87 



The faces in the vertical zone are always deeply striated 

 parallel to the edges of the base, and no even or brilliant 

 faces have been observed. The separate basal lamellae protrude 

 or recede so as to form uneven , step-like surfaces. Some- 

 times corners protrude irregularly from the sides, and the 

 result of a frequent repetition of this are individuals of nearly 

 cylindrical habit. The lamellae here are in twin position with 

 regard to each other, like the embryonic three -sided tables 

 mentioned above. 



These crystals also show another abnormal form, often 

 having one end considerably thicker than the other (Fig. 3, Plate V). 

 This is not a regular pyramidal development, but depends on 

 the fact that the lamellae successively, though irregularly, de- 

 crease or increase in size. This is shown also by the circum- 

 stance that the striated or step-like lateral faces sometimes do 

 not, even roughly speaking, present a plane surface, but are 

 more or less concave, as is seen from Fig. 4, Plate V. Also the 

 basal faces are strongly curved on several of these crystals. 

 Such a deformation extends through the whole mass of the 

 crystal individual. If an individual is convex at one end, it is 

 concave at the other. If such a crystal is cleaved in a direction 

 parallel to the base, each cleavage fragment forms a part of 

 a hollow sphere conformable to the others. This is a fact 

 sometimes observed in certain micas. 



The colour of the mineral is, as its name indicates, gene- 

 rally ash-grey, on the base often pearl-grey. Sometimes the 

 crystals show a tinge of liver-brown. The basal plane and the 

 cleavage planes often show a distinct pearly lustre. Fracture 

 in other directions is only faintly glimmering or quite dull. The 

 powder has the same ash-grey colour as the mineral itself. 



In microscopical sections the mineral is almost colourless. 

 Only a faint tinge of yellowish grey is observable. In sections 

 cut parallel to the principal axis, and consequently perpendicular 

 to the cleavage direction of the mineral, this tinge is not very per- 



