The fragments torn from the sides of volcanic fissures consist of the rocks through which the eruptive forces may happen to have opened their way; pieces of sandstone, limestone, slate, granite, &c., are thus frequently found in considerable numbers among materials which build up volcanic mountains. Thus, some of the volcanic cones in the Eifel are very largely made up of fragments of slate, which have been torn from the sides of the vents by the uprushing currents of steam. At Vesuvius masses of limestone are frequently ejected, and may be picked up all over the slopes of the mountains. These limestone-fragments frequently contain fossils, and Professor Guiscardi, of Naples, has been able to collect several hundred species of shells, transported thus by volcanic action from the rock-masses which form the foundation of the volcano of Vesuvius. The action of water at a high temperature, and under such enormous pressure as must exist beneath volcanic mountains, has often produced changes in the rocks of which fragments are ejected from volcanic vents. The so-called 'lava' ornaments, which are so extensively sold at Naples, are not made from the materials to which geologists apply that name, but from the fragments of altered limestone that have been torn from the rocks beneath the mountain, and scattered by the eruptive forces all over its sides. The chemical action of the superheated and highly-compressed steam on the rocks beneath volcanoes frequently results in the formation of beautifully crystallised minerals. Such crystallised minerals abound in the rock-fragments scattered over the sides of Vesuvius and other volcanoes, both active and extinct. They have been formed in the great chemical laboratories which exist beneath the volcano, and have been brought to the surface by the action of the steam-jets issuing from its fissures. Of still greater interest are those materials which issue from volcanic orifices in an incandescent, and often in a molten, condition, and which are evidently derived from sources far below the earth's surface. It is to these materials that the name of 'lavas' is properly applied. Lavas present a general resemblance to the slags and clinkers which are formed in our furnaces and brick-kilns, and consist, like them, of various stony substances which have been more or less perfectly fused. When we come to study the chemical composition and the microscopical structure of lavas, however, we shall find that there are many respects in which they differ entirely from these artificial products. Let us first consider the facts which are taught us concerning the nature and origin of lavas, by a chemical analysis of them. [Sidenote: CHEMICAL COMPOSITION OF LAVAS.]