114 



The polysyrithetic twinning structure shows in the Green- 

 land catapleiite some particulars. In the following the structure 

 will only be mentioned, when it is seen in sections, parallel 

 to the basis, as all other twin-formations than those after the 

 prisms are very little conspicuous. 



The different types show great deviations, seen especially 

 in two different relations, viz. (1) in the way in which the optic 

 axial plane is orientated in relation to the hexagonal crystalline 

 faces, and (2) in the way in which the twin-lamellæ are orien- 

 tated in relation to the optic axial plane. 



In the crystals of type I, as in the Norwegian catapleiite, 

 the optic axial plane is everywhere perpendicular on the hexa- 

 gonal unit prism m {lOlo}. As the optical axial plane is, 

 according to Brøgger, the monoclinic symmetrical plane, w{l010} 

 answers to the monocUnic faces [lOO} and {lio}. The twin- 

 formation is most frequently the one shown diagrammatically 

 in fig. 6 where the dotted lines show the direction of m(lOlO). 



Fig. 6. Diagrammatic figure of the twin-formation in catapleiite, 

 type 1. The figure shows a part of the crystal chiefly consisting 

 of a single individual (with the axial plane placed vertically). The 

 lines represent thin lamellæ of the individuals in twin-position to 

 the principal individual ; their optical orientation is given by the 

 axial figures drawn on the striæ (one for each direction of the 

 striae). The dotted hexagon represents the direction of the contour 

 of the crystal of catapleiite. 



One individual (with a vertical axial plane) forms the greater 

 part of the surface; the two others are seen as fine striae. 

 The striae that are perpendicular on the directions of m(l010}, 



