32 



NEW YORK STATE MUSEUM 



Fig. 123 



Pinacoids. Planes which are parallel to two orthorhombic axes 

 are known as pinacoids; they consist of two parallel planes and 

 take their names from the axes to which 

 they are parallel, thus: 



The basal pinacoid is parallel to both 

 basal axes a and b. 



The macropinacoid is parallel to the 

 macro and the vertical axis. 



The brachypinacoid is parallel to the 

 brachy and the vertical axis. 

 Fig. 123 shows the intersection of these 

 three pinacoids, the resulting solid being analogous to the cube 

 of the isometric system and the second order prism and base of 

 the tetragonal system. 



Prisms. Prisms cut both horizontal axes, and are parallel to 

 the vertical axis; they are composed of four faces, opposite pairs 

 being parallel. The unit prism for any /. 

 species intersects the basal axes at rela- 

 tive distances which give the axial 

 ratio for that species. In fig. 124, which ' - 

 shows a basal section of the mineral 

 topaz, such a unit prism is indicated, the * \/*' 



axial ratio for topaz being a:b=.529:l. Fig - m 



Prisms occur intersecting the basal axes at distances proportion- 

 ately more or less than the axial ratio subvert to the law of 



Fig. 125 Fig. 126 



rational indexes. These are called respectively macro or brachy 

 prisms according as they are more nearly parallel to the macro 

 or brachy axis than the unit prism. A macro and a brachy 

 prism are shown in fig. 124. 



