ORDER VI. PRISMATIC FELD-SFAR. 253 



Pr 



uneven. Surface frequently streaked, in a ho- 

 rizontal direction, the vertical prisms parallel to 

 the axis, so is also Pr -f- oo ; (Pr-j- GO) 5 is often 

 rough. Most of the other faces are smooth. 

 Lustre vitreous, sometimes inclining to pearly upon 

 the perfect faces of cleavage. Colour white, 

 prevalent, inclining to grey, green, or red ; some- 

 times grey, flesh-red, verdigris-green. Streak 

 greyish-white. Transparent, translucent on the 

 edges. A blueish opalescence observable in 

 the direction of Pr -f- OD, also sometimes of 



- - : most distinctly in transparent varie- 

 ties. The variety called Moonstone, from Cey- 

 lon, appears considerably more red and of a 

 lower degree of transparency, if viewed perpen- 

 dicular to Pr + oo, than in any other direction. 

 Brittle. Hardness = 60. Sp. Gr. = 2-558, a 

 white transparent variety ; limits of the spe- 

 cies = 2-53 ... 2-60. 



Compound Varieties. Twin-crystals. 1. Face 

 of composition parallel, axis of revolution perpen- 

 dicular to a face of ^1. Fig. 79. If this 

 mode of composition be repeated on all the faces of 

 the same form, four-sided prisms consisting of four 

 individuals will be formed, which are nearly rectan- 

 gular, and bounded on their extremities by the faces 



of (r + oc) 3 and ?F, while the faces of *1 



2 2 



constitute their apparently lateral planes, This 



