MlNKUALiKirCAL NOIES — ANDEKSON. 7 



The base here presents an appeai'aiiee quite unlike tliat uf the 

 foregoinfj specimens. It shows numerous hexagonal pits oriented witli 

 their bouudai-ies parallel to the edges rj^i and running into one another to 

 form approximately rectilinear depressions traversing the basal surface 

 with a general trend parallel to these edges. Some of the pits are •'.\ mm. 

 in diameter, and some are so small as to be just visible under high 

 inagnitit'ation ; the smallest pits seem circular Avhen examined by a lens, 

 but under the microscope they are seen to be bounded by sti-aight edges 

 and to have the usual hexagonal form. No tlihexagonal pits are 

 recognisable. The pits descend by steps to the bottom Avhich is generally 

 formed by the basal plane ; this may be in its turn pitted in the same 

 nuinuer, but on a smaller scale, so that we have pits within pits. In 

 other cases the pits terminate downwards in a point. The walls of the 

 pits are sloping and though somewhat striated they reflect light fairly 

 •well. .A sulphur cast of part of the base was made with excellent i-esults, 

 and gouiometric measurement proved that the walls of the pits are 

 formed by faces of the fiist order pyramid p (1011) ; the angle p averages 

 29"^ 51', calculated 29° 58'. The marginal portions of the ciystal are 

 terraced as indicated by the irregular wavy lines resembling contour 

 lines, this effect being apparently a result of the zonal groAvth, thin 

 shells being stripped off in succession. 



The pits on the base are quite similar to previously described natural 

 and artificial etch figures-^, and there is no doubt but that they are the 

 result of the action of some solvent. As the etch pits are bounded by 

 faces oi p (lOll) we may conclude that these planes are produced by 

 the process of etching. 



Crystal xiii. (PI. ii., fig. 2) is a short section of a prism measuring 

 '6 cm. in diameter. It is photographed as a transparency to show the 

 zoned structure and illustrates the common feature that the nucleus and 

 central portions of the crystal are more complex than the exterior layers. 



Crystal xiv. (PI. i., fig. 5), from Heffernan's Mine, is a composite 

 crystal consisting of four portions in parallel position ; the whole 

 measures 5 X !^ cm. The terminations have been corroded and rounded 

 so that no faces are now distinguishable except small areas of the base. 

 It is probable that the whole crystal has been reduced in length as the 

 result of corrosion, and the components of least diameter have been 

 reduced most, as one should expect. 



Crystal xv. (PI. ii., fig. 1) has evidently been strongly etched so 

 that the termination is now reduced to a series of sharp, irregularly 

 distributed pinnacles ; the prism too has been attacked and eaten into by 

 the solvent. This specimen has considerable resemblance to that figured 

 by Lacroix'*, who attributes the peculiar appeai-auce to corrosion. 



a Traube— Neues Jahrb. Min., Beil.-Bd. x., 189(), pp. 4()4-4H8 ; Kuhhiiann— 

 Ih., Beil.-Bd. xxv., 1907, pp. 173-174; Honess— Auier. Jouiu. Sci., xliii., 1917, 

 pp. 22:3-236. 



* Lacroix — Mineralogie de la France, ii., 1897, tig. 2, p. 11. 



