240 PHYSIOGRAPHY. CLASS II. 



ces of crystallisation and of cleavage, exhibit per- 

 fect pearly lustre. Colour white prevalent, va- 

 rious shades, passing into yellow, red, and brown. 

 Streak white. Semi-transparent ...translucent. 

 Brittle. Hardness = 3-5.. .40. Sp. Gr. = 2-161, 

 white crystals from Iceland. 

 Compound Varieties. Twin- crystals : face of 

 composition parallel, axis of revolution perpendicu- 

 lar to one of the faces of Pr, the individuals are 

 continued beyond the face of composition, so that 

 the whole assumes a cruciform aspect.* The crys- 

 tals are frequently aggregated in the form of sheafs. 

 Implanted globules, surface very drusy, composi- 

 tion imperfectly columnar, and strongly cohering. 

 Massive : composition imperfect columnar, indivi- 

 duals broad, straight, and radiating from a com- 

 mon centre, strongly coherent. Often these com- 

 positions are again aggregated into granular masses. 

 Globular shapes formed in vesicular cavities. 



OBSERVATION'S. 



1. The present species was first distinguished from the fol- 

 lowing one, in the Characteristic of the Natural History Sys- 

 tem. Their difference was afterwards pointed out by Mr 

 BROOKE, who gave the latter species the name of Heulanditc. 

 They are chiefly distinguished by their forms, which are 

 prismatic in Stilbite, and hemi-prismatic in the Heulandite ; 

 their hardness and specific gravity agree very near with 

 each other. This difference in the forms is not only suffi- 

 cient, but its necessary consequence is the perfect diversity 



* A beautiful crystal of this variety from Faroe, is pre- 

 served in the cabinet" of Mr ALLAN. H. 



