116 TERMINOLOGY. .113. 



forms are in a transverse position when this plane passes 

 through the terminal edge of the rhombohedron, or through 

 the acute terminal edge of the other pyramid. 



The crystallographic sign of a scalene six-sided pyramid 

 derived from P + n according to m is (P + n) m ; its axis 

 is = 2 n . m. a ; where 2 n . a is the axis of II + n, and a the 

 axis of R. 



The value of m is frequently found in nature to be = 2, 

 = 3, or = 5, all of which occur in rhombohedral Lime-ha- 

 loide. (P + n) 3 is also found in rhombohedral Tourma- 

 line, in rhombohedral Ruby-blende, (P -f n) 5 in rhombo- 

 hedral Iron-ore. Besides these, we have m = 7 in rhom- 

 bohedral Lime-haloide, m = in rhombohedral Fluor- 

 haloide, rhombohedral Quartz, rhombohedral Tourmaline, 

 m = in the two first of these species, m = V and = f 

 in rhombohedral Quartz. 



. 113. THE TRANSVERSE SECTIONS OF THE SCA- 

 LENE SIX-SIDED PYRAMIDS DEPEND ON m. 



For one and the same m, the transverse sections 

 of all forms contained under the sign of (P + n) m 

 are similar to each other. 



Let ABXC, Fig. 47., represent the principal section of 

 the rhombohedron from which the scalene six-sided pyra- 

 mid has been derived ; A3, X the prolongations of the 

 axis, and consequently 9LB, %C the obtuse terminal edges, 

 20, B the acute terminal edges of this pyramid, and 

 aBC its principal section. 



Draw from M, the centre of the axis, in the plane of the 

 transverse section, the line MG parallel to QB. This line 

 will be intersected in F by the obtuse terminal edge 3LB ; 

 and F will therefore be that point in the transverse sec- 

 tion, or MF that line situated in its plane, upon which de- 

 pends the magnitude of the angle of the transverse section 

 at the place of the obtuse terminal edge of the pyramid. 



