PHYSIOGRAPHY. 



Feldspar. 



199 



Cleavage, parallel with P highly perfect, and easily ob- 

 tained ; parallel with M perfect, with T obscure, though 

 sometimes obvious. Fracture conchoidal to uneven. Sur- 

 face frequently streaked in a horizontal direction ; most of 

 the other faces are smooth. 



Lustre vitreous, sometimes inclining to pearly upon per- 

 fect faces of cleavage. Color white, prevalent, inclining to 

 grey, green and red ; sometimes grey, flesh-red, verdigris 

 green. Streak greyish white. Transparent, translucent 

 on the edge. A bluish opalescence observable in the di- 

 rection of T, most distinctly in transparent varieties. The 

 variety called moonstone, from Ceylon, appears considera- 

 bly more red, and of a lower degree of transparency, if 

 viewed perpendicularly to T, than in any other direction. 



Brittle. Hardness = 6-0. Sp. gr. = 2-558, a white 

 transparent variety ; limits of the species 2.53 . . . 2'60. 



Compound Varieties. Twin-crystals. 1. Face of 

 composition parallel with the edge between P and M. 

 (Fig. 189.) Axis of revolution perpendicular to the plane 

 d gfl k h u of the same figure. Angle of revolution = 

 180. See annexed figure. 



Fig. 190. 



If this mode of composition be repeated on all the faces 

 of the same form, four sided prisms, consisting of four indi- 



