DALY. — ETCH-FIGURES ON AMPHIBOLES. 425 



ligure-faces (Plate I. Fig. 3Gd). These, like the smaller elliptical pits, 

 possessed one plane of symmetry transverse to the trace of the cleav- 

 age, and were, of course contrary to expectation, sensibly symmetrical 

 to a plane parallel to the cleavage trace. The other three planes of the 

 unit prism showed figures that clearly indicated the holohedral ortho- 

 rhombic character of anthophyllite. Gedrite (P. 4 at 8 miimtes' exposure) 

 afforded precisely similar phenomena to those of anthophyllite. 



These observations confirm the optical determinations of orthorhombic 

 symmetry by Des Cloiseaux, and meet the objection thereto by Ilintze,* 

 who referred to the possibility that both anthophyllite and gedrite may 

 really be monoclinic, but imitate the optical behavior of an orthorhombic 

 mineral. That these minerals are monoclinic for chemical reasons has 

 more recently been suggested by Retgers : f we have once again an 

 illustration of how etch-figures may come to the help of the chemical 

 theorist. 



In passing, it may be noted that the plane of easy parting (010) in 

 hypersthene (P. 69) and bronzite (P. 67 and P. 68) gave with hydro- 

 fluoric acid extremely sharp pits with six-sided bisymmetric outline, that 

 betokened orthorhombic symmetry for these minerals, though I could 

 obtain no certain residts on etching (110). 



Etch-Figures on the Prismatic Cleavages (110) and (ITO) 

 of Aenigmatite. 



Without destroying the splendid crystals of aenigmatite which Mr. 

 Ussing was good enough to place at my disposal, I was able to etch and 

 orient cleavage pieces of that mineral. The crystals showed (Brogger's 

 orientation) the planes (110), (ITO), (010), (Tol). The standard con- 

 ditions of etching were again used, and, after some trouble, both cleavages 

 were finally attacked with successful outcome. Figure 13 and Plate I. 

 Fig. ola will give an idea of the pits. They are triangular in outline, 

 very analogous to the pits on the cleavage of common hornblende, but 

 with a decided peculiarity of orientation. The attitude of the figure-faces 

 shows a greater resistance to attack in a direction parallel to the edge 

 110 : ITO than at right angles to it, a phenomenon we have already no- 

 ticed in treating of the analogous position of the caustic soda figures on 

 arfvedsonite.J Neighboring pits may overlap, and thus the figure-faces 



* Handbuch, p. 1180. 



t Zeit. fiir phys. Chemie, 1895, Bd. XVI. p. 618. 



I Baumhauer observed the independence of etch-figures and cleavage in the 

 case of several species. Poggen. Annalen, 1872, Bd. CXLV. p. 460. 



