186 



out from epistolite are however, on account of its brittleness, 

 not quite as thin as those of gypsum or mica. There ap- 

 pears also on the base a distinct tendency to cleavage in two 

 directions, after the traces of (llO) and (llO) under the angle 

 of 102° 30'. However it has not been possible to measure the 

 angle of these cleaving faces with the base on account of their 

 unevenness , and it is therefore not certain whether they 

 correspond to the prism or another face in the zone [llO, 00 1]. 

 The epistolite has no special color. The crystal faces are 

 dim, grayish or brownish; the cleaving faces after (oOl), 

 which in most cases is all, that is visible of the mineral, have 

 a very strong silverwhite pearly luster; this is the case especially 

 as regards the pieces from Nunarsiuatiak , which represent the 

 purest variety of the mineral. The deposits from the other 

 localities are more or less yellowish or brownish. When seen 

 under the microscope the small plates only appear transparent 

 to a very slight degree. In the direction of the diagonal cleaving 

 lines two systems of very fine, darker striae are visible, often 

 specially distinct in one direction. Of interpositions aegyrite is 

 most conspicuous and appears in thin rectiliniar needles, 

 generally following two directions ф: the axes à and Ъ , more 

 seldom in other directions. 



The refraction was determined by Thoulet's fluid. In a 

 solution of specific gravity 2'86o the limits of the mineral 

 almost disappear, and the approximate index of refraction is 

 thereby found to be 1"67. The epistolite is optically biaxial; 

 the axial plane being -ф. the plane of symmetry. The apparent 

 angle of the optic axes for faces ф. {OOl}, taken with Adam's 

 apparatus for measuring the axial angle, gives values from 81° 

 to 101°; the average of 7 measurements was 89° 30'. The 

 index of refraction for the glasshemispheres 1 have found by 

 measuring to be 1"53; from this the interior angle of the optical 

 axes is found to vary from 73° 2' to 89° 58', on an average 

 2 F„ = с 80°. The least axis of elasticity is situated in the 



