130 



re-entrant one. The twin-formation is never seen to be repeated. 

 The face è{010} is in the twins generally, especially badly 

 developed; the best forms have given the following values: 



Average 

 value 



Number of Calculated 

 measurements value 



25° 11'— 26° 47' 4 26°22' 



-~Æ 



CL 



Variations 

 Ь:Ь = (010): (010) = 26° 1' 



According to the calculation the 

 faces a, è, è, and a are not lying in 

 one zone. By calculation we see that 

 the two edges («, b) and \ab) form 

 with each other an angle of 22', which 

 fact together with that of the angle 

 Of : Ъ being 76° 49' will involve that 

 the edge between Ь and Ъ gets an 

 inclination of 46V2' towards the plane 

 a{lOO). This position outside of the 

 zone is not, however, seen by the 

 measuring , especially on account of 

 the faces of &{010} giving no good 

 reflexes, but only indistinct and very 

 complicated spots of light; neither 

 are the crystallographic elements 

 especially exact, and comparatively 

 insignificant alterations in them may 

 quite transform these 

 small values. 



The twin-forma- 

 tion cannot be ob- 

 served through the 

 optical relation, as 

 the schizolite, as 

 will be shown here- 

 after, is in this re- 

 spect possessed of 

 Fig. 18. Schizolite. Twin crystal. monolinic symme- 



Fi2 17. Schizolite. 



