Ill LATION OK INDIVIDI'M. CRYSTALS 



139 



twinning :ixis c 180, and 



that the twinning :i.\is is parallel 



to the edge in the prism zone. 



The >haded plane is the contact 



plane and is parallel to the or- 



thopinacoid. The trace of the 



twinning plane in the crystal 



is usually marked by a reentrant 



angle, as xyz, or, where this is 



reduced to a minimum, by stri- 



ations on the crystal face, as 



yy', Fig. 278. 



A reentrant angle is not al- 

 ways present to distinguish the 



crystal as a twin, and often when 



absent, as is shown in the epi- 



dote crystals from Prince of 



Wales Island, Alaska, Fig. 280, 



where the twinning axis is per- 

 pendicular to the orthopinacoid 



and the composition and twin- 

 ning plane is the orthopinacoid, and which after a revolution of 



180, leaves on these crystals no indication of the twinning. 



Striations on the clino- 

 pinacoid due to parallel 

 growth are indicated by 

 the parallel lines, and the 

 effect of twinning on these 

 striations is shown. The 

 striations meet the twin- 

 ning plane, yy', from each 

 individual at the same in- 

 clination, and the trace of 

 the twinning plane on the 

 crystal face bisects the 

 angle between them. 



The complexity of some 

 apparently simple crystals 

 is often only revealed by 



277. (Jypsum Twins from near 

 Paris, France. 



FIG. 278. Gypsum Twins. 



the microscope and polarized light, as in the twinning bands of 

 the plagioclase feldspars, Fig. 281. In enantiomorphic types, 



