380 MINERALOGY 



iron nails. The sharp rhombohedron f occurs as a simple form in 

 the Fontainebleau sand crystals, while the cleavage rhombohedron 

 r occurs as a simple form at Sterlingbush, New York; from this 

 locality individual crystals weighing nearly a thousand pounds 

 have been obtained, mostly rhombohedral in form with but slight 

 modifications. In many localities this rhombohedron is the pre- 

 dominating form, as at Rossie, St. Lawrence County, New York, 

 where r is well developed but with the equatorial belt highly modi- 

 fied with scalenohedrons. Since the cleavage of calcite is so per- 

 fect and in every specimen may be recognized either by the internal 



FIG. 438. Prismatic Habit of Calcite, Wanlockhead, Scotland. The Small 

 Specimen is the Rhombohedron e, Freiberg, Saxony. 



fractures or by the brilliant cleavage surfaces, other forms occurring 

 may usually be oriented by their relation to this cleavage rhombo- 

 hedron r (1011) or the commonly occurring prism m (lOlO). 



2. Prismatic crystals owe their general appearance to the pre- 

 dominating prism m which may be terminated by the simple 

 rhombohedron e, as at Keokuk, Iowa, or at Freiberg, Saxony. 

 Again the termination may be by the rhombohedron and base, as 

 at Patterdale, England; or complicated by the combination of 

 several scalenohedrons and rhombohedrons, as at Patterdale and 

 Derbyshire, England. 



3. When the prism is very short and terminated by the base, or 

 small modifications of a scalenohedron, or rhombohedron in com- 

 bination with these two predominating forms, the crystalline habit 



