ORDER VI. PRISMATIC FELD-SPAE. 261 



for prismatic Feld-spar, N S 3 + 3 A S* for Albite, and 

 N S 3 + 3 C S 3 + 12 A S for Labradorite. Before the 

 blowpipe upon charcoal, prismatic Feld-spar becomes glassy, 

 semi-transparent and white, but melts only with difficulty 

 on its edges into a semi-transparent vesicular glass. It is 

 dissolved by borax, but slowly and without effervescence, 

 into a clear globule. The rest of the species agree in this 

 respect with prismatic Feld-spar. They are not acted upon 

 by acids, except Labradorite, which is entirely dissolved 

 by heated muriatic acid. 



3. The discovery of the difference among the preceding 

 species is too recent to have yet been taken notice of in 

 regard to the geological relations of what formerly used to 

 be called Felspar ; so that wherever only Felspar is men- 

 tioned, we must expect to find one or the other, or perhaps 

 several of the species designated by that common name. 



Common Felspar frequently enters into the composition 

 of rocks, and constitutes, with rhombohedral Quartz, and 

 several species of the genus Talc-mica, the different kinds 

 of granite and gneiss, with hemi-prismatic Augite-spar it 

 forms syenite, green-stone, &c. Generally prismatic Feld- 

 spar and Albite are found at the same time in granite, as in 

 the varieties from Pompey's pillar, and from the block 

 upon which the statue of Peter the Great in Petersburg!! 

 is raised, the Albite being of a greenish-white colour, while 

 the prismatic Feld-spar is flesh-red. Albite is most frequent- 

 ly one of the constituents of syenite and greenstone, as in 

 the neighbourhood of Dresden and Edinburgh. To several of 

 these rocks large crystals of prismatic Feld-spar impart a 

 porphyritic appearance; and it is a characteristic mark of the 

 different kinds of porphyry more properly so called, to have 

 isolated crystals of this species, and also of Quartz, Mica, &c. 

 distributed throughout their compact mass: according to the 

 latter, which in some cases, as in the clinkstone porphyry, 

 itself belongs to the genus Feld-spar, they are distinguished 

 and denominated. Compact Felspar (Labradorite) is fre- 

 quently one of the constituents of greenstone slate, and a 



