GUIDE TO THE MINER ALOGIC COLLECTIONS 



35 



Pinacoids. As in the normal group of the orthorhombic sys- 

 tem, the monoclinic pinacoids are parallel to two axes and con- 

 sist of pairs of parallel planes. 



The basal pinacoid is parallel to both basal axes 

 a and b. 



The clinopinacoid is parallel to the clino and the 

 vertical axis. 



The orthopinacoid is parallel to the ortho* and the 

 vertical axis. 



Fig. 136 shows the intersection of these three 

 pinacoids. Fig. 137 



Prisms. The monoclinic or inclined rhombic prism cuts both 

 horizontal axes and is parallel to the vertical axis. The clino 

 and ortho prisms of this group are entirely ana- 

 logous, in their relations to the unit prism, to 

 the macro and brachy prisms of the preceding 

 system. Fig. 138 shows an inclined rhombic 

 prism terminated by a basal pinacoid. 



Domes. The clinodome (fig. 139) consists of 

 four faces parallel to the clinoaxis cutting the 

 ortho and vertical axes; the faces are parallel 

 in opposite pairs. The four faces which are parallel 

 to the orthoaxis and intersect the other two, by reason 

 of the lark of symmetry constitute two pairs of planes which 



Fig. 138 



Fig. 139 Pig. 140 



are known as hemiorthodomes, the planes lying in the acute 

 angle /S l>-ing known as plus and those in the obtuse angle ;is 

 minus. The pins and minus hemiorthodomes are shown in 

 fig. 140. 



