136 



MINERALOGY 



and will be repre- 

 sented by a striation 

 running across the 

 crystal face in a 

 fixed direction. 

 Wherever these 

 striations appear on 

 the face of a crys- 

 tal, they must be 

 considered as the 

 boundary between 

 two individuals. In 

 Fig. 272 a, a com- 

 plex quartz crystal, 

 the individuals are 

 well marked and 

 apparent ; but in 

 Fig. 272 6, a quartz crystal with striations running across the prism 

 face parallel to the intersection of the prism and rhombohedron, 

 each striation represents a reentrant angle between individuals, 

 or the crystal in its growth may be said to oscillate between the 



FIG. 271. Quartz Crystals in Parallel Position on Or- 

 thoclase. 



b 

 FIG. 272. 



Smoky Quartz from Disentis, Switzerland. 



