40 



CRYSTALLOGRAPHY . 



5. Irregularities in Crystals. — The crystals almost never cor- 

 respond in their diametral dimensions with the calculated axial 

 dimensions. They are always lengthened, widened, shortened, 

 or narrowed abnormally, bat without affecting the angles. Ex- 

 amples of diversity in this kind of distortion are given in figs. 

 1 to 7, of barite. 



6. Distinctions. — In the trimetric system the angle 135° does 

 not occur, because the three axes are unequal. There are pyra- 

 mids of four similar planes in the system, but never of eight ; 

 and the angles over the terminal edges of the pyramids are 

 never equal as they are in the dimetric system. The rectangu- 

 lar octahedron of the trimetric system is made up of two hori- 

 zontal prisms, as shown in fig. 6, and is therefore not a simple 

 form ; and it differs from the octahedron of the dimetric sys- 

 tem corresponding to it (fig. 16, p. 32) in having the angles 

 over the basal edges of two values. 



IV. MOXOCLDsIC SYSTEM. 



1. Descriptions of Forms. — In this system the three axes are 

 unequal, as in the trimetric system ; but one of the axial inter- 

 sections is oblique, that between the axis a 'vnd the vertical axis 

 c. The following examples of its crystalline forms, figs. 1 to G, 

 show the effect of this obliquity. On account of it the front 

 or back planes above and below the middle in these figures 

 differ, and the anterior and posterior prismatic planes are une- 

 qually inclined to a basal plane. 



<0~V 



PYROXENE. 





3 







L 









A 



6 



s 



7s 



rV 





I 



2z 



sj/ 



FJ 



£LDS 



PAl 



\. noii.SE 



EjEI 



sDE 



The axes and their relations are illustrated in figs. 7 and 8. 

 Fig. 7 represents an oblique rectangular prism, and fig. 8 

 an oblique rhombic. The former is the diametral prism, like 

 the rectangular of the trimetric system. The axes connect 

 the centres of the opposite faces, and the planes are of three 



