100 



substances , a highly refracting and double refracting yellow 

 one , and a more slightly refracting and double refracting 

 colourless one. The former substance is formed as ridges 

 running in all directions and penetrating the whole face as a 

 network. A photographic representation of this fact is seen in 

 fig. 7, where, for the sake of distinctness, a part has been 

 chosen, in which the ridges are strongly marked and placed 

 comparatively far from each other. From the walls of these 

 ridges sidebranches are sent off in all directions, often of a 

 stalactitic appearance; these branches sometimes fill up the 

 whole space, so that the interjacent mass disappears, and the 

 substance looks as a fine-grained yellow aggregate; generally, 

 however, some few spots of the colourless substance are found. 

 The yellow substance is most frequently of a very strong colour; 

 it is very conspicuous in Canada balsam, and consequently it 

 must be much more refracting than this substance. In polar- 

 ized light it is seen to consist of quite small individuals 

 (ca. Vi"»™'")? quite irregularly orientated, and showing inter- 

 ference colours of the first or second order, and consequently 

 the double refraction must be rather considerable. On account 

 of the smallness of the individuals axial figures cannot be seen. 

 The interjacent substance is colourless, and is of about 

 the same refraction as the Canada balsam; it occurs in com- 

 paratively large individuals commonly penetrating two or more 

 adjacent interspaces. The double refraction is not very strong; 

 in not very thin sections strong interference colours of the first 

 order are found; as a comparison it may be noted that the 

 analcite often found outside the crystals of erikite, in the same 

 sections at most shows white of the first order. On account 

 of the slight extent in which this substance is found without 

 being interrupted by the yellow one, it has not been possible 

 to find any axial figure in it; I have, however, succeeded in 

 finding such a figure in a mineral, situated in a somewhat 

 larger cavity in one of the crystals, and shoving interference 



