Dihexagonal Alternatiiuj Type. 2jJj 



Natural Etch Figures on Calcite. 



Natural etchings are quite often observed on the rhom- 

 bohedrons, but well-defined figures upon the scalenohe- 

 dvon and prism of calcite are much' less common; hence 

 a brief description of the natural figures found on the 

 Cumberland crystals may be of interest. Three forms 

 are etched; the rhombohedron e (0112), the scalenohe- 

 dron v (2131), and the unit prism (1010). The figures 

 occurring upon the rhombohedron are the simple triangu- 

 lar pits, frequently found on that form, but the figures of 

 the scalenohedron are unique. They are blade-like and 

 extend horizontally across the crystal face (see Dia. A). 

 The figures are composed of two large faces, and a very 

 small one ; the larger faces lie in the same zone (v:r) ; the 

 upper face dips very abruptly to connect the lower one, 

 which descends very gradually, producing a basal groove 

 lying in the uppermost part of the figure. The lower 

 face forms a very ragged intersection with the surface of 

 the crystal. One end of the figure is pointed, the other 

 occupied by a very small face, which dips abruptly to 

 meet the basal groove. The figures reveal no planes 

 of symmetry except through the middle edge of the 

 scalenohedron. 



The unit prism contains many elongated triangular 

 forms, with curved contours ; they are composed of 

 three faces as are the artificial figures of the rhombohe- 

 dron r. The figures are symmetrical to a vertical plane 

 and indicate by their position the alternating c axis. 



The etchings of the second-order prism are asym- 

 metric quadrilateral forms, having straight margins, two 

 of which extend approximately at right angles to the 

 prism edge (see Dia. A). These etch figures reveal the 

 asymmetrical character of the face. 



Thus, it is readily observed that the etch figures as 

 obtained naturally or artificially, upon the various forms 

 of this series of minerals, are in perfect type and that 

 the etching itself is of the greatest importance in the role 

 which it plays, as a check upon the symmetry of crystals 

 as derived geometrically. 



Princeton University, Princeton, N. J. 



