous strata, or by enclosing them in schistus rocks with which they are almost always mixed: but we are indebted to the labour of volcanoes for informing us that they are among the most common substances in the bowels of the earth; and they are never so much disguised by the subterranean fire as to be mistaken in the lavas of which they form the basis."
For an account of volcanic productions, see MINERALOGY Index.
The quantity of matter thrown out from volcanoes under the name of lava is prodigious. After the great eruption of Aetna, in 1669, Borelli went from Pisa to Sicily to observe the effects of it. The matter thrown out at that time amounted to 93,830,750 cubic paces; so that had it been extended in length on the surface of the earth, it would have reached more than four times round the whole earth. All this matter, however, was not lava, but consisted also of sand, stone, gravel, &c. The lava he computed at 6,300,000 paces, which formed a river, according to our author, sometimes two miles broad; but according to others, it was six or seven miles broad, and sometimes 20 or 30 yards in depth. Sir William Hamilton informs us, that the lavas of Aetna are very commonly 15 or 20 miles in length, six or seven in breadth, and 50 feet deep. The most considerable is scarce less than 30 miles long and 15 broad. The most considerable lavas of Vesuvius do not exceed seven miles in length. The same author, however, tells us, that the lava which issued from Vesuvius in 1767, was six miles long, two in breadth, and in most places 60 or 70 feet deep. In one
Lava. one place it had run along a hollow way made by currents of rain not less than 200 feet deep and 100 wide; and this vast hollow it had in one place filled up. He says, he could not have believed that so great a quantity of matter could have been thrown out in such a short time, if he had not examined the whole course of it himself. Even this quantity, however, great as it is, appears very trifling in comparison of that thrown out in Iceland in the year 1783, which covered a space of ground 90 miles in length and 42 in breadth, to the depth of more than 100 feet. Dr Van Troil, in his Letters on Iceland, tells us, that he and his companions travelled over a tract of lava upwards of 300 miles in length: and in 1728, we are told that an eruption of lava took place, which continued for two years to run into a great lake, which it almost filled up.
As the lavas are thrown out from the volcanoes in the highest degree of ignition, it may easily be supposed that such vast bodies will retain their heat for a long time. It would indeed be well worth observing, what length of time is required to cool a lava perfectly; as from thence we might in some measure judge how far those philosophers are in the right, who argue concerning the length of time required to cool an ignited globe of the size of our earth or larger. Sir William Hamilton tells us, that in the month of April 1771, he thrust sticks into some of the crevices of the lava which had issued from Vesuvius in October 1767, and they immediately took fire. On Mount Ætna, in 1769, he observed the lava that had been disgorged three years before to smoke in many parts. No particular observation, however, hath been made in what proportion the heat of lavas is gradually lost.
Sir William Hamilton informs us of a curious fact relating to a lava in the island called Lacco. Here is a cavern shut up with a door; and this cavern is made use of to cool liquor, and fruit, which it does in a short time as effectually as ice. Before the door was opened, he felt the cold on his legs very sensibly; but when it was opened, the cold rushed out so as to give him pain; and within the grotto it was intolerable. He was not sensible of wind attending this cold; though upon Mount Ætna and Vesuvius, where there are caverns of this kind, the cold is evidently occasioned by a subterranean wind: the natives call such places ventaroli. From old lavas there also frequently happens an eruption of noxious vapours called mosetes. These likewise break out from wells and subterraneous places in the neighbourhood of a volcano before an eruption. Our author tells us, that the vapour affects the nostrils, throat, and stomach, just as the spirit of hartshorn or any strong volatile salt; and would soon prove fatal if you did not immediately withdraw from it. These mosetes, he says, are at all times to be met with under the ancient lavas of Vesuvius, particularly the great eruption of 1631.
Sir William Hamilton informs us, that the lavas of Ætna and Vesuvius are much the same, but those of Ætna rather blacker and more porous than those of Vesuvius. Some kinds of lava take a fine polish, and are frequently manufactured into boxes, tables, &c. In Naples, the inhabitants commonly make use of it for paving the streets, and even the subterraneous cities of Pompeii and Herculaneum have been paved with
the same substance. A fine large cubic piece of lava is preserved in the hall of the British Museum.