54 



CRYSTALLOGRAPHY. 



Occasional curved crystals occur, as in fig. 26, representing 

 crystals of quartz, and fig. 2/ of a crystal of chlorite. The 



QUARTZ. 



CHLORITE. 



feathery crystallizations on windows, called frost, are examples 

 of curved forms under this system. 



VII. DISTINGUISHING CHARACTERS OF THE SEVERAL 

 SYSTEMS OF CRYSTALLIZATION. 



1. Isometric System. — (1) There may be symmetrical groups 

 of 4 and 8 similar planes about the extremities of each cubic 

 axis; and of 3 or G similar planes about the extremities of each 

 octahedral axis. (2) Simple holohedial forms may consist of 

 6 (cube), 8 (octahedron), 12 (dodecahedron), 24 (trapezohedron, 

 trigonal trisoctahedron, and tetrahexahedron), and 48 (hexoc- 

 tahedron) planes. 



2. Dimetric System. — (1) Symmetrical groups of 4 and 8 

 similar planes occur about the extremities of the vertical 

 axis only. (2) Prisms occur parallel only to the vertical axis; 

 and these prisms are either square or eight-sided. (3) The 

 simple holohedial forms may consist of 2 planes (the bases), of 

 i planes (square prisms), of 8 planes (eight-sided prisms ami 

 ssjuaie octahedrons), of 16 planes (double eight-sided pyra- 

 mids). 



3. Trimetric System. — (1) Symmetrical groups of 4 similar 

 planes may occur about the extremities of either axis, but 

 those of one axis belong equally to the others. (2) The prisms 

 are -rhombic prisms only, and these may occur parallel to either 

 axis, the horizontal as well as the vertical. (3) Simple hole* 



