258 PHYSIOGRAPHY. CLASS n. 



bination between _ and , that is, to the line a b. 

 2 2 



The degrees of transparency of the varieties hitherto ob- 

 served are lower than those of prismatic Feld-spar or Albite. 



iv. Common Felspar from Baveno. 



These varieties are particularly distinguished from the 

 rest by that highly perfect cleavage parallel to _, 



which far surpasses the rest observable in the same species 

 parallel to (r + co) 3 , r + cs and Pr -f co, and by their 

 specific gravity, which is much inferior to all the species of 

 the present genus, having been found in a crystal = 2-392. 

 The limits appear to be 2-39 and 2-45. The combinations 

 are hemi-prismatic, and from observations with the com- 

 mon goniometer they seem not to be very different from 

 those of prismatic Feld-spar. They possess very low de 

 grees of transparency, and often occur massive, having 

 distinct lamellar particles of composition. 



In the older systems, all these different species, and 

 moreover Chiastolite or Made, were comprised Avithin the 

 common name of Felspar, and variously subdivided into sub- 

 species and kinds. First those grey varieties which possess 

 bright iridescent colours were separated from the rest, and 

 established into a particular subspecies under the name of 

 Ldbradore Felspar. It comprehends varieties of prismatic 

 Feld-spar, and of Labradorite, as has been observed above. 

 The most transparent and pure varieties, generally in im- 

 planted crystals lining the walls of narrow veins in ancient 

 rocks, were likewise considered as a particular subspecies, 

 and called Adularia, in reference to the supposed oldest lo- 

 cality known of this variety. It is made up of varieties 

 of prismatic Feld-spar and of Albite. The less transparent 

 varieties were divided into common and compact Felspar, 

 the first of which contained, though not exclusively, easily 

 cleavable crystals, the second imbedded crystals having no 



