184 THE POPULAR SCIENCE MONTHLY. 



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 form- 

 ing petrifactions in other localities. The delicate, feathery crystalliza- 

 tions 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, ame- 

 thyst, cairngorm, and other valuable crystallized forms of quartz, are 

 often found in connection wilh the same rocks, or in others of a more 

 purely metamorphic character. All these varieties of quartz are sec- 

 ondary formations, deposited from watery solutions. The exact mode 

 in whieh 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 atten- 

 tion to this subject, has observed that " jasper will collect itself piso- 

 litically 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 exter- 

 nally, 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 



