122 



MINERALOGY 

 Forms 



I. Monoclinic pyramids differ from those heretofore considered, 

 as the octants subtended by the faces are large and small, yield- 

 ing faces, represented by the same parameters or indices, of two 

 sizes, of which the larger faces form the minus and the smaller form 

 the plus pyramids. Monoclinic pyramids do not inclose space; 



FIG. 240. The Minus Pyramid. The Plus Pyramid. Combination of the Plus and 



Minus Pyramid. 



the combination of the plus and minus pyramids is equivalent to a 

 single orthorhombic pyramid, and incloses space, Fig. 240. 



As in the orthorhombic system, there are three series of pyramids 

 holding the same relation to the axes as in 

 that system. 



Unit series of pyramids, & : b : me, (hhl), 

 (hhl). 



Ortho series of pyramids, ft : nb : mC, 

 (hkl), (hkl). 



-> <.." Clino series of pyramids, nt : b : me, (khl), 



(khl). 



The unit or fundamental pyramid is the 

 connecting link between the three series. 

 When the form represented by the spherical 

 projection is a pyramid, the poles will fall 

 within the triangle, Fig. 239. 

 FIG. 241. The Unit n . Prisms. When the poles lie in the 

 circle at right angles to the c axis, the form 



is a prism, of which there are two series ; when the pole is nearer 

 the b axis, it is a prism of the clino series, na : b : oo c, (hko). When 



