Determination of Diallage. 133 



ture, taken from Calcareous-spar. Here the faces of composition 

 are parallel to those of R 1, the rhombohedron truncating the 

 terminal edges of R, or the fundamental form ; they are always 

 the consequence of thin laminae, or films of the same substance, 

 engaged in the mass in an opposite sense, whose exact di- 

 rections we can geometrically construct, by supposing one part 

 of the mass contained between two faces parallel with each other, 

 and with one of the faces of R 1, to be revolved 180 round 

 a line perpendicular to these faces, considered as their axis 

 of revolution. Not even the perfectly transparent varieties from 

 Iceland are free from such laminae; their action upon light 

 has led Dr BREWSTER to ascertain their existence. If they 

 occur in greater number parallel to only one of the faces of 

 R 1, they give rise to a variety, in which only one of the faces 

 of cleavage, parallel to R, is very deeply striated in the direction 

 of its horizontal diagonal. The seeming difference in the angles, 

 owing to that composition, has induced BERNHARDI to consi- 

 der these varieties as belonging to a particular species, which he 

 calls Streifenspath, or Striated-spar. It is necessary, therefore, 

 to be very careful, in ascertaining whether an observed face in 

 the interior of a crystal or of a cleaveable mass, be really produced 

 by cleavage, or whether it owe its existence to regular composi- 

 tion. 



Certain faces of cleavage, not parallel to the primitive form, 

 have been said by HAUY to pass alongside of the molecules, with- 

 out touching or intersecting them, which faces, for that reason, 

 he called des joints surnumeraires. This explanation applies re- 

 markably well to the above mentioned faces of composition, 

 which have been so accurately described by HAUY * in a variety 

 of green Diallage, where their pearly lustre is peculiarly bright. 



* Traite, 2de Ed. v. ii. p. 462. 



