ORDER VI. HHOMBOHEDRAL KOUPHONE-SPAR. 233 



of composition parallel, axis of revolution perpen- 

 dicular to R oo, the individuals being continued 

 beyond the face of composition. Fig. 128. Aussig, 

 Bohemia; Fig. 173. Giant's Causeway. 2. Face 

 of composition parallel, axis of revolution perpen- 

 dicular to one of the faces of R, the individuals ter- 

 minating at the face of composition. Fassa, Tyrol. 

 Massive : composition granular, of various sizes ; 

 feces of composition uneven. 



OBSERVATIONS. 



1. According to the optical researches of Dr BREWSTEE, 

 the rhombohedron, given above as the fundamental form 

 of rhombohedral Kouphone-spar, is composed in a remark- 

 able symmetrical manner of several solids, possessing each 

 of them two axes of double refraction. They are often 

 disposed in laminae round a kernel, which has only one 

 optical axis, coincident with the principal axis of the rhom- 

 bohedron. 



2. According to VAUQUELIN, the present species con- 

 sists of 



Silica 43-33. 



Alumina 22-60. 



Lime 3-34. 



Potash with Soda 9-34. 



Water 21-00. 



Alone before the blowpipe it melts into a white spumous 

 mass, and is not acted upon by acids. 



3. Also this species occurs, like several others of the 

 present genus, chiefly in the cavities of amygdaloidal rocks, 

 the sides of which are often coated with Green-earth. It 

 is accompanied by other species of the genus Kouphone- 

 spar, by rhombohedral Lime-haloide and rhombohedral 

 Quartz. It occurs also in narrow veins in different trap 

 rocks. 



