If we visit those parts of the earth's surface where great masses of fused volcanic rock have slowly cooled down in contact with sedimentary materials, we shall not be disappointed in our expectations. Diamonds, rubies, sapphires, emeralds, topazes, garnets, and a host of equally beautiful, if less highly prized, crystalline substances, are found in such situations, lying in the subterranean chemical laboratories in which they have been formed, but now, by the action of denuding forces, revealed to our view. In some cases it is not necessary to penetrate to these subterranean laboratories in order to find these beautiful gems and other crystallised minerals; for the steam jets which issue from volcanic fissures carry up fragments of rock torn from the side of the vent, and in the cavities and fissures of such ejected masses beautiful crystallised products are often found. Such rock-fragments containing minerals finely crystallised are found abundantly on the flanks of Vesuvius and other active volcanoes, and among the materials of the Laacher See and other extinct volcanoes. [Sidenote: FORMATION OF MINERAL-VEINS.] But it is not only the finely crystallised minerals and gems which we owe to volcanic action. The various metallic minerals have nearly all been brought from deep-seated portions of the earth's crust and deposited upon the sides of rock-fissures by the agency of the same volcanic forces. It is these forces which have, in the first instance, opened the cracks through the solid rock masses; and, in the second place, have brought the metallic sulphides, oxides, and salts--either in fusion, in solution, or in a vaporised condition--from the deep-seated masses within the earth, causing them to crystallise upon the sides of the fissures, and thus form those metallic lodes and veins which are within reach of our mining operations. There is still one other important class of minerals which owe the existence, though indirectly, to volcanic agencies. The cavities of igneous rocks, especially the vesicles formed by the escape of steam, constitute, when filled with water, laboratories in which complicated chemical reactions take place. The materials of the lava are gradually dissolved and re-crystallised in new combinations. By this means the most beautiful examples of such minerals as the agates, the onyxes, the rock-crystals, the Iceland-spars, and the numerous beautiful crystals classed together as 'Zeolites' have been formed. No one can visit a large collection of crystalline minerals without being struck with the large number of beautiful substances which have thus been formed as secondary products from volcanic materials. CHAPTER VI. THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS