. 179. OF COMPOUND MINERALS. 251 



2 1 P. Pr}; of which the former indicates the face of com- 

 position to be parallel to both the faces of the horizontal prism 

 Pr, wjiile the latter refer to faces of composition perpendi- 

 cular to those edges of P, in which the pyramid is touched 

 by the faces of Pr. The' continuation of the individuals 

 beyond the faces of composition produces the identity of 

 the results of the two laws. 



Prismatic Feld-spar gives an interesting example of an- 

 other law of regular composition, in which the face of compo- 

 sition is parallel to a face of crystallisation^ but the axis of revo- 

 lution lies in the same face^ and coincides with an axis of t/ie 

 crystalline form. Its crystalline form is the combination 



1?L? (y> _ *L (P). (Pr + 0=) 3 (r, 0- Pr + oo (M). 



Vol. II. Fig. 61. The face of composition is Pr + co ; the 

 axis of revolution is situated in it, and parallel to the prin- 

 cipal axis of P. After having placed^ two crystals of this 

 form in an upright and parallel position, we must turn the 

 one round its vertical axis, while the other remains un- 

 moved. In the position thus produced, the junction of the 

 two individuals will produce a twin-crystal. But we may 

 join them either with those faces of Pr 4- co (M\ which are 

 situated to the right, or with those which are situated to the 

 left of the face y, which is conceived to be turned towards 

 the observer, and contiguous to the upper apex of the 

 form. The product in the first case will be the twin- 

 crystal represented, Vol. II. Fig. 80., and its crys- 



tallographic sign IJ-1_?. .. (Pr 4- os) 3 , Pr + OB, 



{ r, Pr -f 03 : Pr + oo. Pr + os } ; the product of the second is 

 the twin-crystal, Vol. II. Fig. 81., and its sign * + 2 . 



. (Pr+co) 3 .Pr+03,{l,Pr + c : Pr + oo. Pr + w}. 



The first part of the expression within the braces, indicates : 

 the right or the left face of Pr + oa to be the face of com- 

 position, while the second expresses the direction of he 



