242 Pen field and Ford — Developments of Calcite Crystals. 



twins from Egremont, fig-. 10. A prominent feature of the 

 twins is the vertical zone r, /"and x of the individual to the 



ii 



right, extending over the twinning plane to x, fi, and r of the 

 left hand individual, and so on around the crystal. Thus with 

 this method of twinning four x faces, two in front and two 

 behind, form as it were a vertical prism, analogous to the 

 prism formed by four faces of the scalenohedron v, fig. 4, 

 when the flat rhombohedron e, 0112, is the twinning plane. 

 In figs. 4 and 12 the rhombohedral symmetry is not appar- 

 ent, and the habit is like that of twin crystals of the monoclinic 

 system, having the vertical faces v and x, respectively, as 

 prisms and a pinacoid as twinning plane. The twin crystals 

 represented by fig. 12 are so attached that only a portion of 

 the lower x and v faces are visible. 



4. Crystals from the Stank Mine, Lancashire, England. 

 This locality is represented in the Brush collection by a suite 

 of eleven groups of crystals. A form which appears on all of 

 the crystals, and which seems to be especially characteristic for 

 the locality, is the negative scalenohedron ±>, 2*8 , 10 - 3, having 

 the same middle edges as the rhombohedron f, 0221. On all of 

 the specimens the faces of the scalenohedron have a decided 

 vicinal character. Some of the specimens are simply groups 

 of scalenohedral points without modifications. Generally, 

 howejver, as shown by fig. 13, the flat negative rhombohedron 

 e, 0112, is slightly developed, and long narrow faces of the 

 positive scalenohedron v, 2131, modify the pole edges, while 

 some portions of the negative rhombohedron s, 0551, may be 

 seen near the attachment of the crystals. Another prominent 

 type which seems to be common is represented by fig. 14. In 



