TERMINOLOGY. 



If the angle of revolution is 90, we have a different twin crystal, 

 as represented in Fig. 148, which is also a frequent composition 

 among the crystals of Staurotide.* 



Fig. 147. 



Fig. 148. 



In detecting twin crystals, we are frequently assisted by the obser- 

 vation of the re-entering angles : though it very often happens that 

 such angles are found in simple crystals, as was just mentioned, and 

 also in irregular aggregates of crystals. The re-entering angle in 

 twin crystals is sometimes not visible, also in consequence of its be- 

 ing closely enveloped in the gangue in which the crystals are enga- 

 ged, and in other instances it is effaced, apparently by the increase 

 of the crystal upon certain planes, after the individuals have assu- 

 med their situation as a regular composition. When the re-enter- 

 ing angle does not permit us to detect such forms, we are led to their 

 discovery from the impossibility of explaining them without assu- 

 ming that they are formed from the composition of two individuals, 

 according to some one of the methods above explained. 



* The two twin crystals just described, differ in other respects, than in 

 the angle of revolution. In Fig. 148, it is perceived that the face M, 

 of one crystal constantly corresponds to the face M, of the other crys- 

 tal, the face o, to the faceo'; and in Fig. 147, this correspondence does 

 not occur : in the upper part, truly, the face o, corresponds to the face o' y 

 and in the lower part, the face M, to the face M' ; but, laterally, the 

 face o' of one crystal, corresponds to the face M of the .other, and th 

 same of the rest. 



