DALY. ETCH-FIGURES ON AMPHIBOLES. 421 



to secure etchings of this plane for augite nor of (TOl) for any py- 

 roxene. 



3. As was to be expected, the pits on the unit-prism of actinolite and 

 diopside are unlike in outline. I made the experiment of testing the 

 cohesional property of the actinolite substance on a plane making with 

 (010) an angle essentially equal to half the cleavage angle of diopside. 

 A new prism-face was thus ground on a crystal of Zillerthal actinolite, 

 then carefully polished and etched in the usual way, with cone, hydro- 

 fluoric acid after two minutes' exposure. The face was found to be 

 covered with etch-hills, the effect of strong attack (much quicker on the 

 artificial face than on any natural face in the prism-zone) ; but, among 

 them, a few pits which were surprisingly like those on diopside (see 

 Photograph 18), and on aegerine. Further comparison could be made 

 by etching an artificial face on diopside lying 62° 15' out of the plane 

 of symmetry, and also by using the caustic alkalies in this round of 

 experiment. 



4. From the close association of the pyroxenes with our group, it is 

 important to recognise that recently attempts have been made to remove 

 the diopsides from the holohedral class into a hemihedral (" domatische," 

 Groth) class of the monoclinic system. In 1889, G. H. Williams sug- 

 gested this hypothesis on purely crystallographic grounds, interpreting 

 the imperfect development of the planes about the extremities of the 

 vertical axis of crystals from Orange County, N. Y., and Canaan, Conn., 

 as an evidence of hemihedrism.* In this, he was followed by Dana in 

 the " System " (1892, p. 352). But that the failure of planes about one 

 end of an axis need not mean true structural lack of symmetry is well 

 known, and has lately been exemplified by etch-figures on cuprite that 

 restore it to the holohedral category. Pelikan rightly rejected this argu- 

 ment, but still, on the basis of etching results, he considers it probable 

 that the diopsides are nevertheless hemihedral in Williams's sense.f He 

 was led to this conclusion chiefly by the study of the HF pits on (010). 

 He figures some of these from a Nordmarken specimen which are un- 

 symmetrical in that lines drawn from one side to another through the 

 centre of a pit would not be bisected at that centi-e ; there is, in other 

 words, a lack of that antimetric (dimetric) character necessarily charac- 

 teristic of (010) if diopside be holohedral. In particular, the asymmetry 

 of the face is supposed to betray itself in the fact that one corner of the 

 rhomboidal figure may be truncated by a fifth figure-face while the oppo- 



* Amer. Jour. Science, 1889, Vol. XXXVIII. p. 115. t Op. cit., p. 19. 



