MONOCLINIC SYSTEM. 41 
distinct kinds, being parallel to unlike axes and diametral sec- 
tions. In the latter, as in the trimetric rhombic prism, the 
lateral axes connect the centres of the opposite sides. More- 
over, this rhombic prism may be reduced to the rectangular by 
the removal of its edges by planes parallel to the lateral axes. 

MONAZITE. MIRABILITE. 
The axis a, or the inclined lateral axis (inclined at an oblique 
angle to the vertical axis c), is called the clinodiagonal ; and the 
axis 6, which is not inclined, the orthodiagonal (from the Greek 
for right, or rectangular). The vertical section through the 
former is called the clinodiagonal section ; it is parallel to the 
plane 72 (fig. 1-6). The vertical section through the latter is 

the orthodiagonal section ; itis parallel to planesi-<. Owing to 
the oblique angle between @ and c, the planes above a differ 
in their relations to the axes from those below, and hence comes 
the difterence in the angle they make with the basal plane. 
The halves of a crystal either side of the clinodiagonal section 
—the vertical section through a and c——are alike in all planes 
and angles. Another important fact is this: that the plane 2-2, 
or that parallel to the clinodiagonal section, is at right angles not 
only to O and 7-7, but to all planes in the zone of O and 11; 
