QUARTZ, a mineral, which is essentially silicic acid, or oxide of silicon (see SitacoN), although it is often combined or mixed with other substances. It is a very abundant and widely diffused mineral. It is almost the sole constituent of quartz rock, in which gold is far more frequently found than in any other matrix; and it is a principal con stituent of granite, syenite, protogine, eurite, pegmatite, granulite, elvanite, all the different kinds of sandstor e, and many other rocks. It is also a common mineral in trap rocks, limestone, etc., and the sands of the sea-shore and of deserts are chiefly formed of it. It is found both massive and crystallized; the primary form of the crystals is a rhomboid, but it far more frequently occurs in six-sided prisms, terminated by six-sided pyramids; or in six-sided pyramids; or sometimes in dodecahedrons, formed by six-sided pyramids base to base. It is hard enough to scratch glass easily, and it gives fire with steel. It becomes positively electrical by friction; and two pieces, rubbed together, give light in the dark. When pure it is quite colorless; but, owing to the presence of foreign substances, it often exhibits great variety of colors; and many minerals, known by different names, and consisting chiefly of quartz, have little or nothing to distinguish them but their color. Thus rock crystal, chalcedony, carnelian, cairngorm, agate, amethyst, prase, chrysoprase, jasper, etc., are mere varieties of quartz. Opal (q.v.) is very nearly allied to it.
Quartz rock, or quartzite, is a sedimentary sandstone, converted into a very hard corn rock by metamorphic action, It is distinctly granular; the grains, however, seem to melt into each other, or to be enveloped in a homogeneous silicious paste. It is
frequently brittle, and in weathering, it breaks up into small irregular cubes.
Quartz veins occur in metamorphic rocks. The structure of the veins is compact and homogeneous, and very different from that of quartzite. Veins not only differ in width, but the same vein is very variable throughout its course, sometimes thinning to a very fine film, and then swelling out to great thicknesses, Quartz veins are more metallifer ous than the mass of the rocks in which they occur. They are the principal natural repositories of gold, for though the precious metal is chiefly obtained from alluvial sands and gravels, these are the weathered and abraded fragments of the under-lying, or neigh boring palnozoic rocks. Small quantities of gold have been found in the quartz veins trav ersing the Silurian and Cambrian rocks of Wales and Scotland; and in Victoria the great veins are so highly auriferous, that they are mined for the precious metal. Wherever the lower Silurian rocks make their appearance on the surface throughout the colony, are everywhere intersected by enormous numbers of quartz veins, which often reach a thickness of 10 to 15 feet. As yet only a very small proportion of these have been explored; but the results have been so remunerative, that mining in the solid rock for gold is extensively pursued. One mine has been driven to a depth of 400 ft., and, con trary to the generally-received opinion, the vein at this depth continued to be auriferous.