MINERALOGY AND PETROLOGY. 939 



Quartz.) 



We consider quartz as wholly a secondary mineral, when considered petro- 

 logically; that is to say, whether found in the acid rocks, such as granite, or in 

 small amounts in the basic rocks, it has been through some earlier state and has 

 been introduced into these places by some more or less fortuitous or accessory con- 

 ditions in nature. In the acid crystalline rocks it is believed to have existed in clastic 

 condition prior to the metamorphism, and in the basic rocks it originated either in 

 the same way by the metamorphism of a basic clastic rock containing less quartz, or 

 by contact with and the inclusion of some of the acid rocks. This opinion is based 

 not only on the observations recorded in Part II of this volume, but on the results 

 hitherto reached by experimentation in the artificial production of quartz by igneous 

 fusion of other minerals and of rocks.* Never has quartz yet been found to result 

 from the cooling of fused mineral matter, but its production through the action of 

 natural pneumatolitic agencies is a matter of common observation. Artificially also 

 quartz crystals have been produced in the presence of water at a high temperature 

 under great pressure. Daubree produced fine quartz crystals by subjecting common 

 glass to heat in a closed vessel at a temperature of 320 degrees centigrade, the test 

 extending through several weeks. The glass was transformed into a leafy kaolinic 

 mass on the surface of which could be seen standing out so as to be perceived by the 

 hand-glass hexagonal pyramids of quartz. 



Friedel and Sarasin also produced quartz by employing a solution of gelatinous 

 silica in a slightly alkaline liquid. In another experiment by the same chemists, 

 conducted likewise in the presence of water under pressure, with a silicate of potash 

 for the purpose of obtaining artificial orthoclase, crystallized quartz was formed. 

 Tridymite and opal have also been formed artificially by various experimenters when 

 operating in the wet way and under pressure. The only instance (which yet needs 

 verification) of the production of quai'tz in the dry way is that of Hautefeuille, who 

 produced a substance supposed to be quartz, but having different crystalline forms. 



Quartz is therefore to be considered a mineral of that zone of the earth's crust 

 to which water and the vapor of water are accessible. It seems to be precluded from 

 the deep-seated original rocks, not only by the facts of field and microscopical 

 observation, but also by the most obvious inference from the experiments that have 

 been made for its production by dry heat. In all cases where, in the foregoing 

 descriptions (Part II), quartz has been mentioned in an "igneous" rock, the rock 

 containing it is either in immediate or approximate known contact with acid rocks, 

 or the environment is such that such proximity of acid rocks is a reasonable infer- 

 ence (Nos. 672, 686). In numerous instances it occurs in the metamorphic clastic 



*La plupart ties nombreuses varieK-s de silice u structure rristalline, qui se rem-ontrent dans la nature, n'ont pu jusqu'au 

 present 6tru reproduites artifieiellement. On n'a pu refairc ni li- quartz en large plages des granites, ni le quartz granulitique 

 des granulites et microgranulitiques, ni le quartz idolmlaire, ni le quart/, cunC-iforme des pegmatites et inicrnpe^matites, ni le 

 quartz bipyramidc a forme* raecourcies a angles' arrondis, si roimnuii el si canirteristiques dans les microgranulit.es, dans les por- 

 phyres et dans les roches volcaniques acides. Synthese des mineraux ct des ruches, p. 81- Fourjue et Michel Levy, Paris, 1882. 



