MONOCLINIO SYSTEM. 



41 



distinct kinds, being parallel to unlike axes and diametral sec- 

 tions. In the latter, as in the trimetris lhoinbic 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. 



6. 



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 tho 

 former is called the clinodiagonal section y it is parallel to the 

 plane il (fig. 1-6). The vertical section through the latter is 



the orthodiagonal section / it is parallel to planes i-i. Owing to 

 the oblique angle between a and c, the planes abcve a differ 

 in their relations to the axes from those below, and hence comes 

 the difference in the angle they make with the basal plane. 



The halves of a crystal either side of the clinodiagonal section 

 — tho vertical section throiigh a and c — are alike in all planes 

 and angles. Another important fact is this : that the plane i-i, 

 or that parallel to the clinodiagonal section, is at right angles not 

 only to and i-i, but to all planes in the zone of and ii * 



