TI.Ti;.\<;u\.\l. SYSTKM 



67 



FIG. 115. Pyramid of the Second Order, (101), of 

 Cassiterite. 



:ui edge. There is a -eri< s nf pyramids ;A' the first order, their 

 acuteness and general appearance depending upon the value of m. 



III. Tetragonal pyramid of the second order ; a : oo a : me ; (hoi). 

 When the value of n is s. , it> maximum limit, or if the pole is 



moved to coincide with the crystallographieal axes, then in the 

 resulting form of eight 

 Ibices the Mxes will ter- 

 minate in the center of 

 the equatorial edge, 

 yielding a pyramid of 

 the second order, Fig. 

 115. In shape this pyr- 

 amid in no way differs 

 from the pyramid of the 

 first order, with which 

 it becomes congruent by 

 a revolution of 45 around the c axis. There is a series of pyramids 

 of the second order, depending upon the value of m. 



IV. Ditetragonal prism; na : a : oo c ; (hko). 



When the value of n is between its limits and m is infinity, or if 

 the pole in the spherical projection is moved to the primitive circle 

 between the extremities of the didigonal axes, the resulting form 



is the ditetragonal prism, Fig. 116. It is bounded 



by eight similar faces. 



Each face will cut one 



of the lateral axes at 



unity, the other at a 



distance greater than 



unity, and will be par- 

 allel to the c axis; it 



will therefore be an 



open form extending 



to infinity unless ter- 

 minated by combining 



with another form. 



All prisms are open 

 forms. There is a series of ditetragonal 

 prisms, the value of the interfacial angles 

 depending upon the value of n. 



V. Tetragonal prism of the first 

 order; a : a : coc; (no). 



FIG. 117. Prism of the First 

 Order, (110). 



