Tin; MONorUNir SVSTI.M 



123 



it> position is warcr the extremity of the ft axis, it is of the ortho 

 >. ii s, ft : nb : ooc, (kho). Prisms are not plus and minus forms, as 

 each face subtends two octants, one above and one below, Fig. 241. 



III. Clinodome, oo ft : nb : me, (ohl). 



When the pole lies in the plane at right angles to the ft axis, the 

 faces are parallel to the clinoaxis and the form is the clinodome. 



FIG. 242. The Plus and Minus Ortho- 

 domes. 



FIG. 243. Combination of 

 the Three Pinacoids. 



IV. Orthodome. If the poles lie in the equatorial plane the 

 faces will be parallel to the orthoaxis and the form is the ortho- 

 dome, of which there are two forms: 



the plus orthodome, ft : oo b : me, (hoi), 

 formed by the two faces subtending 

 the four small octants ; and the minus 

 orthodome, ft : b : me, (hoi), formed by 

 the two faces subtending the four large 

 octants, Fig. 242. 



V. Orthopinacoid, ft: oob: ooc, (100), 

 when the poles lie in the plane of sym- 

 metry at 90 from c. 



VI. Clinopinacoid, oo ft : b : oo c, (010), 

 when the poles lie on the b axis. 



VII. Basal pinacoid, oo ft : oo b : c, 

 (001), when the poles lie on the c axis. 

 Figure 243 is a combination of the 

 three pinacoids. 



Combinations. The possible forms 

 to combine in this type are : 



Fro. 244. Combination of 

 m( 110)^8(100), b(010), u(lll), 

 and y(101), in Augite. 



Pyramids, three series, plus and minus, (hkl), (hkl). 

 Prisms, two series, (hko). 



