PHYSIOGRAPHY. 277 



Idocrase. 



= 118 8'. (encadree. H.) Fig. 259. Ponr=10S 18'. 

 d on r=. 161 42'. r on * = 153 30'. r on 5 = 152 58'. 

 r on #=150 35'. x on a?=154 28'. z on *=139 52'. 

 (enneacontaedre. H.) 



Cleavage, parallel with M not very distinct, still less so 

 parallel with P. Fracture imperfectly conchoidal, uneven. 

 Surface, P sometimes uneven and curved ; the lateral faces 

 striated parallel to their common intersections, the rest of 

 the faces smooth. 



Lustre vitreous, inclining to resinous, sometimes the lat- 

 ter very distinctly. Color various shades of brown, passing 

 into leek-green, pistachio-green, olive-green and oil-green. 

 Streak white. Semi-transparent . . . faintly translucent on 

 the edges. If viewed in the direction of the axis, the colors 

 incline more to yellow, perpendicular to it, more 1 to green. 



Hardness =6-5. Sp. gr. = 3-399. 



Compound Varieties. Massive : composition granular, 

 of various sizes, sometimes considerable, and often strongly 

 connected. There occur also columnar compositions, 

 generally of thin individuals, straight and divergent or irreg- 

 ular, faces of composition irregularly streaked. 



1. Before the blow-pipe, on charcoal, it melts easily into a pale green 

 glass, rarely attended by effervescence. With borax, it easily melts into 

 a clear glass stained with iron. 



2. Analysis. 



By KLAPROTH. By BORKOWSKY. 



" fr. Vesuvius. fr. Silesia. . fr. Eger, Bohemia. 



Silica . 35-50 . . 42-00 . 41-00 



Alumina . 33-00 . . 16-25 . 22-00 



Lime . 22-25 . . 34-00 . 22-00 



Magnesia . 00 00 . 3 00 



Oxide of iron . 7-50 . . 5-50 . 6-00 



Oxide of manganese 0-25 . . a trace. . 2-00 



Potash . 0-00 . . 000 . 1-00 



24 



