The foregoing account of volcanic rocks applies, principally, to one olass only of lavas, the stony or common lavas, the most abundant product of volcanoes; but there is a second, the vitreous or glassy lavas, of which the mineral called obsidian, or volcanic glass, is the type, and of which pearlatone and pitehetone and a few other minerals are examples, but partly in a different condition. While the lavas of the greater number of volcanoes belong to the former class (though mingled we believe in those of every one with some glassy lava), these of certain volcanoes are exclusively vitreous, of which that of the volcano in the Island of Bourbon is an example. In the flowing state the nature of these two classes of lavas is very different. The vitreous lavas are then essentially in a state of dry igneous fusion, comparable to that of artificial glass, or of metals ; but the stony lavas, even while fresh erupted and flowing, are not properly in a melted state, being in fact as truly aggregates of distinct mineral substances while in that state as they are when they have ceased to flow, acquired the character of ordinary solids, and become cold. They are, while they retain the flowing property, as Mr. G. Poulett Serope first observed and maintained (in his work referred to below) more than a third part of a century since, a sort of mud, consisting of the crystalline grains of the minerals found to constitute them, allowed to slip or glide over each other by the intervention of water or of aqueous vapour in a peculiar condition of condensation and adhesion to the surfaces of the solid particles, though at a red- or even at a white-heat. Mr. Soropo's views, though almost contemptuously rejected at the period of their enunciation, have received great support from the subsequent discovery of the possible existence of water in a liquid state at high temperatures (as they might have received it from a contemporary one of its convertibility into vapour of its own volume), and from the now admitted necessity of the agency of water in the production of all the crystalline aggregates constituting the Plutonic rocks, including granite itself, which has been referred to by Mr. Scrape in a confirmatory paper communicated to the Geological Society in 18513. A remarkable proof, among many others, that the stony lavas are not really in a fused condition, is afforded by the fact urged by Mr. Scrope, that the radiation of heat by them is comparatively so alight, evincing that they cannot possess that intensely high temperature which would be requisite to impart to their mineral and chemical elements a truly liquid state, independently of the agency of water.
The views of this consummate volcanic geologist, however, are now probably to be regarded as forming only a first approximation to a true theory of the nature of lava. The researches which have been subsequently pursued in the chemistry of geology, and in the artificial synthetic production of minerals, as well as in the geology of the metamorphic rocks, and reap ectiug the action of the igneous rocks upon the sedimentary strata, tend to modify the original conclusions of Mr. Scrope, but still only in the manner which has characterised the progress of all great discoveries in science. The necessary agency of water, together with intense heat, in the production of the crystalline rocks, has been urged by Scheerer and E. de Beaumont. Its operation through a great range of temperatures, both in producing and in alter ing rocks, has been demonstrated or illustrated by Bunsen and Delease, and more recently by a remarkable series of experiments by Professor Daubree of Strasburg. He has shown "that the molecular state of the water in lama, 'be it what it may,' has had a great effect in the formation of silicates, even when anhydrous. It causes them to separate, and to crystallise at a temperature much below their point of fusion ; it enables them to crystallise in an order of succession different from that of their fusibility ; thus, for example, leucite, an infusible silicate of alumina and potash, occurs in lavas in well-formed crystals, often of large size. To this, Ludwig, in his German trans
lation of Daubrice's essay, adds that the crystals of leucite often con tain fragments of lava, and even small crystals of the very fusible mineral augite." • These results are most apposite to, and beautifully elucidatory of, the true nature of lava as originally observed by Mr. Scrope. His views, perhaps, would have sooner been entertained by other geologists, he more explicitly distinguished between the vitreous and the stony lavas when flowing. The former when in the purest and most characteristic condition present us with the state of igneous fusion in its moat perfect form ; solidifying into glass when rapidly-, and crystallising when very slowly, cooled. Taking volcanic phenomena as a whole, there doubtless exists every gradation between this and the hydroplastic condition of lava such as that of Vesuvius and Etna, which, however rapidly cooled, does not solidify into glass, but into an aggregate rock or stone, identical with that resulting from its slow cooling in nature. It is important to observe that the glassy lavas are converted by slow cooling not into stony lava, but merely into the crystalline state of the combination of silicates of which they consist. The stony lavas, however, when truly fused, whether naturally or artificially, become the glassy lavas if quickly cooled, or the corresponding vitrite or crystalline substance if allowed to cool slowly. Such also is evidently the origin of the vitreous lavas io volcanoes, their observed transitions from the glassy to the crystalline or stony form depending partly on their mineral constitution and their condition when in the flowing state, and partly on the circumstances and rate of their refrigeration. Of all this we now possess ample evidence, both geological and experimented For some of the former we may refer to the facts detailed by Mr. Darwin in his 'Geological Observations on the Volcanic Islands,' &c.; and as an example of the latter, to the results of fusing basalt, which is an ancient lava, and when melted and rapidly cooled becomes a glass scarcely distinguishable from obsidian, but when gradually cooled does not return to its original condition of a stony lava, an aggregate rock consisting of several mineral substances, but becomes a kind of pearlstone—the peculiar crystalline condition of the glass. The importance of these latter facts in reference to the acceptance and right understanding of Mr. Scrope's views has already been urged by Mr. Brayley [Were, Geecoue, in Moo. Div.], and been frequently adverted to by him in lectures on igneous Water undoubtedly is second only in quantity among the products or educta of volcanic action, to the solid earthy matter, oxides, like it, some of non-metallic combustible, some of metallic bases. Its func. times in the origination of Plutonic and volcanic action wcro first sug gested, as we shall find, by the discoverer of the chemical nature of that earthy matter, Sir H. Davy. Those which it possesses in volcanic phenomena themselves, partly in the condition of vapour, and partly it is probable in a peculiar intermediate condition not yet under stood, were first truly discovered by Mr. Scrope, and established in his ' Considerations on Volcanoes.' The liquid products of voleauoe..s also contain, though rarely, the mulphorio and muriatic acids ; and among the substances of moat interest in aidiug to complete the theory of chemical actions, are sublimatious of conunon salt, and muriato of ammonia. The origin of these where the volcanoes are situated by the seaside cannot be doubtfuL Boracic acid is another product of this kind occurring in the crater of volcanoes (Daubeny); but this we now know is an abundant element of the earth's crust, widely dissemi. milted, though but rarely in great quantities in a single locality. Com. pounds of boron. however, do not appear to play an important part in the history of volcanoes.