30 



CRYSTALLOGRAPHY. 



them rough instead of polished planes. This has arisen from 



oscillation between octahedral and cubic conditions. 



In some cases crystals are filled out only along the diagonal 

 planes. Fig. 65 represents a crystal of common salt of thia 

 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 forma 

 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- 



1. 



^rfe. 



eJW 



TOOCRASE. 



APOPHYLLITE. 



zikcon. 



iuf rock and have perfect terminating planes only at the other, 

 Bcjuare prisms, with or without pyramidal terminations, square 

 sctahedxons, 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 



