26 



MINERALOGY 



pairs of parallel faces, and for that reason they are known as the 

 diagonal-faced hemihedrons, represented in the Miller system of 

 indices by ic(hkl). 



II. By selecting one half of the pairs of faces, taking those which 

 intersect in the axial planes, as represented in Fig. 37 ; these when 



FIG. 37. 



FIG. 38. 



extended will produce the hemihedral class, Fig. 38, known as the 

 parallel-faced hemihedrons, designated in the Miller system by 



III. By selecting every other face around the extremity of an axis, 

 and those alternating with it around the adjacent axis, as repre- 

 sented by the shaded faces in Fig. 39 ; these when extended will 



FIG. 39. 



FIG. 40. 



produce the gyroidal, or plagiohedral, class of hemihedrons, repre- 

 sented in Fig. 40. These are denoted in Miller's system by r(hkl). 



