140 



MINERALOGY 



where there are right- and left-handed forms, it is not possible to 

 revolve one individual around an axis into "a congruent position, 



FIG. 279. Simple Crystal of 

 Epidote. 



FIG. 280. Twinned Epidote from Prince 

 of Wales Island, Alaska. 



and a twinning axis is therefore not always possible; but such 

 crystals are twinned by reflection, as some twins of quartz. Twins 

 formed by the union of plus and minus, upper and lower, or right 



FIG. 281. Twinning Lamellae of Plagioclase, between Crossed Nicols. 

 Much enlarged. 



and left forms are supplementary twins; and when as contact 

 twins, and equally developed, the individuals will possess a pseudo- 



