DALY. ETCH-FIGURES ON AMPHIBOLES. 391 



Similar results were found with crystals of Actinolite (V. IG), (V. 17), 

 and of tremolitic anaphibole (V. 8). Tlie series are, respectively, (TOl), 

 (Oil), (100), (110), and (Oil), (010), (110).* 



These conclusions illustrate the now well established method of inter- 

 preting molecular structure by solubility in different crystallographic 

 directions.! The series of facial attackability correspond to the dimen- 

 sional variations in the pits on the various faces of an amphibole. They 

 explain, for example, the very general elongate form of the pits on planes 

 of the prism zone, an elongation in a vertical sense and coupled with a 

 greater resisting power in the prism zone itself. I may further note, in 

 passing, that here the different zones behave, as always, in a way to indi- 

 cate the holohedral character of all the amphiboles. The " Lusungsober- 

 fiiiche " (Becke) of amphiboles is monosymmetric and centrosymmetric. 



Description op the Etch-Figures. 



We may now proceed to the characterization of the etch-figures 

 themselves. It has been thought most convenient, and as tending towards 

 an easier survey of the facts, to group them with reference to crystallo- 

 graphic planes primarily and to consider in order the behavior of each 

 species on etching each of tliese planes with hydrofluoric acid. TVe may 

 hope thus to lessen the repetition of detail necessary in some degree ; at 

 the same time, the essential features of likeness or unlikeness of the dif- 

 ferent varieties will appear with most clearness. Following this more 

 or less bald statement of fact, which is abbreviated as far as consistent 

 with our immediate aims, there will be an attempt to correlate the facts 

 both in way of summary review and as related to certain others which 

 shall be especially introduced in the general discussion. 



Etch-Figures on (HO), Actinolite Type. 



Throughout the whole series of non-aluminous amphiboles (excepting 

 riebeckite) which I have yet studied (actinolite, tremolite, smaragdite, rich- 

 terite, astochite, etc.), cleavage pieces give figures that are practically 



* One fresh crystal of Zillerthal actinolite etched with an alkali, caustic soda, 

 displayed greater resistance to attack on (010) tlian on (110). This may be another 

 illustration of tiie rule enunciated by Bauraliauer, that, in certain cases, the direc- 

 tions of rapid solution are reversed for acids and alkalies. Thus, he found it to be 

 true for Linneite when he compared its behavior in the process of etching using 

 nitric acid with its behavior in Becke's experiment of etching with caustic potash 

 (Resultate der Aetzmethode, p. 20). The same holds true for magnetite (Ibid., p. 21). 



t Cf. Baumhauer, Resultate der Aetzmethode, p. v. 



