55 



hedral forms may consist of 2 planes (the bases, and each pair 

 of diametral planes), of 4 planes (rhombic prisms in the three 

 axial directions), and of 8 planes (the rhombic octahedrons). 

 (4) The forms may be divided into equal halves, symmetrical in 

 planes, along each of the diametral sections. 



4. Monoulintc System. — (1) No symmetrical groups of 

 similar planes ever occur around the extremities of either axis. 

 (2) The prisms are rhombic prisms, and these can occur paral- 

 lel only to the vertical axis and the clinodiagonal. (3) The 

 planes occurring in vertical sections above and below the basal 

 section, either in front or behind, are all unlike in inclination 

 to that section, excepting the prismatic planes in the ortho- 

 diagonal zone ; in other words, true prisms occur in no vertical 

 section excepting the orthodiagonal. (4) Simple forms consist of 

 2 planes (the bases, the diametral planes, and hemiorthodomes), 

 of 4 planes (rhombic prisms in two directions and hemioctahe- 

 drons). (4) The forms may be divided into equal and similar 

 halves only along the clinodiagonal section. No interfacial 

 angle of 90° occurs except between the planes of the clinodiag- 

 onal zone and the clinopinacoid. 



5. Triclinic System. — In triclinic crystals there are no 

 groups of similar planes which include more than 2 planes, and 

 hence the simple forms consist of 2 planes only. The forms 

 are not divisible into halves having symmetrical planes. There 

 are no interfacial angles of 90°. 



b. Hexagonal System. — Symmetrical groups of 3, 6, and 12 

 similar planes may occur about the extremities of the vertical 

 axis. (2) Prisms occur parallel to the vertical axis, and are 

 either six or twelve-sided (3 in a hemihedral form) and equi- 

 lateral. (3) Simple holohedral forms may consist of 2 planes 

 (the basal), of 6 planes (hexagonal prism), of 12 planes (twelve- 

 sided prisms and double six-sided pyramids), of 24 planes 

 (double twelve-sided pyramids). Simple rhombohedral forms 

 may consist of 2 planes (the basal), of 6 planes (rhombohedrons), 

 and of 12 planes (scalenohedrons) . 



2. TWIN, or COMPOUND CKYSTAIJ3. 



Compound crystals consist of two or more single crystals, 

 united usually parallel to an axial or diagonal section. A few 

 are represented in the following figures. Fig. 1 represents a 

 crystal of snow of not unfrequent occurrence. As is evident 



