HARD WICKE 'S S CIENCE - G O SSI P. 



97 



QUARTZ J ITS VARIETIES AND MODES OF FORMATION. 



No. II. 

 By the Rev. J. MAGENS MELLO, M.A., F.G.S. 



M O X G S T the most 

 varied and beautiful 

 forms of quai^tz which 

 have had a purely- 

 aqueous origin, are all 

 the varieties of crys- 

 talline 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 forma- 

 tions, 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 colours (the colours depending on the pre- 

 sence of metallic oxides), together with jasper, rock 

 ciystal, amethyst, &c., 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 

 ciystalline 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, 

 No. 149. 



has observed* that "jasper will collect itself pisoliti- 

 cally out of an amorphous mass into a concretion 

 round central points, but does not actively terminate 

 its external surface by spherical curves ; while chalce- 

 dony 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 col- 

 lecting into spheres, unless provoked to do so by the 

 introduction of a foreign substance, or encouraged to 

 do so by accidentally favourable 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 would consist of 

 chalcedony or jasper in the inner portions, and have 

 distinctly crystallized exteriors. There is another class 

 of agates composed of external bands of chalcedony 

 or jasper, stalactitically deposited in a cavity which 

 may either have a hollow centre, or one filled up with 

 crystals of quartz. There appear, however, to be 

 intermediate varieties in which concretionary or 

 stalactitic formations have been combined with, or 

 interrupted by, other modes of growth. 



Some of the most curious and beautiful agates are 

 those containing dendritic ciystallizations ; in these 

 we see, in the more or less transparent chalcedony, 

 which in these agates is not banded, wonderful 

 mossy or confervoid-like growths, often very closely 

 resembling vegetable forms. The valuable stones 

 from Mocha contain ferruginous brown or black 

 inclosures, whilst some of the dendritic agates from 

 India are filled with a bright green network of 

 what appear to be filaments of confervce. These 

 dendritic forms in the moss agates are mostly the 

 oxides of iron or manganese ; or in the green Indian 

 pebbles, delessite or chlorite. The question of their 

 origin is a difficult one. In some agates the den- 

 drites may have resulted from a segregation of the 



* Geological Magazine, vols. iv. and v. 



