WOME XI. GRANITE AND GRANITIC PORPHYRY. 



the laminar texture of felspar, causing its loss, 

 without depriving the felspar of the faculty of 

 assuming the exterior forms of its usual crystal- 

 lisation. This is perceived in those felspars, 

 which constitute the large spots in green por- 

 phyry, called serpentine antico ; and still more in 

 the felspars, which mingled with green horn- 

 blende form the granites called Egyptian greens, 

 It frequently happens that their compact frac- 

 ture no longer presents any indication of a lami- 

 jiar texture, though they still affect the quadran- 

 gular prismatic form, which belongs to their 

 mode of crystallisation. 



" Just as in the magma of mother- waters, re- 

 duced to a state of paste by evaporation, there 

 are particles which, escaping from the viscidity 

 of the medium in which they are engaged, agp 

 gregate and form crystals, which are found bu- 

 ried in the mass : in the same manner, in these 

 kinds of magma of the great precipitation, it is 

 rare that some isolated crystals are not found 

 among them ; and which have acquired so much 

 [more bulk and regularity, as they have had 

 more facility of aggregation. They are distin- 

 guished from the paste which contains them, by 

 their form, their tissue, and almost always by 

 their colours, brighter than that of the base. 

 Thus are formed rocks called porphyries j and 



vol. ii. N 



