98 



Heddle') obtained from the Treshinish Islands six-sided 

 plates with the plane angles с 128° 50' and 115°; 1 cannot 

 understand how he has got these angles, which harmonize so 

 little with the fact that the mineral is uniaxial. Heddle de- 

 scribes, too, one or two bewelling faces on the edges, which 

 he mentions as very brilliant and smooth; unfortunately he 

 has not measured the position of these faces. 



For the gyrolite from Niakornat I have found the 

 following: 



Crystalline form rhombohedral tetartohedral (rhombohedral 

 class of Groth); the tetartohedrism is only to be seen by the 

 etching figures. 



Axis с = 1-9360. 



The following forms have been found: c{000l), r{loTl}, 

 «{10I2}, determined by the measurements: 



Average ., . . Number of Calculated 



value anations measurements value 



r:c =(l0Tl): (0001) = 65° 54' 64° 19'-67° 23' 5 

 г<:с =(l0T2):(000l) = 48°3' 47°25'-48°42' 3 48° 1 Г 

 r:r =(l0Tl): (Olli) = 75° 58' 1 75° 32' 



As will be seen the values are divergent to a rather high 

 degree, so that they cannot be considered as very exact. This 

 arises from the fact that the faces are not well adapted for 



measurements; the 

 faces of с {oOOl} are 

 large and shining but 

 mostly curved in dif- 

 ferent directions so 

 that they give a number of signals in the goniometer. The rhom- 

 bohedral faces are sometimes shining enough, but they are al- 

 ways very small and furthermore somewhat curved ; the reflec- 

 tions, which they give, are of course mostly very indistinct. 

 There cannot, however, exist any doubt about the correctness 



») Min. Mag. 8, 1889, p. 272. 



