ITS MODIFICATIONS. 181 



Class /. Acute edges of the prism replaced by 

 single planes. 



Modifications e, and /, incline unequally on the 

 adjacent lateral planes. 



The characters which distinguish this class of 

 prisms from right rhombic prisms, are, first, the un- 

 equal inclinations of any secondary lateral plane on 

 the two adjacent lateral primary planes ; and secondly, 

 the occurrence of similar modifying planes upon two 

 only of the terminal edges. 



There is a remarkable general character of obliquity 

 in the planes resulting from modifications c, and </, 

 which tends to distinguish the secondary forms be- 

 longing to this class of primary forms from those 

 belonging to the right rectangular prism. 



Epidote affords a good illustration of this character. 



ITS MODIFICATIONS. 



Primary form. An oblique rhombic prism. 



The figure is supposed to be oblique in the direc- 

 tion O A, so that the terminal plane forms an obtuse 

 angle with the edge H. The planes M M', may meet 

 at an acute, or an obtuse angle. For the convenience 

 of description, the solid angle at A, will, in either 

 case, be called the acute solid angle ; that at O, the 

 obtuse solid angle ; and those at F, the lateral solid 

 angles. 



The edges B will be called the acute terminal edges, 

 and those at D the obtuse terminal edges. The edge 

 H, and its opposite, are the oblique edges oftheprism^ 

 and G and (J' the lateral edges of the prism. 



