690. Volcanoes are secondary combustions of masses that have originated through the primary combustion, and are therefore only of local occurrence. Such combustible masses are without doubt bodies belonging to the class of Inflammables, and thus carbons, sulphur, sulphuretted metals. Simply burning gases would throw up on high the masses of earth, but not heat them to the degree of fusion. 691. By the heat of these combustions the masses of earth have been fused, forming lavas. The silicious lavas are obsidian, pitch-stone. The argillaceous lavas are the kinds usually met with. Next come the talcose lavas. The calcareous lavas are probably dolomite. B.--_METALLIC ORES AND INFLAMMABLES._ 692. Metallic-ores and Inflammables are products of the planet, when completed, and have not originated along with the origin of the latter, like the earths. The question accordingly arises, what have been the forces by which the metals and Inflammables were produced. a. _METALLIC VEINS._ 693. Fissures in rocks, so narrow that they cannot be illuminated by the sun, are called passages or _veins_. They are rarely found in granite, appear generally for the first time in gneiss, more rarely in the later kinds of rocks, and almost cease to be met with in the stratified chain of mountains. They are found principally in mountains, and thus in masses of earth which project above the level land. We must thus arrive at the conclusion that they have there originated by actual fissure, and that indeed for this reason, that masses which project or stand freely out would admit of yielding asunder more easily than the masses of the plains. This fissure may take place by mechanical disruption, by land-slips, or even also by desiccation. 694. The veins are not prolonged into the kind of rock that underlies them, as e. g. gneiss veins into granite, and so on; they have hence originated from above. 695. They are open and wider above and strike out below; they have not therefore originated by a force acting from beneath. 696. In the schistose rock they form generally transverse fissures. 697. There was a time in which the veins stood empty, as well as a time in which the primary valleys were empty, namely unreplenished with gneiss, mica-schist, and such like minerals. b. PRODUCTION OF ORE. 698. Geogeny takes two directions; the one passes upon the periphery into the splitting action of light, the other into the abyss, where darkness reigns. 699. The valleys were the condition that conduced to the differentialization of the earths, because in them light had power to produce the highest polarity. By the valleys the Earthy has been separated into its principles; silex has separated into clay and talc, to which finally carbonate calcareous earths and salts succeeded.