COMPOUND MINERALS. 



87 



4. Fig. 145 represents a crystal, in which the face of composition 

 ar'r r b is perpendicular to the axes of the aggregated crystals. By 

 causing the part above the face of composition to revolve upon an 

 axis perpendicular to that face, through 60, the twin crystal, Fig 

 145, is generated. This form is met with among the crystals of 

 Carbonate of Lime. Setting aside the new planes tt and ft 1 at the 

 summits of this twin crystal, it may be seen, by consulting Fig. 131, 

 how it differs from the crystals to whose composition it is due. 



5. If we imagine two crystals of the form of a right rectangular 

 Prism, terminated by four sided pyramids, to be placed with their 

 broad lateral planes parallel, and made to penetrate each other, so 

 that their axes shall coincide : and if one of the crystals, by revolv- 

 ing upon the common axis, through 90, be conceived to emerge 

 from its concealment in the other, it will originate the twin crystal, 

 Fig. 146, which is a very frequent composition among the crystals 

 of Harmotone. 



Fig. 145. 



Fig. 146. 



6. Let us imagine one of the irregular six sided prisms, Fig. 147, to 

 consist of two similar individuals, whose axes (by the mutual penetra- 

 tion before described,) coalesce. Now if we suppose one of them 

 to emerge, by revolving upon an axis perpendicular to the prismatic 

 axis of the aggregated crystals, through 60, we have the twin crys- 

 tal there represented, which is a common form of Staurotide. 



