Penfield and Ford — Developments of Caloite Crystals. 211 



are introduced in the present article for the sake of comparison 

 with the two types of rhombohedral twinning previously 

 described. The twinning plane in these crystals is the rhom- 

 bohedron r, 1011, and the common scalenohedron v, 2131, thus 

 twinned and drawn, as in previous cases, with the twinning 

 plane vertical, is represented by fig. 9. Fig. 10 represents a 



9 10 



crystal of the butterfly twin type in the Brush collection, and, 

 by comparison with fig. 9, it may be seen that the upper faces 

 of the crystal result from the development of the two front, 

 lettered faces of fig. 9 and corresponding faces behind, to the 

 complete obliteration of the reentrant angle. The faces _at 

 the lower extremity of fig. 10 are those of the prism m, 1010. 

 It is a matter of interest to observe how the scalenohedron 

 when twinned as described according to the three rhombohe- 

 dral laws gives apparently simpler shapes by distortion, or 

 unequal development of some of its faces, than if the distor- 

 tion had not taken place. 



3. Crystals from Pallaflat, Cumberland, England. 



A feature of the crystals from this locality, as represented by 

 specimens in the Brush collection, is_the prominent development 

 of the negative scalenohedron x, 13-11. This form, as shown by 

 fig. 11, has its shorter pole edges bevelled by the common 

 scalenohedron v, 2131, and has the same middle edges as the 

 negative rhombohedron,y, 0221. Fig. 11 was drawn by Mr. 

 W. Yalentine of the Sheffield Laboratary. It presents noth- 

 ing new, and is practically identical with fig. 674 of von 

 Bournon's Traite de Miner alogie published in 1808. The 

 figure is introduced in the present article, because by under- 

 standing its simple zonal relations, the same forms can be 

 easily identified as they occur on a twin crystal to be described. 



Fig. 12 represents the development of two beautiful twin 

 crystals in the Brush collection, both occurring on the same 

 hand specimen. The twinning plane is the unit rhombohe- 

 dron, and the development is analogous to that of the butterfly 



