182 THE POPULAR SCIENCE MONTHLY. 



nary quartz. It is very varied in color : some beautiful red, brown, 

 and green-banded stones are obtained in Siberia, in Egypt, and else- 

 where. 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 com- 

 bined Avith 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, be- 

 comes soluble in water, and from this solution gelatinous silica is pre- 

 cipitated by hydric chloride. Years ago it was noted that silica when 

 combined with an alkali is soluble in water, and that thus the decom- 

 position of feldspar might in some instances be a source of silica in 

 solution. The residue of decomposed feldspar, when it has been exam- 

 ined, 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 crystal- 

 line 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 occa- 

 sionally 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 



