86 



TERMINOLOGY. 



entering angle, in this twin crystal, is not always observable ; 

 times it is effaced by the undue enlargement of the faces rr and r ! r' f 9 

 so that they join and form one termination, while the planes P P f 9 

 whose faces correspond to the obtuse lateral edges, form the oppo- 

 site termination. 



3. Another twin crystal, common among the forms of Oxide of 

 Tin, may be conceived of in the following manner. Supposing, as 

 in the former instances the mutual penetration of two crystals, Fig. 

 142, is a composition of two right square Prisms, surmounted by 

 four sided pyramids, whose prismatic axes coincide : the face of 

 revolution ahey, is situated parallel to the edge om of the pyra- 

 mid. If the part situated above the plane ahey is made to under- 

 go a semi-revolution, around an axis perpendicular to that plane, 

 the twin crystal, represented by Fig. 143, is produced. 



Fig. 142. 



The regular composition just explained 

 is never so simple as here represented : 

 a large number of additional faces are 

 usually present, which we have omitted, 

 in order to render the explanation more 

 easy. 



Fig, 144 is a twin crystal of Titanite, 

 whose formation, it will be seen, admits 

 of a similar explanation. These crystals 

 are sometimes called geniculated forms. 



Fig. 144. 



