386 E. S, Dana— New twins of Staurolite and Pyrrhotite. 



occurriiipr prism on staurolite be made i-|, the twinning planes 

 will be then planes of simple axial ratios in accordance with 

 the usual law, namely l-l (instead of |-?) and 1 (instead of |-|). 

 re described would then have 



already bsen mentioned that when the twinning 

 plane is f | the vertical axes cut each other at an angle of 

 about 60°. It would be expected from this fact that compound 

 crystals of three individuals would be found, the entire circum- 

 ference being thus divisible by six, and this has been the case. 

 Figure 2 represents one of a considerable number of crystals of 

 this description in which three interpenetrating individual 

 crystals cut each other respectively at angles of 60° and 120°, 

 and, being symmetrically developed, form thus a six-rayed star. 

 Still another form is shown in figure 3 ; it is interesting as being 

 a combination of two methods of twinning in the same com- 

 pound crystal which has been rarely observed in any species. 

 The vertical axis of two of the single 'crystals are at right angles 

 to each other, and that of the third cuts each of the other two 

 at an angle of 60^. This latter fact explains the occurrence of 

 this form. 



The crystals figured were from the cabinet 

 Brush. 



2. On a twin of Pyrrhotite. 

 The annexed 

 of pyrrhotite, wh 

 the kindness of Dr. Harrington of Montreal. 



The crystal itself is somewhat more than three times the 

 4. size of the cut, and is almost as sym- 



metrically formed. The pyramidal 

 planer (r) are uniformly deeply striated 

 in a horizontal direction, parallel to the 

 basal section ; the faces are thus made 

 quite uneven. Furthermore these same 

 planes show a considerable number of 

 minute longitudinal depressions, or etch- 

 ings, parallel to the vertical axis. This is a prominent feature 

 of all the pyramidal planes alike, thus confirming the accepted 

 hexagonal nature of the species. The transverse crystal, as 

 seen in the figure, is quite irregular, being made up of a 

 group or bundle, of small crystals in parallel position ; this 

 is especially :rue of one ol the extremities. These small crys- 

 tals are some of them free from the striations alluded to and 

 allow of exact measurement. The angle for ry^r (basal) thus 

 obtained is 163^ ( O/sr=98°30') ; this corresponds to the form ^\ 

 which requires 162° 40' (0/^ Y=98° 20'). The twinning plane 



