MODIFICATIONS. 



259 



Decrements on the edges, may be represented by the 

 general symbol P B P , 



If p 1, the symbol represents mod. i. 

 p ^> 1 class k. 



Some of the secondary crystals of Blende are pro- 

 duced by defective modifications of this primary form, 

 and are such as might result from regular modifi- 

 cations of the tetrahedron. 



The Octahedron with a square base. 

 Fig. 310. 



Simple and mixed decrements on the terminal edges. 



General symbol, P A P . 

 p 



If p =. 1 ? the symbol represents mod. a. 

 p ^> 1, class b. 



Intermediary decrements on the terminal solid angles. 



These are of two kinds, and require two general 

 symbols. 



1st. (Bp B' q b'q br) represents class c ; 



2d. (B P B x q b'r bs : B P B'r b' q bs) represents class d. 



Simple or mixed decrements on the lateral angles. 

 General symbol P E P . 



1, the symbol represents class e. 

 p > 1, . class h. 



