TWINNING. 



63 



A peculiarity notod by Mr. Hess on several crystals similar to the one 

 shown in figure 18 is that in front of the twinned crystal, on the larger 

 untwinned unit, is a small cavity bounded by rough planes which 



FIGURE 14. Contact twin on (02"3). Forms: &{010} 

 o{100>,77z{110},Z{210}. 



FIGURE 15. Contact twin on (023). Forms: 

 c{001>, 6{010>, a{100>, m{110>, Z{210}. 



FIGURE 16. Line of contact of two contact twins. 



however suggest that they may be slightly developed terminal faces, 

 such as /{Oil}. (See PL VIII, A, p. 19, center.) They may be 

 caused, however, by crystals of scheelite, now removed. 



