If the observations on this curious topic, which has so long occupied scientific men, should in this and other parts of the work sometimes appear contradictory, let it be regarded as a proof of the author's candour, and not of his inattention to a subject far from being ascertained.

The account of the volcanic substances will extend to considerable length, and some degree of prolixity may be found in the minuteness of the details, which was necessary for the sake of accuracy; especially as these substances have been objects of repeated disputes and contestations among the mineralogists and geologists.

NOME I. COMPACT LAVA.

The volcanic substances are of such various kinds, that their arrangement becomes more difficult. By far the most important substance is the lava, which must be considered chiefly as it is compact or porous, the former requiring particular attention. In Karsten's catalogue there are only two bits of lava; and as Buffon had prejudices against certain rocks which contradicted his system, so Werner seems absolutely to shut his eyes upon the grandeur and importance of volcanic productions. Hence they are treated with great neglect, and may be said to be excluded from German cabinets; while, to the impartial observer, they convey sublime ideas of the wonderful power of nature.

As the opinion that basaltin is at least sometimes volcanic, appears to gain ground, it must, when identified by its geognosy, be admitted as the most compact of all lavas. Like porous lava, it very often contains grains, or even nodules, of olivine, or what has been called chrysolite; and zeolite forms likewise a common parasitic substance. Neither of these, it would appear, is found in siderite, or in the basalt of the

Basaltin.

ancients; whose most common admixtures are quartz and felspar, and in some porphyries chalcedony. This observation, if exact, would seem of itself to indicate a different origin; for if basaltin were merely the more earthy and compact appearance of the siderous substances, hornblende, and grunstein, as asserted by the Wernerians, it seems difficult to imagine why its parasites should thus totally differ. Chrysolite or olivine also occurs in the masses of native iron, and other stones said to have fallen from the atmosphere; and which are well known to appear in the form of fiery meteors, and to bear other palpable marks of fusion by heat*.

Arrangement. In this division, the terms HYPONOME and MICRONOME, implying greater and smaller subdivisions of the Nome, will become still more necessary, and more strictly applicable, as, though the subjects resemble each other, they are widely different in a geological point of view. The want of such denominations has obliged the writers on volcanic products to di vide them into new and unusual classes, genera, and species; in violation of the other provinces of mineralogy, where these terms bear quite a

Perhaps in a heated state the magnesia may combine with the silex, and the potash evaporate; so that felspar and magnesia may become olivine.

different interpretation. Hence the genera of Dolomieu are, 1. Compact lava. 2. Porous lava. 3. Scoriæ, &c. &c.; while the genera of Werner are Flint, Clay, Lime, &c. Here, on the contrary, basaltin remains a mode among the siderous substances, being only a different combination; while among the volcanic it becomes a hyponome, being amidst the accidential, not the elemental, rocks; not in a series of similar combinations, but in a mere assemblage of substances of quite distinct natures, but all altered by fire.

HYPONOMe i. VOLCANIC BASALTIN.

Volcanic basaltin from Etna, Vesuvius, the isle of Bourbon, &c.

The same, with olivine, from the isle of Bourbon.
The same, with zeolite, from Etna.

Micronome 1. The same, with various substances involved in the volcanic torrent.

Micronome 2. The same, with fragments of ejected rock.

Micronome 3. Compact lava, with melted

gar

nets, from Vesuvius. The appearance is rather vitreous.

Brochant's account.

HYPONOME II. POROUS BASALTIN.

The three very compact homogenous lavas of Dolomieu are probably original rocks; for he speaks of their occurrence in blocks*; and the grand error of his volcanic treatises is, that he confounds antecedent rocks and ejections with lavas.

The siderous compact lavas are thus described by Brochant; who has, however, in this part of his valuable work, followed the arrangement and ideas of Dolomieu.

"These lavas are commonly of a black colour, more or less deep, seldom grey or brown: their fracture is imperfectly conchoidal, their contexture very compact; they are harder, but more brittle than trap, rather sonorous, very heavy; they melt, under the blow-pipe, into black scoriæ; they attract the magnet; they give, by breathing on them, an earthy smell: this lava is one of the most common in volcanic regions, above all in the currents which have issued from Etna, and which are almost entirely composed of it.

* Etna, 185.