Page:Popular Science Monthly Volume 23.djvu/196

This page has been proofread, but needs to be validated.

thermal springs, as, for instance, the geysers of Iceland, and by others in Kamchatka and in New Zealand: this silica often incrusts mosses and other substances in the same way that we may see calc-tuff forming petrifactions in other localities. The delicate, feathery crystallizations of silicious sinter are extremely beautiful.

The quartz of veins appears generally to have been deposited from aqueous solution, and will be seen, as has been already remarked, to contain innumerable cavities inclosing water. Occasionally these watery cavities are of large size, and may be observed without any instrumental aid.

Among the most varied and beautiful forms of quartz which have had a purely aqueous origin are all the varieties of crystalline and amorphous silica, which frequently coat the interiors of geodes and other hollow spaces in the igneous rocks, and which consist chiefly of an intermingling of chalcedony and jasper, and are conveniently grouped under the general name of agates. Pure rock-crystal, amethyst, cairngorm, and other valuable crystallized forms of quartz, are often found in connection with the same rocks, or in others of a more purely metamorphic character. All these varieties of quartz are secondary formations, deposited from watery solutions. The exact mode in which agates have originated is a question full of interest, and not easy in every case to answer. A wonderful history of mineral growth is written in the folded leaves, if one may so denote the bands of a single agate. A very large number of agates consist of more or less concentric layers of chalcedony of various colors (the colors depending on the presence of metallic oxides), together with jasper, rock-crystal, amethyst, etc., in many cases.

Chalcedony is sometimes described as a reniform condition of silica, and though apparently amorphous, when it is microscopically examined, it generally, if not always, exhibits a minute and definite radiated crystalline structure. It frequently forms stalactites, and many of the most exquisite of the banded agates are sections cut from stalactitic formations. Jasper may be looked upon as chalcedony, which, as it consolidated, caught up a certain amount of alumina, or sometimes of lime or oxide of iron. Professor Ruskin, who has paid some attention to this subject, has observed that "jasper will collect itself pisolitically out of an amorphous mass into a concretion round central points, but does not actively terminate its external surface by spherical curves; while chalcedony will energetically so terminate itself externally, but will, in ordinary cases, only develop its pisolitic structure subordinately, by forming parallel bands round any rough surface it has to cover, without collecting into spheres, unless provoked to do so by the introduction of a foreign substance, or encouraged to do so by accidentally favorable conditions of repose."

According to the same observer, some agates appear to be of the nature of concretions formed from within, round a nucleus; these