F E L 



[ no] 



F E L 



an earthy, vitreous, or compact 

 state, it forms the basis of all lavas, 

 and of the quarter number of trap 

 rocks. Associated with augite, 

 and usually in a vitreous form, it 

 constitutes some of the well-known 

 modern volcanic basalts, in which 

 the greater or less preponderance of 

 the latter mineral confers the more 

 or less black, dense, and ferrugi- 

 nous character, which they often 

 assume. Mixed with hornblende, 

 it forms a large class of ancient 

 rocks, also called basalt when the 

 minerals are intimately blended, 

 or greenstone when each is dis- 

 tinguishable. In another condition, 

 felspar, in a glossy but loosely 

 aggregated state, composes a rock 

 of porous, rough, and earthy aspect, 

 called trachyte. In a compact 

 state, the same mineral is the base 

 of many of the porphyries ; and in 

 a more or less crystalline form, 

 associated with quartz, mica, and 

 other minerals, it composes the 

 great class of granite rocks. As 

 felspar is not found in any of the 

 aqueous sedimentary deposits, ex- 

 cept in a decomposed or regenerated 

 state, it may therefore be considered 

 the most characteristic ingredient 

 of all igneous rocks. Professor 

 Jameson divides felspar into five 

 species, namely, 1. Khombohedral 

 Feldspar, or Nepheline. 2. Pris- 

 matic Feldspar, or Common Feld- 

 spar. 3. Tetarto-prismatic Feld- 

 spar, or Scapolite. 4. Polychro- 

 matic or Labrador Feldspar. 5. 

 Pyramidal Feldspar, or Scapolite. 

 1. The rhombohedral feldspar, 

 or Nepheline of Haiiy and Werner, 

 is of a white or grey colour, and 

 occurs both massive and crystal- 

 lised; it is externally splendent, 

 internally vitreous and shining. 

 Cleavage fourfold. Fracture con- 

 choidal, melts with difficulty before 

 the blow-pipe. Its crystals form 

 druses. It occurs in drusy cavi- 

 ties. Its constituent parts are, 



according to Gmelin, silica 43-46, 

 alumina 33*49, soda 13-36, potass 

 7'13. Other authors, however, 

 give a different analysis, stating 

 lime and oxide of iron to form a 

 portion of its constituents. 



2. Prismatic feldspar, or common 

 feldspar. The prismatischer feld- 

 spath of Mohs. Potash feldspar. 

 Of this there are many varieties, 

 namely, adularia, or moonstone, a 

 transparent variety with a silvery 

 or pearly opalescence ; glassy feld- 

 spar, a translucent variety, with 

 various shades of colour, such as 

 white and red, which from its 

 abundance has obtained its name ; 

 amazon-stone, a blue or green 

 variety ; Norwegian Labrador feld- 

 spar, a dark-green variety with a 

 beautiful changeableness of colour, 

 obtained from Frederickswarn, in 

 Norway ; compact feldspar, a 

 feebly translucent variety, with a 

 splintery fracture; slaty feldspar, 

 or clinkstone, a slaty variety ; por- 

 celain earth, earthy feldspar, and 

 claystone, varieties, in a compara- 

 tively loose state of aggregation, 

 without lustre or transparency, and 

 varying in their degree of com- 

 pactness. 



3. Tetarto-prismatic feldspar, or 

 albite. See AlUte. 



4. Polychromatic or Labrador 

 feldspar. Lime feldspar. The 

 polychromatischer feldspath of 

 Mohs. This beautiful mineral 

 was first discovered on the coast of 

 Labrador, as a constituent part of 

 syenite. When light falls on it in 

 certain directions it exhibits the 

 most beautiful changeability of 

 colour. It occurs massive and 

 disseminated. Cleavage splendent. 

 Fracture glistening. It has been 

 subsequently found in different 

 parts of Europe. It contains about 

 eleven per cent, of lime and four 

 of soda. It breaks into rhom- 

 boidal fragments. In its change- 

 ability of colour, it exhibits patches 



