CARBONATES 



381 



is tabular, as at Andreasbrr^ in the Harz, also in West Cumberland, 

 England. 



4. Calcite crystals of the scalenohedral habit, designated " dog- 

 tooth spar," is pn>l>al>ly (lie most common. In this form the 

 BCftlenohedron v (2131) is the predominating form. At Tavetsch, 

 S\vit/erland, this sealeiiohedron occurs uncombined. Nearly 

 simple scalenohedrons of large size but slightly modified on the 



FIG. 439. Calcite Rhombohedron r, and Epidote from Sulzbachthal, Tyrol. 



apex by rhombohedrons and very flat scalenohedrons occur at 

 Joplin, Missouri, also at Mineral Point, Wisconsin. Very acute 

 BOalenohedlpns occur at Thunder Bay, Lake Superior. Probably 

 the clearest and most beautiful specimens of this habit are those of 

 West Cumberland, England. 



Twinning. 1. Twins in which the vertical axis c is the twinning 

 axis and the base is the composition face. The reentrant angle 

 appears in the plane of the lateral axes as in Fig. 295, which are 

 twinned scalenohedrons from Guanajuato, Mexico. 



2. When the twinning axis is the normal to e, and the face e (OlT2) 

 is the composition plane. In these twins the vertical axes of the 

 two individuals. stand at an angle of 52 30' 30". Fig. 294 are 

 scalenohedrons from Mexico, twinned after this law. This style 

 of twins also forms the polysynthetic twins of rock-forming calcite 

 and the parting noticed in some calcites as that from Franklin, 



