40 STRUCTURE OF MINERALS. 



When the planes a! in figure 64 are a little more extended, 



the form is changed to figure 65, or a double six-sided pyramid. 



It is in this way that the pyramidal form of crystals of quartz 



s produced from the primary rhombohedron. In figure 66, 



66 67 68 6B 



a', as is seen, is a different plane from a' in figure 64. By 

 enlarging the planes a', till the planes R are obliterated, 

 figure 67 is obtained, an acute rhombohedron. This may 

 appear a singular result : but it will be understood on con- 

 sidering that there are six lateral angles ; and three of the 

 planes a' incline upward, and three, alternate, incline down- 

 ward ; they must therefore produce an oblong solid, bounded 

 by six equal faces, which is nothing else than a rhombohedron. 

 In figure 68, the lateral edges are beveled by the planes e'. 

 The planes e enlarged to the obliteration of the faces 

 R ft lead to the form in figure 69 — a twelve-sided figure, or 

 dodecahedron, and called from the shape of its faces, a 

 scalene dodecahedron. It is the form of dog-tooth spar, a 

 variety of calcareous spar. In figures 28, 29, the planes e 

 and a are each parallel to the vertical axis, and they con- 

 sequently produce prisms when extended, as explained on 

 pages 31, 32. 



In figure 3, under Tourmaline, we have an instance of a 

 hemihedral modification in the hexagonal, system. The ex- 

 tremities of the prism, as will be observed, have different 

 secondary planes, there being- in addition to the three faces 

 R, three small triangular planes above, and three narrow 

 linear planes below. Topaz crystals are also differently 

 modified at the extremities, and are examples of hemihedral 

 modifications in a right rhombic prism. 



Another law gives still greater interest to the study of 

 crystallography : but it can only be briefly alluded to in this 

 place. When speaking of the right square prism it was 



Mention some instances of hemihedral modifications, and explain. 



