COMPOUND MINERALS. 



89 



. 73. REGULAR COMPOSITION OF MORE THAN Two 

 INDIVIDUALS. 



The regular composition of three, four, five, or more 

 individuals may be reduced to, and explained by, the regu- 

 lar composition of two individuals. 



For example, Fig. 149 represents a regular composition of three 

 individuals which occur in Chrysoberyl. The individual 1 forms 

 with 2 a twin crystal, as in Fig. 147, and 2 may be conceived to 

 form with 3 a second composition of the same kind. 



Fig. 150. 



Fig. 150 is a regular composition of Titanite, consisting of a greater 

 number of twin crystals; each of which is readily conceived of, by 

 an attention to the faces of the crystal. The face upon which the 

 crystal stands may be said to form with M one twin crystal ; M with 

 M 3 , a second ; M 2 with M 5 , a third ; M 5 with M 6 , a fourth} 

 M 6 witL MS a fifth. 



In the absence of re-entering angles, the striae upon some of the 

 planes of regularly aggregated crystals, are frequently of such a 

 nature, as to enable us to understand their composition. Thus, the 

 longitudinal markings upon the broad lateral planes of Fig. 149, ren- 

 der evident the different individuals entering into the form ; and the 

 lines which connect its re-entering angles, point out the faces of 

 composition. In the crystal, whose composition is represented in 

 Fig, 150, we observe on plane M s as well as its opposite, distinct 

 lines, bisecting the angles a b c, b c d, and c d e, which meet near 

 the centre of the face at 0, and point out the faces of composition, 



As respects the cleavage of compound crystals, the individual 

 crystals possess the same cleavages as individuals not thus aggre- 

 gated, 



S* 



