230 



8. Weldman — Amphibole Hudsonite. 



has not been so commonly noted. In the hudsonite the twin- 

 ning lamellae vary considerably in width, having a maximum 

 breadth of 0'02 mm in the several sections examined. The 

 larger lamellae are usually continuous across the section, while 

 the smaller ones often thin out into mere lines or end abruptly 

 at a fracture. The appearance of the twinning lamellae in a 

 portion of a slide parallel to the clinopinacoid (010) is shown 

 in figure 1. The short and thin lamellae or parting planes are 

 concentrated along the fractures, and thus bear witness that 

 the twinning and parting, like the fractures, have a secondary 

 dynamic origin. The larger and more continuous lamellae are 

 probably connected with larger fractures in the mineral not 

 seen in the slide. Figure 2 represents the optical orientation 

 and its relation to the structural planes of prismatic cleavage 

 and basal twinning and parting as seen in the plane of the 

 clinopinacoid. 



+c 



3 



T * 



.' (001) , 



/' 



1 A 



^ 76i 1 



\ 



^ 



r 



, (00/) - 



-a 



t c 



The occurrence of transverse twinning lamellae and parting 

 planes in amphibole, like those described in the hudsonite, cor- 

 responding to similar structure planes in pyroxene, where they are 

 considered parallel to the basal pinacoid, is only another instance 

 showing the wisdom of the method of giving the same symbols to 

 these analogous planes in both groups as first suggested by 

 Tschermak in his Lehrbuch der Mineralogie, in 1884, urged by a 

 number of writers, and finally fully adopted by Tschermak 

 and Dana in their text-books. The several cogent reasons for 

 this change were well stated by Miigge* and Williamsf a num- 

 ber of years ago and need not be repeated here. Because of 

 this change adopted in the orientation of amphibole so as to 

 correspond throughout with pyroxene, there is necessarily a 

 lack of uniformity in literature in describing the relations of 



*Neus. Jahrb. f. Min., 1889, i, 244. , f This Journal, xxxix, 356, 1890. 



