Dihexagonal Alternating Type. 209 



lent figures were produced. The etchings produced by the 

 three solvents HO, HN0 3 and the mixture of the two 

 acids, are very much alike, but the solvent composed of 

 the mixture of the two seems to favor the HN0 3 in the 

 form of the figures. The fourth solvent was citric acid, 

 and this being very unlike the other acids, produced fig- 

 ures which are unique and unlike, in form, any of the 

 preceding figures. However great the difference in the 

 etchings they are alike in that they all reveal a vertical 

 plane of symmetry, and indicate by their relative posi- 

 tions the alternating axis c, which corresponds with the 

 calcite type. 



Second Order Prism, 1120 — The 1120 prism occurring 

 on the scalenohedral crystals from Cumberland, England, 

 was not well developed and too much striated for etching, 

 consequently, simple crystals from Patterdale, Cumber- 

 land, which possesed only the second-order prism and 

 the base, were used in the investigation of this form. 



10-percent HC1 etched the second-order prism and the 

 base after about 20 seconds. The figures of the 1120 form 

 were rather slow in developing, but the face was quite 

 readily attacked by the acid, as deep solution lines run- 

 ning parallel to the cleavage lines could be easily 

 distinguished long before the figures had attained the 

 mature stage of development. The grooves are of two 

 kinds : one, very narrow and shallow, and very straight ; 

 the other, deep, and broad with edges quite irregular. 

 These elongated depressions are formed by the inter- 

 growth of etch figures, and extend in a diagonal direction 

 (fig. 4). The lighter grooves extend parallel to two 

 of the four edges of the etch figures, and meet the prism 

 edge at an angle of 45°, and on the adjacent face it con- 

 tinues at right angles to its former direction. 



The large grooves are also formed by figures, but in 

 this case, solution has continued across the face at right 

 angles to the longer diagonal of the rhombic figures, 

 which can be clearly distinguished in the bottom of the 

 groove. In the other case it will be remembered the solu- 

 tion proceeded in a direction parallel to two sides of the 

 rhombohedron, consequently the elongated depressions 

 vary in size. The deeper groove makes an angle of about 

 60° with the prism edge, and about 69° with the smaller 

 .QToove. The rhombohedral cleavage cutting the prism 

 face measures about 45°, so it is probable that the light 



