54 CRYSTALLOGRAPHY. 
Occasional curved crystals occur, as in fig. 26, representing 
crystals of quartz, and fig. 27 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. Isomerric System.—(1) There may be symmetrical groups 
of 4 and 8 similar planes about the extremities of each cubic 
axis; and of 3 or 6 similar planes about the extremities of each 
octahedral axis. (2) Simple holohedral forms may consist of 
6 (cube), 8 (cctahedron), 12 (dodecahedron), 24 (trapezohedron, 
trigonal trisoctahedron, and tetrahexahedron), and 48 (hexoc- 
tahedron) planes. 
2. DimEeTRIc SystemM.—(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 holohedral forms may consist of 2 planes (the bases), of 
4 planes (square prisms), of 8 planes (eight-sided prisms and 
Square octahedrons), of 16 planes (double eight-sided pyra- 
inids). 
3. TRimMETRIC 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 holo- 
