74 



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



forms of quartz usually occurring as concretions in ' 

 limestone rocks ; sometimes, however, as bands 

 of considerable thickness. The black colour so 

 common to the flints of the chalk fonnation and 

 to the chert nodules and bands in the mountain 

 limestone is due to the presence of carbon. Horn- : 

 stone is merely a variety of chert. ; 



Chalcedony, Agate. — Chalcedony has been described : 

 as a mixture of crystalline and amorphous quartz ; j 

 its tendency is to assume a botiyoidal or stalactitic j 

 form ; and its numerous variations of colour and 

 modes of occurrence have led to the adoption of 

 different distinguishing names. Camelians and sardes 

 are only colour distinctions of chalcedony ; and the 

 immense family of agates, including the onyx and 

 sardonyx, is more or less composed of chalcedony, 

 disposed in layers, regular or irregular, and combined 

 with other forms of quartz, such as amethyst, jasper, 

 &c. This latter name is applied to an aluminous 

 variety of quartz : it is opaque, and has a less 

 crystalline appearance than ordinary quartz. It is 

 very varied in colour : 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, coloured by 

 metallic oxides. 



Opal. — 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, Hun- 

 gary, 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 occur- 

 rence, we will next turn to the history of their forma- 

 tion. We shall find that quartz may have been 

 formed by more than one process in the grand labora- 

 tory of Nature. 



According to Cotta, there are two modifications of 

 chemical composition in quartz, which are dis- 

 tinguished by their different degi^ees 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 tliat silica when combined 

 with an alkali is sokible in water, and that thus the 

 decomposition of felspar might in some instances be a 

 source of silica in solution. The residKe of decom- 

 posed felspar, when it has been examined, has been 

 found to contain only a portion of the silica ilue 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 felspar 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 ac- 

 quaintance 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 con- 

 dition. 



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 por- 

 phyries, 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 fre- 

 quently tiny crystals of other minerals ; in the quartz 

 of granite these are very often found to be alkaline 

 chlorides, or sometimes the cavities are filled up with 

 glassy mineral matter ; as, for instance, in the quartz 

 of some of the Icelandic trachytes. Other cavities 

 are found, especially in the granitic quartz, filled with 

 gas, or sometimes with water, or liquid carbonic acid. 

 In these latter cavities small bubbles will be found 

 which are movable ; the smaller ones, indeed, appear 

 to be endowed with a kind of perpetual motion of 

 their 'own. The quartz in these rocks must have 

 crystallized at a very high temperature, — indeed, where 

 glass cavities occur, from a state of true igneous fusion. 

 Mr. Sorby has shown, in a recent paper,* that the 

 solvent power of liquid water at the temperature of 

 about 412 deg. C. is very great : its action on glass 

 has been such as to produce quartz ciystals from it. 



There seems to be clear proof that the quartz of 

 the granite rocks which contains partially filled fluid 

 cavities, and cavities inclosing crystals of common 

 salt, &c., has been formed in a partially melted mass 

 of rock, and began to crystallize when that mass was 

 exposed to the solvent action of liquid water, at a 

 temperature not far below 400 deg. C, but yet not 

 sufficiently high to expand the water into steam. 

 Mr. Sorby concludes that "by far the larger part of 

 the quartz in granitic rocks was set free and crystal- 

 lized through the action of liquid water, at a tem- 

 perature of a dull red heat, just visible in the dark. 

 Tlie exact temperature may, however, have varied 

 considerably, since if the pressure were not sufficiently 

 great, the water might remain in the form of steam 

 until the rock had cooled somewhat more." It has 

 been noticed as somewhat remarkable that the quartz 

 in granite should have been usually the last mineral 



• Miiurral Ma^asine, No. 2, 187^. 



