68 



MINERALOGY 



When the value of n is unity and that of m is infinity, or let the 

 pole be moved on the equatorial plane to coincide with the inter- 

 mediate axes, then the resulting form is the tetragonal prism 



j of the first order, Fig. 117. It will be 



bounded by four faces, cutting the c axis at 

 infinity, the a axes at unity. The lateral 

 axes terminate in the middle of the edges. 



VI. Tetragonal prism of the second 

 order; ooa:a:ooc; (oio). 



When the value of both n and m is at 

 infinity, their maximum limit, or if the pole 

 be moved in the equatorial plane to coin- 

 cide with the crystallographic axes, then a 

 rectangular prism results, the tetragonal 

 prism of the second order, which in shape 

 Fia. 118. The Tetragonal differs in no way from the prism of the 



Prism of the Second Order. firgt Qrder except the ft axeg terminate in 



the center of the faces. It becomes congruent with the first order 

 prism by a revolution of 45 around the c axis, Fig. 118. 



VII. Basal pinacoid; oca: ooa:c; (ooi). 



The only possible position of the pole remaining is when it coin- 

 cides with the c axis, when all 

 eight faces above the equato- 

 rial plane will form one face 

 and all the faces below will fall 

 in one plane, yielding a form 

 of two faces, the tetragonal 

 base or basal pinacoid which 

 extends to infinity on all sides, 

 Fig. 118, c. All pinacoids cut 

 but one axis and are parallel to 

 the other two ; in combination 

 with prisms they inclose space. 

 The fixed forms of the tetrago- 

 nal system are the base and the 

 prisms of the first and second 

 orders ; these correspond to the 

 fixed forms of the isometric system, as do also the position of their 

 poles in the triangle, viz., the corners or angles; as here there is 

 evidently only one position for the pole, there is only one form 

 possible. 



FIG. 119. Combination of (111) 

 (110) (100) of Cassiterite. 



(101) 



