{graham] 



DAWSONITE 



169 



where the stoutness of the needles is much exaggerated; the same com- 

 bination is represented in the spherical projection of fig. 2. 



Fig. 1. — Common Habit of Dawsonite. 



Fig. 2. — Spherical Projection. 



In general, the crystal faces are bright and yield sharp reflections 

 of the collimator signal, excepting in the prism zone, where 

 the faces are striated longitudinally and sometimes give rise to a string 

 of images. There is, however, a perfect cleavage parallel to the rhombic 

 prism M(ilO), which enables the ratio between the a and h axes 

 to be determined with fair accuracy. 



In addition to the forms enumerated above, some of the crystals 

 exhibit faces, usually solitary, which probably belong to the following 

 forms : — 



(130) observed on 4 crystals. 



(230) " "3 " 



(101) (210) .... " "1 '' 



The needles are elongated along the c-axis and generally striated 

 in the same direction. The habit of the crusts on the dyke walls is 

 platy or bladed, due to a parallel growth of these needles or to an 

 o.«cillatory combination of M with A or B, which produces striœ parallel 

 to the prism edge. The blades penetrate one another at all angles, but 

 no twin crystals were found. 



Optical Properties. 



Examined under the microscope in parallel ligbt, the needles show 

 straight extinction and compensate when tbe quaxtz wedge is inserted 

 along their length. If they are lying on a cleavage face, as is usually 



