TRANSLATIONS AND NOTICES 



OF 



GEOLOGICAL MEMOIRS, 



On Silica in Igneous Eocks. By Baron EiCHTOFEiir. 



[Proceed. Imp. Geol. Instit. Vienna, March 15, 1859.] 



In quartziferous and trachytic porphyries silica occurs constantly in 

 the shape of perfect crystals ; while in the different varieties of gra- 

 nite, crystals of other mineral substances are generally impasted in 

 the quartz. The chemical and mineralogical composition* being iden- 

 tical in the three quartziferous types of the granitic, porphyritic, 

 and trachytic series, the dijfferences in their exterior evolution can 

 only be accounted for by the circumstances in which they underwent 

 solidification. In the granite the orthoclase and oligoclase were 

 solidified previously to the quartz: an anomaly explained by the 

 difference between the point of solidification and the point of fusion, 

 and by a protracted viscosity of the quartz : an opinion quite ad- 

 missible if applied to the slow refrigeration of a liquid magma on the 

 surface of the ground. In the quartziferous and trachytic porphyries, 

 quartz was first segregated ; then orthoclase and sanidine, and lastly 

 oligoclase. These two groups of rocks having been protruded at a 

 far later period, when refrigeration of the earth's crust had greatly 

 progressed, two phases of solidification must be distinguished : one 

 of very slow refrigeration, coinciding with the general refrigera- 

 tion of the globe ; the other more accelerated, beginning only after 

 protrusion on the surface. The first gave origin to the most refrac- 

 tory minerals ; the other to the felspathic paste : the sharp outlines 

 of the crystals imbedded in it prove the change of circumstances to 

 have taken place without transition. 



The quartziferous and trachytic porphyries of the Old Eed and 

 Trias periods exhibit other peculiarities. In the first of these rocks, 

 silica is constantly crystallized in double pyramids without any 

 prismatic planes ; in the second of them, the prismatic planes, 

 though constantly subordinate to those of the pyramid, generally 

 make their appearance. In some varieties of undulated structure 

 silica remained partly within the fmidamental paste and unequally 

 distributed within it. In the trachytic porphyries the siKca forms 

 well-defined milk-white veins resembhng chalcedony. 



[Count M.] 



^ The slight differences between orthoclase and sanidine not being regarded, 

 VOL. XV. PAET 11, '" E 



