216 Kraus and Cook — Iodyrite from Tonopah. 



Type 8. Figure 9 illustrates clearly the characteristic devel- 

 opment of twins belonging to this group. Here one of the 

 individuals is fully developed and generally somewhat tabular 

 in habit. Hemimorphism is pronounced. Attached to the 

 side of the fully developed crystal is the second smaller indi- 

 vidual and in the position demanded by the common twinning 

 law e {3034 1 . The second individual is also more or less tab- 



Figs. 6-9. 



C'^^^^^J 



/ ' ] 



J^ 



?n 



m 



\ i' 



— i 



\l m 



— ^H-- 





























ular. Twins of this character were not as frequently encoun- 

 tered as those of type 7. 



Type 9. Quite a number of contact twins, the individuals of 

 which possess a rhombohedral development, were observed. 

 They resemble very closely the twins from Lake Yalley, New 

 Mexico, which were described in 1885 by Genthand vom Rath. 

 The figure given by them is reproduced in Dana's System of 

 Mineralogy.* 



In the writings of the earlier contributors to our knowledge 

 of the crystallography of iodyrite much confusion exists as to 

 the occurrence of the pyramidal forms about the upper and 

 * 6th. edition, 1892, 160. 



