Pen field and Ford— Developments of Calcite Crystals. 239 



Twins, with the rhombohedron e, 0112, as twinning plane . — 

 The habit resulting from this kind of twinning is like that of 

 the well known Guanajuato calcites, fig. 4, described by Pirs- 

 son,* and it should also be stated that as early as 1837 Levyf 

 also described and figured calcite twins of this same type from 

 Streifenberg, Nertschinsk, Siberia. Fig. 4 is analogous to the 

 figures of Pirsson and Levy, though drawn in a different posi- 

 tion, and represents the common scalenohedron v, 2131, drawn 

 with the twinning plane vertical and having a position like 

 that of the side face of a cube, or the pinacoid 010 of any of 

 the three axial systems. This position has been adopted for 

 representing the twin crystals as it gives the best idea of their 

 peculiar development. Fig. 5 represents the scalenohedron v L , 



7"4'11'3 twinned without distortion, a type which has not been 

 observed on any of the specimens, but the figure is introduced 

 in order to show how, by the extension of the two lettered 

 faces in front and the parallel faces behind, together with the 

 suppression of the four small faces below, the Guanajuato 

 type, fig. 4, results. Most of the Union Springs crystals of the 

 Guanajuato type show in addition to the scalenohedron certain 

 modifications at the reentrant angle, fig. 6. The faces forming 

 the_reentrant angle are the pyramid of the second order y, 

 8 - 8'16'3, and a rhombohedron designated as A, apparently in 

 the zone with v x and y, which would cause it to have the sym- 

 bol 8083. The surfaces forming the gash or reentrant angle, 

 however, are curved to such an extent that exact symbols can- 

 not be assigned to portions of them. 



Twins with the rhombohedron f. 0221, as twinning plane. — 

 The rhombohedron^is one of the rare twinning planes of calcite, 

 and the habit presentod by the crystals frona Union Springs is 

 very striking. The scalenohedron <y : , 7'4'll-3, twinned about 

 /, and drawn with the twinning plane vertical, as previously 



*This Journal III, xli, p 61, 1891. 



f Description d'une Collection de Minereaux formee par H. Heuland, vol. i. p. 

 10, fi£. 5, Plate 1. 



