. 179. OF COMPOUND MINERALS. 



of a single one ; and the whole is nothing else but a parti- 

 cular case of those mentioned in the explanation of the striae 

 (. 175.), that is to say, it is an alternating repetition of 

 the faces of the simple forms contained in the combination. 



On the other hand, we find real hemitrope or twin- 

 crystals, which yet do not contain any re-entering angles, 

 because the existence of these depends upon the situation 

 of the faces of the simple forms. It is not the re-entering 

 angles, therefore, that form the essential character of the 

 twin-crystals, but the situation of their different parts, 

 which cannot be explained without assuming that they are 

 formed from the composition of two individuals. 



The law according to which twin-crystals are formed, may 

 also be expressed by crystallographic signs ; since it is re- 

 quired only to mention the situation of the axis of revolu- 

 tion and the plane of composition in reference to the crys- 

 talline forms occurring in a species. For this purpose 

 the crystallographic sign of the face, parallel to which the 

 regular composition takes place, is included in braces ; 

 the direction of the axis of revolution is added to it, and 

 separated by the sign : , if it should not be perpendicular 

 to the face of composition. 



After this general consideration of twin-crystals, a few 

 examples will be sufficient for illustration, in so far as is 

 required for our purpose, and for exemplifying the em- 

 ployment of the crystallographic signs. 



Suppose the face of composition to be parallel to a face of 

 crystallisation, and the axis of revolution, perpendicular to it, to 

 beat the same time an axis of crystallisation. 



If we join two octahedrons in a parallel position in such 

 situations, that they come into contact with their own faces, 

 the face of composition will be parallel to one of the faces 

 of the octahedrons, and one of the rhombohedral axes will 

 be common to both these forms. Although the faces of 

 one of them produce re-entering angles with some of the 

 faces of the other, yet the assemblage of the two octahe- 

 drons cannot be considered as a twin-crystal, because the 

 faces of the two forms are exactly parallel to each other. 



