LAVA. sort being less compact, and containing more earthy particles, would certainly be much sooner fit for vegetation than one composed of the more perfect vitrified matter." Mr Bergman, who has accurately analyzed some Icelandic lavas, informs us, that one kind is very coarse, heavy, and hard, full of bladders, almost black, intermixed with white grains resembling quartz, which in some places have a figure not very unlike a square. This black matter is not attracted by the magnet; but if a piece of it is held against a compass, the needle visibly moves. When tried in the crucible, it yields from ten to twelve pounds of iron in every hundred weight. It does not dissolve in the least with sal soda, and very difficultly with borax, and scarce at all with urinous salt. It seems to contain a great deal of clay in its composition, which may be extracted by all acid solvents. This last he is likewise, from experiments, assured is the case with the lava of Solfaterra in Italy.
The white lava, which possesses more or less of those transparent grains or rays with which lavas are generally chequered, does not seem to be of the nature of quartz, as it cannot be attacked by sal soda; it is, however, soluble with some difficulty by borax and fusible urinous salt, or microcosmic acid. These effects are perfectly similar to those produced upon the diamond, ruby, sapphire, topaz, and hyacinth. The chrysolite, garnet, tourmalin, and schoerl, can neither be dissolved by sal soda, though they are somewhat attacked by it when reduced to a fine powder; and upon the two last-mentioned ones it produces a slight effervescence; on which account, says Mr Bergman, it is possible that the precious stones found upon Mount Vesuvius, which are sold at Naples, are nearer related to the real precious stones than is generally imagined. He found no such grains in a finer kind of lava, quite porous within, and entirely burnt out, and considerably lighter than the former ones.
The Iceland agate is of a black or blackish brown colour, a little transparent at the thin edges like glass, and gives fire with steel. It cannot easily be melted by itself; but becomes white, and flies in pieces. It can hardly be dissolved in the fire by fusible urinous salt; but it succeeds a little better with borax, though with some difficulty. With sal soda it dissolves very little, though in the first moments some ebullition is perceived, and the whole mass is afterwards reduced to powder. Hence Mr Bergman concludes, that this agate hath been produced by an excessive fire out of the black lava formerly mentioned.
In the Iceland pumice-stone, quartz and crystals are often found, particularly in the black and reddish brown kind. The stones thrown out of the volcano, whether gray, or burnt brown, seemed to consist of a hardened clay, mixed with a siliceous earth. They were sprinkled with rays and grains resembling quartz, and some few flakes of mica. They fused with great difficulty in the fire; with sal soda they showed some effervescence at first, but which ceased in a short time. The parts resembling quartz produced no motion at all; from whence Mr Bergman concludes, that the black lava already mentioned proceeds principally from this mass. Several other stones which were sent him from Iceland, Mr Bergman supposed to have no con-
nexion with the eruptions, but to have been produced some other way.
In Mr Ferber's travels through Italy, we are informed, that he has seen species of lava so exactly resembling blue iron slags, that it was not to be distinguished from them but with great difficulty. The same author tells us likewise, "that the Vicentine and Venetian lavas and volcanic ashes contain enclosed several sorts of fire-striking and flint horn-stones, of a red, black, white, green, and variegated colour, such as jasper and agates; that hyacinths, chrysolites, and pietre obsidiane, described by Mr Arduini in his Gior- nale d'Italia, are found at Leonedo; and that chalcedony or opal pebbles, and noduli with enclosed water drops, (chalcedonii opalii enhydri), are dug out of the volcanic cineritious hills near Vicenza.
M. Dolomieu considers the chemical analysis of la M. Dolomieu's op- tion. va as but of little account. When subjected to the fire a second time, they are all of them reducible to the same kind of glass; from which it has been concluded, that all volcanic products have been formed of the same kind of materials, and that the subterraneous fire has always acted on and variously modified the same kind of stone. But an analysis by fire, he justly observes, is of all others the most fallacious. The substances are all fusible, and we have no proper methods of measuring the intensity of our fire; so that the same substance which to-day may come out of our furnaces untouched, may to-morrow be found completely altered, even though the fire employed should not appear to us to be any more violent than the former. Analyses by different menstrua have not been more successful. Mr Bergman has indeed analyzed some lava with acids, and gives with astonishing pre- analysis cision the following result, viz. that a hundred parts of lava. of lava contain forty-nine of siliceous earth, thirty-five of argillaceous earth, four of calcareous earth, and twelve of iron. These experiments, however, our author observes, give us no information with regard to laws in general. They only show the composition of the particular specimens that he tried; and even after the descriptions that he has given, we are a good deal at a loss to discover the species of lava which he subjected to analysis. "It would be as ridiculous (says M. Dolomieu), to apply this analysis to every volcanic product, as it would be to believe that the component parts of a fissile rock were the same with those of every rock composed of laminae or thin strata." For these reasons he is of opinion, that, in order to understand the nature of lavas, we should consider not only that of volcanoes themselves, but of the bases on which they rest. Had this been done, we would have found that the volcanic fires generally exist in beds of argillaceous schistus and horn-stone; frequently in a species of por- Of the seat phry, the gluten of which is intermediate betwixt of volcanic horn-stone and petrosilex; containing a large quantity fies. of schoerl, feldt-spar, and greenish quartz or chrysolite, in little rounded nodules. These substances, he tells us, would have been found in those mountains which are called primitive, and in strata buried under beds of calcareous stone; and, among other things, would have convinced us, that the fluidity of lavas does not make them lose the distinctive characters of their bases. In the mountains called Primitive, those rocks which