Ch. XXXni.] MINERAL COMPOSITION OF GRANITE. 705 



Ordinary granite, as well as syenite and eurite, usually contains two 

 kinds of felspar: 1st, the common, or orthoclase, in which potash is 

 the prevailing alkali, and this generally occurs in large crystals of a 

 white or flesh color; and 2 dly, felspar in smaller crystals, in which 

 soda predominates, usually of a dead white or spotted, and striated 

 like albite, hut not the same in composition.* 



As a general rule, quartz, in a compact or amorphous state, forms a 

 vitreous mass, serving as the base in which felspar and mica have 

 crystallized ; for although these minerals are much more fusible 

 than silex. they have often imprinted their shapes upon the quartz. 

 This fact, apparently so paradoxical, has given rise to much ingenious 

 speculation. We should naturally have anticipated that, during the 

 cooling of the mass, the flinty portion would be the first to consoli- 

 date ; and that the different varieties of felspar, as well as garnets and 

 tourmalines, being more easily liquefied by heat, would be the last. 

 Precisely the reverse has taken place in the passage of most granite 

 aggregates from a fluid to a solid state, crystals of the more fusible 

 minerals being found enveloped in hard, transparent, glassy quartz, 

 which has often taken very faithful casts of each, so as to preserve 

 even the microscopically minute striations on the surface of prisms 

 of tourmaline. Various explanations of this phenomenon have been 

 proposed by MM. de Beaumont, Foumet, and Durocher. They refer 

 to M. Gaudin's experiments on the fusion of quartz, which shows that 

 silex, as it cools, has the property of remaining in a viscous state, 

 whereas alumina never does. This " gelatinous flint " is supposed to 

 retain a considerable degree of plasticity long after the granitic mix- 

 ture has acquired a low temperature ; and M. E. de Beaumont suggests 

 that electric action may prolong the duration of the viscosity of silex. 

 Occasionally, however, we find the quartz and felspar mutually im- 

 printing their forms on each other, affording evidence of the simul- 

 taneous crystallization of both.f 



According to the experiments and observations of Gustave Kose, 

 the quartz of granite has the specific gravity of 2*6, which charac- 

 terizes silica when it is precipitated from a liquid solvent, and not 

 that inferior density, namely, 2*3, which belongs to it when it cools 

 in the laboratory, in what is called the dry way, or from a state of 

 fusion. It has been, therefore, inferred, perhaps somewhat rashly, 

 that the manner in which the consolidation of granite takes place is 

 exceedingly different from the cooling of lavas, even of those which 

 are the most crystalline. It has also been still more hastily inferred, 

 that the intense heat formerly supposed to be necessary for the pro- 

 duction of mountain masses of plutonic rocks may be dispensed with. 

 The first question to be decided is, whether or not silica can be ob- 



* Delesse, Ann. des Mines, 1852, t. iii. p. 409, and 1848, t. xiii. p. 675. 

 f Bulletin, 2e serie, iv. 1304 ; and D'Archiac, Hist, des Progres de la Geol. 

 i. 38. 



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