166 



Scientific Proceedings, Royal Dublin Society. 



characteristic cruciform twinning ; but a more obscure lamellar 

 form, of the same mineral probably, is intermingled with the 

 larger crystals. These larger crystals have a high vitreous lustre, 

 and are white, translucent, transparent. The lamellar forms are 

 duller in lustre and are white, nearly opaque. 



Cry stallo graphic character. — The accompanying figure, in iso- 



metric projection, shows the nature of their crystallographic appear- 

 ance. It differs somewhat from that ascribed to harmotome by 

 Dana, Des Cloizeaux, &c, due to the conspicuous development of the 

 hemihedral form 1 (copying the notation of Dana), while still pre- 

 serving the holohedrism of the prism I. It will be seen that this 

 development has reduced one set of the prism faces to minute 

 dimensions. Indeed they can hardly be seen on the specimen 

 without the use of a lens. In any other specimens I have 

 examined the prism is either predominant — 1 having the appearance 

 of a mere bevelling of the edge 01, or otherwise, it is eliminated 

 altogether, I becoming a hemihedral form. The effect is that the 



