46 



to be subjected to the К undt test for pyroelectricity; nor 

 have I been able to produce any distinct and decisive etching 

 figures on them. It must therefore be presumed that the 

 cordyiite crystals belong to the bipyramidal (holohedral) class 

 of the hexagonal system and that in this case there is nothing 

 but a parallel growth. 



The colour of the mineral is pale wax-yellow; sometimes 

 it is nearly colourless, sometimes inclining to brownish yellow. 

 In the fresh condition it is nearly clear and transparent, but by 

 a superflcial alteration the crystals usually become ochre-yellow 

 and dull. This is generally the case with the prismatic faces 

 and with the pyramidal faces that lie nearest to them. The 

 faces of the base and the fundamental pyramid, on the other 

 hand, generally are, as has already been stated, free from this 

 alteration. On the fresh fracture the mineral has a vitreous 

 to adamantine, on the base generally a pearly, lustre. 



In microscopical sections the mineral is colourless or 

 shows a hardly perceptible tinge of yellow. Sections oriented 

 parallel to the vertical axis show extinction parallel to this 

 axis without perceptible pleochroism. Sections oriented parallel 

 to the base are optically perfectly isotropic. In converging 

 polarized light a normal uniaxial interference figure with ne- 

 gative character is exhibited. The rings are rare, which in- 

 dicates that the mineral has weak double refraction. No at- 

 tempts have been made to determine the indices of refraction, 

 because the crystals are so small that a prism cannot be 

 made. 



A section parallel to the base shows a zonal structure. 

 The zones run parallel to the hexagonal outline of the section 

 and seem to depend upon submicroscopic inclusions (fig. 10, 

 PI. II). In the middle of the section there is a minute hexa- 

 gonal area that is clear and without inclusions. This area 

 is surrounded by a hexagon of some opaque yellowish matter, 

 under high magnifying powers it separates into concentric zones 



