quartzite, clays become hardened and lustrous, and coals assume a form like coke and graphite. Crystalline minerals and gems are formed, the rarest ones under a combination of powerful forces of heat and pressure which has been imitated by man only in the feeblest degree, most notably in the production of minute artificial diamonds by Mr. Hannay. The steam-jets which issue from volcanic fissures carry up fragments of rock torn from the sides of the vent, in the cavities of which beautiful crystallized products are often found. The various metallic minerals have nearly all been brought from away down in the earth's crust and deposited upon the sides of rock-fissures, the same volcanic forces opening the cracks through the solid rock, and then bringing up the metallic compounds and causing them to crystallize on the sides of the fissures. The cavities of the igneous rocks, when filled with water, constitute laboratories in which real chemical reactions take place—where the materials of the lava are gradually dissolved and recrystallized in new combinations, and the agates, the onyxes, the rock-crystals, the Iceland spars, and the class of zeolites have been formed. "No one can visit a large collection of crystalline minerals without being struck with the great number of beautiful substances which have thus been formed as secondary products from volcanic materials."
Hot springs, geysers, and carbonic-acid springs, which are also volcanic in their origin, afford another variety of curious products. Hot springs often contain in solution large quantities of silica, which has been taken up at the moment of its separation from the alkalies or alkaline earths with which it has been combined, and which is deposited when the water, having reached the surface, is relieved from pressure and cools down. Thus are formed the basins of the geysers of Iceland,
and the curious structures with fanciful names which distinguish the geysers of the Yellowstone. The outlines of some of these structures are given in the figure (Fig. 9). The deposited silica is known to geologists as "sinter." Carbonic-acid water, in a similar manner,
