250 TERMINOLOGY. . 179. 



composition, but that they are continued beyond it, which 

 causes their cruciform aspect. In the crystallographic sign 

 of similar compositions, the number 2 is prefixed to the 

 sign of the face of composition. Thus the twin-crystal of 

 paratomous Kouphone-spar is expressed by P. Pr + cc.. 

 Pr -f 05, 2{ P -|^]; that of prismatoidal Garnet by P -co. 



P + cc. fr + co, 2 -j ? > . Other systems present similar 



occurrences, as, for instance, one of the most remarkable in 

 the hexahedral pentagonal-dodecahedrons of hexahedral 

 Iron-pyrites. 

 The crystallographic sign of the cruciform pentagonal. 



dodecahedron is 1, 2 {s} the face of composition being 



one of the faces of the dodecahedron (. 63.). 



Another kind of twin-crystals results, if the face of com- 

 position is perpendicular to an edge of the crystalline form, and 

 the axis of revolution parallel to this edge^ or, which is the same 

 thing, perpendicular to the face of composition. Rhombo- 

 hedral Ruby-blende affords examples of this law ; the crys- 

 talline form R 1. P + co is composed in a face perpen- 

 dicular to one of the terminal edges of R 1, or to the 

 inclined diagonal of R 2, the axis of revolution being 

 paralleltothisline.Vol.il. Fig. 139. The crystallographic 

 sign is R 1. P + co,!*""'^*" 1 }; where R 2. R 1 



indicates those edges, which at the saml time lie in the 

 planes of R 1, and of R 2. 



According to this law may be explained the twin-crystals 

 of di-prismatic Lead-baryte, prismatic Lime-haloide, pris- 

 matic Melane-glance, &c., whenever they assume a cruci- 

 form aspect. Under these circumstances, the present law 

 is as it were complementary to the preceding one, and 

 either of them may be applied, although in many cases the 

 preceding one will be found more simple. The law of the 

 composition of di-prismatic Lead-baryte, Vol. II. Fig. 38., 

 may therefore be expressed either by 2{Pr}, or by 



