Dihexagonal Alternating Type. 215 



loss the same over the entire face, producing a glossed, 

 fused appearance. Good results were obtained only by 

 hot dilute citric acid ; although figures appeared after the 

 first few seconds, immersion for approximately one min- 

 ute was necessary to produce well-defined forms (see 

 figs. 10 and 11). The etch figures are somewhat similar to 

 those produced by the HC1 on the 1010 prism ; the basal 

 plane is very similar in the two figures, varying only 

 about 4° in the apex measurement from the reading of the 

 HC1 figure, this being approximately 30°. The deeper 

 portion of the figure is the apex, from which there arises 

 the five bounding planes of the pit; the basal plane 

 ascends very gradually to the surface of the crystal, its 

 intersection being almost invisible. The two smaller 

 faces rise abruptly from the pit to form the point of the 

 figures, which angle is about 65° and turned upward. 

 From the apex of the figure, the two lateral bounding 

 planes extend downward, gradually becoming narrower, 

 due to the rise of the basal plane which it intersects, and 

 finally becomes invisible without a definite bounding line. 

 These two planes differ in size, the left being longer than 

 the right. The left side of the figure extends parallel to 

 the intersection of the 1010 and 2130, while the right side 

 extends parallel to the left scalenohedral edge upon 

 which it occurs. The figures are asymmetric and asym- 

 metrically placed. 



Rhomb ohedr on — The unit rhombohedron r was suc- 

 cessfully etched with practically all of the common 

 acids, HN0 3 , HC1 and citric acid giving the best results. 

 The HC1 figures were obtained after two or three sec- 

 onds' immersion, and are very simple, triangular forms, 

 with the sides slightly curved, giving the figure a rounded 

 appearance (fig. 12). The apex is turned upward. The 

 figures are usually formed of three planes or faces, but 

 occasionally one possesses a basal plane ; the figures are 

 symmetrical to a vertical plane cutting the rhombohe- 

 dral face. 



At this point it may be well to consider the rhombo- 

 hedral figures of the other minerals of the series before 

 giving the results of the other solvents on the calcite 

 rhombohedron. 



Magnesite is not so readily soluble as calcite, conse- 

 quently strong HC1 diluted to one-tenth its strength with 

 water attacked this carbonate but slightly, and where 



