115 



and, accordingly, form angles of 60° with the optic axial 

 plane, are considerably more conspicuous than those running 

 parallelly to w? (lOTo) and forming angles of 30° with the 

 optic axial plane. The former kind of striae belong to twins 

 after the monoclinic {зю}, which is, accordingly, here the most 

 common law; the latter kind are twins after {ио}. 



As far as it has been possible to observe the facts, the 

 crystals of types 2 — 4, contrary to the preceding ones, have 

 always the optic axial plane placed parallelly to the hexagonal 

 ^{lOlo}. To account for this remarkable fact three possibilities 

 may be found. 



1. The monoclinic single individuals are orientated in 

 another way in relation to the hexagonal crystal, than they 

 were in type 1, the planes of symmetry being in the two cases 

 placed at right angles to each other. Nothing corresponding 

 to this fact is, I think, known in any other pseudo-hexagonal 

 mineral; that the same monoclinic faces may in one case cor- 

 respond to hexagonal faces of the first series, in the other 

 case to faces of the 2"^ series, seems to be contradictory to 

 all theories of crystalline structure. 



2. The optical axial plane may be differently orientated in 

 the monoclinic single individual, in this case accordingly per- 

 pendicular on the plane of symmetry. Such an alteration in 

 the position of the axial plane is exceedingly improbable, and 

 the more so, as in both cases the axial angle is very nearly 

 of the same size. ' 



3. The apparently homogeneous lamellae may be composed 

 of still finer, hypomicroscopical lamellae in the same way as 

 mentioned under type 2; in this case the optic axial plane 

 is turned 30°, and thus becomes parallel to m(lOÏo} if before 

 it was perpendicular on it, and vice-versa. This explanation, 

 however, is in most cases exceedingly improbable, and cannot 

 well be used where the single individuals have distinct limits 

 and definite extinction. 



