the crater beneath. Some of the volatile products of volcanoes are of economical value, like the sal-ammoniac, sulphur, and boracic acid of Vulcano, which have given rise to extensive chemical works.
Solid substances are ejected and accumulate around the orifices, where they frequently form large mountains. They are fragments of rock torn from the formations through which the eruptive stream passes—crystallized minerals, or matters which, derived from sources far below the earth's surface, issue in an incandescent or molten condition, and to which the name of lavas is properly applied. The lavas are composed of the silicates of aluminium, magnesium, calcium, iron, sodium, and potassium in different degrees of combination or mixture. Oxygen, in silicic acid or in a metallic oxide, makes up nearly half the weight of all of them, the metalloid silicon one fourth, and aluminium one tenth of the most of them. Silica or silicic acid—rock-crystal or flint—is present in proportions varying from one half to four fifths of the whole mass. Those lavas in which silica is present in larger proportion are called "acid lavas"; those in which the base is in greater proportion, "basic lavas." Between these are the "intermediate lavas," in which the proportion of silica is lower than in the acid lavas, and the proportion of bases is lower than in the basic lavas. Of the five great groups into which geologists have divided the lavas, the rhyolites are acid, the basalts are basic, and the trachytes, andesites, and phonolites are intermediate.
The structure of lavas can be more clearly ascertained by studying them in the condition of transparent or semi-transparent slices under the microscope. Most of them are made up of crystals of different minerals, varying in size from those which are hardly visible to the naked eye to those of an inch or more in length. Others appear glassy in structure. Under the microscope, they are shown to be made up of two kinds of materials, a base or ground-mass of a glassy character, and distinct crystals irregularly distributed through this glassy base, like the raisins in a cake. In some cases the vitreous part makes up the whole mass of the rock; in others smaller or larger numbers of crystals are seen to be scattered through a glassy base; while in others again the crystals are so numerous that the presence of an intervening vitreous ground-mass can be detected only by the aid of the microscope. When slices of the glassy materials are examined with high powers, cloudy patches are seen diffused through the substance, which under still higher powers resolve themselves into innumerable particles having very definite outlines, some transparent and some opaque. At the same time fresh cloudy patches are brought into view, requiring still higher powers for their resolution; and so the process may go on, as it does in examining the nebulæ with the telescope. The minute particles thus brought into view are called microliths or crystallites. Sometimes, instead of being indiscriminately diffused, they are collected in groups of very definite form, resembling the frost-work on