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, but 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 hort- 
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. MONOCLINIC 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 and the vertical axis 
c. The following examples of its crystalline forms, figs. 1 to 6, 
show the effect of this obliquity. On account of it the front 
or back planes above and below tl.e middle in these tgures 
differ, and the anterior and postericr prismatic planes are une- 
qually inclined to a basal plane. 

PYROXENE. FELDSPAR. WORNBLUNDE. 
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 cpposite faces, and the planes are of three 
