TKTKACONAL SVSTKM 



73 



na : a : me 



ir(hkl) 



Forms 

 I. Tetragonal pyramid of the third order ; 







(hkl). 



The faros represented by the poles in Fig. 131 form a pyramid 

 milur in shape to the tetragonal pyramids of the first and second 



orders, differing 



only in that the 



lateral axes 



terminate in the 



equatorial edge 



between the tet- 



rahedral an^k 



and the middle. 



A diagram of the 



equatorial plane 



is represented in 



Fig. 134; aa is 



e pyramid of the first, ee is the pyramid 



>f the second order, both being fixed forms ; 



b is the ditetragonal pyramid, a variable form lying between the 



:ed forms as its limiting forms ; the dotted line represents the ex- 



nsion of one half of the faces, as ab to d, where it intersects at 



MO. 131. Type 24: Tetragonal 

 Equatorial. 



Fio. 132. 



Fio. 133. Pyramid of the Third 

 Order of Scheelite, (313). 



with the extension of the alternate face ad ; note that the axes 

 tliis pyramid terminate not in the angle, nor in the middle of the 

 Ige, but at a point on the edge somewhere between them. This 

 lingnim will represent the relation of the prisms equally well. 



