Of a few of the external forms of lava we have already given some notice. The really ornamental lavas are chiefly found in the Somma, and constitute the remarkable dykes or veins to which we have also referred; they consist of a compact basis imbedding many beautiful and perfect crys tals of leucite, a mineral, nearly, if not altogether, confined to the volcanoes of Italy: they are called partridge-eyed lavas, and have a beautiful appear ance. The Vesuvian lavas, for the most part, are hard and compact, and frequently approach in ap pearance to a perfect identity with some of our greenstones and basalts. Hence it is much used for paving, and the road between Naples and Portici is a perfect specimen of this application. It is quarried from the coulec of 1794, near where it flowed into the sea, and here a striking tendency towards the assumption of prismatic forms may be observed, and it is even probable that excellent ca binet specimens, having the columnar structure, might here be obtained; nothing indeed can more perfectly resemble a quarry of basalt; and the chain is so complete through the medium of the lava of ,Capo di Bove near Rome, and the extinct volca noes of Auvergne, that the question of their simi larity of origin is probably for ever set at rest. The varieties of lava found in Vesuvius is very great, and not less than t‘sentv-two very distinct ones have been enumerated in the Monte Somma alone. In general, however, they are rough, °puke, and without lustre, very rarely indeed presenting the vitreous lustre. Hence obsidian is one of its most uncommon products, though met with in conside rable quantity in the neighbouring island of Ischia; it is only, we believe, found in small ovoidal cavi ties, principally confined to the leucitic lavas of the Monte Somma, and from that point of ancient emission the small quantity of pumice which is found in detatched masses on the mountain, seems also to have proceeded; these two minerals being probably very nearly allied in their formation. Lava highly cellular is not au unfrequent produc tion of Vesuvius, and small spheres of it are fre quently ejected along with volcanic sand, and by their lightness reach great distances. The cavities are occasionally coated with muriate of copper, as in specimens from the " Cratere del Francese." In the crater, lava exhibits a very different form; the action of heat and moisture, but especially the acid vapours which are abundantly evolved, soon effect the disruption of those affinities by which the compact condition of the rock was maintained. The most powerful agent is sulphurous acid, which at the same time, forming a neutral salt with the iron, which is by far the most abundant colouring matter of the rock, leaves, ere its action is finished, the dark, rugged, intractable lava in the plastic condition of a white powdery clay. At the Solfa tara this operation goes on with regular energy, and rocks may there be seen in a condition almost identical with the claystone porphyry of the trap formations. The quantity of potassa contained in the lava of Mount Vesuvius proves eminently fa vourable to the soil formed by its disintegration.
The Brecciated lavas of Vesuvius are chiefly in teresting from the adventitious fragments which they contain, belonging to the primitive or older secondary strata, which, being elevated by volcanic agency, are imbedded in a basis of the lava of the eruption to which they may belong. Granite and gneiss are sometimes found to be ejected from the crater, but what in some respects is most interest ing, the masses of coarse Apennine limestone so thrown out are found in a remarkably changed form, being converted into a fine granular white marble, nearly of the nature of dolomite. But we cannot enlarge farther on this curious topic.
Volcanic tufa may be said to form the connect ing link between many ancient volcanic formations and those of modern date. We see in the Bay of
Naples the old tufas forming whole hills and dis tricts, which, for time immemorial, have existed in their present form, and which, for many cogent reasons, may be conjectured to have been elevated from the bottom of the sea. Again we see this rock identified in the stony crust which covers Hercula neum, and which, far from being a lava, as has often been alleged, appears to have owed its fluidity to moisture rather than fire. Its origin is revealed by modern eruptions, in which the formation of tufa occasionally occurs. We have in our possession a real specimen of this nature, produced by the erup tion of 1822, and retaining the impression, or ra ther the substance of the leaf of a tree; it is nothing else than consolidated mud, proceeding from the partial solution and mechanical mixture of the solid materials of the mountain uniting with water at a high temperature, perhaps derived from the ocean. Tufa is therefore probably identical with the pro ducts of the salses or mud volcanoes of Modena, Sicily, and other places.* The eruptions of volcanic sand or dust which oc casionally take place, we have already alluded to. Its comminution is extreme, and it is hence car ried by the wind to distances almost incredible. It reached Egypt and Syria in A.D. 79, Constantino ple in 472 and 1631, and was dispersed all over Ca labria in 1139, and 1794. In the first year above mentioned it covered Pompeii to the depth of many yards, in union with grosser fragments of scoria and cellular lava, and in 1822 it lay finger deep in Naples at a distance of six miles. That this im palpably divided matter should reach to such a dis tance as Egypt, we need not reject as fabulous, since we have authentic accounts of dust raised from the deserts of Africa by no extraordinary phenomenon, reaching hundreds of miles to lee ward, a:nd covering the sails and decks of vessels. In the case of Vesuvius the quantity is sometimes so enormous as to create a great degree of dark ness at considerable distances: of this Pliny has given us an interesting testimony in his account of the first recorded eruption. Some time since it was asserted by different Neapolitan chemists, that gold had been detected in the volcanic dust of Ve suvius: this fact, had it been confirmed, though curious, would not have been unexampled, 1- but it appears to be completely disproved by the Analysis of Rose and of Monticelli and Covelli.
The following is the analysis of the volcanic sand of 1822, from a very recent memoir on this sub ject.$ The gases evolved during eruptions have justly been looked upon by chemical philosophers as highly important indices of the nature of the inter nal action, and from these, some beautiful specula tions referring the source of volcanos to the oxida tion of the alkaline and earthy bases, have most ingeniously been derived; but the distinguished au thor of that hypothesis was more lately disposed to found upon it some only of the more circumscribed phenomena of volcanic energy. The principal elastic fluids evolved by Mount Vesuvius are the sulphurous and muriatic acids; the former more particularly accompanying the stage of complete inflammation; the latter its incipient progress. There is a singular fact respecting the gases evolved by Vesuvius, mentioned by Brcislak and confirmed by later writers, that while the muriatic and sulphurous acid vapours have proved most de structive to vegetable life, the olive and pear tree have been found free from its influence. It is cer tain from the experiments of Drs. Turner and Christison that different plants are very differently affected by the same gases, and the phenomena are such as well deserve farther investigation, not only in a chemical and physiological, but in an economi cal point of view.