. 179. OF COMPOUND MINERALS. 49 



tion. The crystallographic sign becomes P + 1 { 5 } , or 



p + ].[]?+ ],{;}. 



Sometimes the faces of infinite forms are faces of com- 

 position. Thus, in paratomous Augite-spar the crystalline 



-p w 

 form (Pr + co) 3 . Pr + co is composed in the face of 



Pr + 05, the axis of revolution being perpendicular to this 



face. The re-entering angles are produced by the hemi- 



p 



prismatic form The crystallographic sign of this re- 

 gular composition is (Pr + co) 3 . Pr + co, {Pr + cs}. 



Hemi-prismatic Augite-spar gives a similar example. The 

 form is - . _. (Pr + co) 3 . Pr + co, the face of composi- 

 tion is parallel to Pr + co, and the axis of revolution per- 

 pendicular to it. This composition, however, by a curious 



anomaly, commonly does not present any re-entering angles, 



p 

 although they should occur, both on account of _ and 



of-*. 

 2 



To this class likewise belong the well known cruciform 

 twin-cr} T stals of paratomous Kouphone-spar and prismatoi- 

 dal Garnet. The crystalline form of the first is P. Pr -f co. 

 Pr + co ; the face of composition is P 4- co, and the axis of 

 revolution is perpendicular to it, Vol. II. Fig. 40., without 

 the faces t. The form of the other is P co. P + co. 

 Pr + co ; the face of composition is one of the faces of f Pr, 

 and the axis of revolution is perpendicular to it. 



We may observe, in respect to the latter two twin-crys- 

 tallisations, that they represent in some measure -the double 

 of what we have seen in the preceding cases, so that a more 

 symmetrical assemblage of the two individuals is produced, 

 which is not the case with the greater number of the other 

 twin-crystals. This peculiarity is owing to the circum- 

 stance that the individuals do not terminate at the face of 



