Page:Popular Science Monthly Volume 23.djvu/194

182 quartz. It is very varied in color: some beautiful red, brown, and green-banded stones are obtained in Siberia, in Egypt, and elsewhere. Bloodstone is considered to be a mixture of chalcedony and jasper, colored by metallic oxides. One of the most beautiful forms of quartz is opal, which is nothing more than amorphous silica combined with water, which has filtered out from the rocks, usually igneous ones, and is found in cavities and fissures in those rocks. Bohemia, Hungary, Auvergne, and Queensland yield opals, some of them of great beauty and value.

Having thus briefly pointed out the principal varieties of quartz, and the modes of their occurrence, we will next turn to the history of their formation. We shall find that quartz may have been formed by more than one process in the grand laboratory of Nature. According to Cotta, there are two modifications of chemical composition in quartz, which are distinguished by their different degrees of solubility. "The one is insoluble in water and in every acid except hydrofluoric, and the other is soluble in water at high temperatures, especially in the presence of other acids and alkalies." The insoluble variety of quartz may, it is said, in process of time become "converted into the soluble by the contact-influence of infiltrated moisture." It may, however, be noted that ordinary quartz, if fused with carbonate of soda, becomes soluble in water, and from this solution gelatinous silica is precipitated by hydric chloride. Years ago it was noted that silica when combined with an alkali is soluble in water, and that thus the decomposition of feldspar might in some instances be a source of silica in solution. The residue of decomposed feldspar, when it has been examined, has been found to contain only a portion of the silica due to it, the remainder having been dissolved. In a similar manner mica is another mineral which may be a source of supply for pure silica. A fact of some importance in studying the mode of the formation of quartz is that, unlike feldspar and other minerals, which in crystallizing pass at once from the fluid to the solid state, quartz passes through an intermediate viscous or colloid condition before it assumes the crystalline form. It is, comparatively speaking, only very recently that we have had any practical acquaintance with this colloidal form of silica. The late Mr. T. Graham, by his most valuable experiments in dialysis, succeeded in obtaining pure silica dissolved in water, which rapidly assumed a gelatinous condition.

The three principal agencies that have taken part in the formation of quartz are heat, water, and organic life. When we examine, by the aid of the microscope, certain forms of quartz, such, for instance, as the crystals occurring in some of the quartz porphyries, and occasionally in the pitchstones, as well as much of the quartz of granite rocks, we find that they contain minute cavities which inclose very frequently tiny crystals of other minerals; in the quartz of granite these are very often found to be alkaline chlorides, or sometimes the