22 W. E. Ford — RemarTcable Twins of Atacamite. 



a complex group is represented in figure 7, No. II being in 

 tvsrin position in respect to No. I, wliile No. Ill is in twin 



fosition in respect to No. II, e of III being parallel to e' of 

 I, etc. 

 The second type of twin crystals, although governed by the 

 same law, is very different in habit. It consists of a normal indi- 

 vidual having a rather stout development, with its dome faces 



Fig. 8. 



at either end nearly if not entirely covered by four individuals in 

 twin position. Two of these twin individuals have one of their e 

 faces parallel to one of the planes of the crystal in normal posi- 

 tion, one twin being revolved toward the front and the other 

 toward the back. The other two twin individuals have a similar 

 relationship and ai-rangement except one of their e faces is par- 

 allel to the second e face of the normal crystal. These twinned 

 individuals usually show only the faces of the jjrism and the 

 brachypinacoid, which, lying on the top of the normal crystal, 

 form shallow troughs running in the direction of the brachy- 

 axis on either side above the position of its dome faces. Occa- 

 sionally e faces of the twins are found either at the side forming 

 a projection over b (010) of the normal crystal or sometimes in 

 slight reentrant angles along the crest of the crystal. These 

 relations are shown on the stereographic projection, figure 8, 

 which gives the forms of the crystal in normal position and 

 the prism and pinacoid faces of the four different twin posi- 



