Ch. XXXIIL] poephyeitic geanite. 563 



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 ex- 

 planations of this phenomenon have been proposed by MM. de Beau- 

 mont, Foumet, and Durocher. They refer to M. Gaudin's experiments 

 on the fusion of quartz, -which show that silex, as it cools, has the prop- 

 erty of remaining in a viscous state, whereas alumina never does. This 

 " gelatinous flint" is supposed to retain a considerable degree of plas- 

 ticity long after the granitic mixture 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 imprinting their forms on each other, afford- 

 ing evidence of the simultaneous crystallization of both.* 



It may here be remarked that ordinaiy 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 2dly, felspar in 

 smaller crystals, in which soda predominates, usually of a dead white or 

 spotted, and striated like albite, but not the same in composition.! 



Porphyritic granite. — This name has been sometimes given to that 

 variety in which large crystals of common felspar, sometimes more than 

 3 inches in length, are scattered through an ordinary base of granite. 

 An example of this texture may be seen in the granite of the Land's 

 End, in Cornwall (fig. 685). The two larger prismatic crystals in this 



Fig. 685. 



Porphyritic granite. Land's End, Cornwall. 



drawing represent felspar, smaller crystals of which are also seen, similar 

 in form, scattered through the base. In this base also appear black 

 specks of mica, the crystals of which have a more or less perfect hex- 

 agonal outline. The remainder of the mass is quartz, the translucency 

 of which is strongly contrasted to the opaqueness of the white felspar 

 and black mica. But neither the transparency of the quartz, nor the 

 silvery lustre of the mica, can be expressed in the engraving. 



* Bulletin, 2d serie, iv. 1304; and Archiac, Hist, des Progrfes de Geol., i. 38. 

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



