99 



crystals generally remain whole, when the surrounding pegma- 

 titic mass is cut through. The colour is brownish, always 

 rather varying in the same crystal, so that the faces appear to 

 be striated or spotted; the colour varies between a rather light 

 yellowish brown and a rather dark grayish brown. The streak 



Fig. 7. Erikite. A microscopical section of the mineral. The highly pro- 

 jecting ridges and most of the dark interjacent mass consist of the yellow 

 substance: the white parts are mostly formed of the colourless substance; 

 the largest white spot represents a cavity. After a photograph by K. J. V. 



S teenstrup. 



is colourless. The crystals are opaque ; in thin sections they 

 are only slightly transparent. 



The most peculiar feature with regard to the physical 

 nature of the crystals is their structure, as they are never 

 homogeneous, but always show a compoimd construction. In 

 microscopical sections they are seen to consist of two different 



7* 



