34 



NEW YORK STATE MUSEUM 



gous to those described under the prisms. Some combinations 

 in this group, which includes many important species, are shown 

 in fig. 128-34. 



Hemimorphic group 



The comparatively few species crystallizing in this group 

 occur in forms which are symmetric to two planes of sym- 

 metry passing through the basal axes and intersecting in the 

 vertical axis which is an axis of binary symmetry. 

 The two extremities of the vertical axis are not 

 modified in the same way, giving a different 

 termination to the two extremities of the crystal. 

 The crystal of calamin shown in fig. 135 gives a 

 good example of this type. 



Fig. 135 



referred to 



MonocHnic system 



Crystal forms in the monoclinic system are 

 three unequal uninterchangeable axes, two 

 of which are inclined at an angle to each other, the third 

 being perpendicular to the plane of the other two. The inclined 

 axis which is placed vertical is designated by c, the other in- 

 clined axis by a and the normal or orthodiagonal axis by b. 



A monoclinic crystal is represented con- 

 ventionally with the orthoaxis (b) extending 

 from right to left and the clinoaxis (a) dip- 

 ping downward from back to front, the 

 acute angle between the vertical and clino 

 axes being designated by j (fig. 136). The 

 statements regarding axial ratio under the 

 discussion of the orthorhombic system 

 apply in the case of monoclinic species with 

 the additional note that the angle ,5 varies 

 for every species and constitutes one of the factors to be de- 

 termined. 



Normal group 



Forms of the normal group are symmetric to one plane of 

 symmetry, which is the plane of the clino and vertical axes, and 

 to one axis of binary symmetry, which is the orthoaxis (fig. 137). 



Fig. 136 



