Page:Popular Science Monthly Volume 41.djvu/674

656, as in the processes of glass-making, but in the simple presence of air and water at ordinary temperatures, it remains unaffected through long ages. "When the silica is united with a metal, such as aluminum, in kaolin and the ordinary clays, the compounds are still very stable, but they are less so than the simple oxide. When, further, there are several metals included in the compound, as in the mica minerals and their allies, the silicate decreases in stability as it increases in complexity, and we have, as with carbon, a readily decomposable compound.

The world has chosen rock as the symbol of. stability, but it has not chosen very wisely, for the majority of rocks are anything but stable.

In the case of the mica family the readiness with which the minerals take up water and part with the more soluble of their components is shown in the many gradations by which they pass through the hydrous micas to the clays and soapstones. It is very noticeable in the mica regions themselves. A mica mine is, indeed, an instructive object-lesson in soil formations. One can almost see the decay of the crystalline rocks going on before one's eyes.

Were the micas only important as a rock constituent, they would doubtless receive very careful study by reason of the many interesting problems which their occurrence and alteration bring up, but in addition to this, their characteristic physical qualities, their transparency, elasticity, laminar structure, luster, comparative infusibility, and electrical non-conducting power, give them a number of applications in the arts, and make them the object of industrial mining. The mica of the market is in nearly all cases the common white mica or muscovite. From its chemical composition it is sometimes known as potash mica, to distinguish it from lithia and other micas, but these names are more common in the laboratory than in trade. Although mica is so widely distributed in Nature, it is only in a few localities and under well-defined conditions that it occurs in large enough plates to be profitably mined. Granite and gneiss both consist of a mixture of the three minerals, mica, quartz, and feldspar (another silicate of potash and alumina), but as ordinarily found, the mica is too thoroughly mixed with the other ingredients, and is in too small masses, to be available. It is only when fissures in the rock have been filled with very coarsely crystallized granite that the mica can be mined with profit.

Such fissure veins occur in a number of localties, notably in Siberia and Norway on the other side of the water; and in our own country, in New Hampshire, in North Carolina, in Wyoming, in New Mexico, in the Black Hills of Dakota, and probably in paying quantities in Alaska. Of late years the importation of