30 CRYSTALLOGRAPHY. 
them rough instead of polished planes. This has arisen from 
oscillation between octahedral and cubic conilitions. 
In some cases crystals are filled out only along the diagonal 
planes. Fig. 65 represents a crystal of common salt of this 
kind, having pyramidal depressions in place of the regular faces. 
Octahedrons of gold sometimes occur with three-sided pyram- 
idal depressions in place of the octahedral faces. Such forms 
sometimes result when crystals are eroded by any cause. 
II. DIMETRIC, ok TETRAGONAL SYSTEM. 
1. Descriptions of Forms.—In this system (1) the axes cross at 
right angles; (2) the vertical axis is either longer or shorter than 
the lateral; and (3) the lateral axes are equal. 
The following figures represent some of the crystalline forms. 
They are very often attached by one extremity to the support- 

IDOCRASE. APOPHYLLITE. 
ing rock and have perfect. terminating planes only at the other. 
Square prisms, with or without pyramidal terminations, square 
octahedrons, eight-sided prisms, eight-sided pyramids, and espe- 
cially combinations of these, are the common forms. Since the 
lateral axes are equal, the four lateral planes of the square 
prisms are alike in lustre and surface-markings. For the same 
reason the symmetry of the crystal is throughout by fours; that 
is, the number of similar pyramidal planes at the extremity is 
either four or eight; and they show that they are similar by 
