DALY, — ETCH-FIGURES ON AMPHIBOLES. 3T5 



Sir David Brewster observed his " O^Jtical Figures " on a few hornblendes 

 characterized by natural pits of corrosion.* Since nothing has been 

 done towards a comparative review of etching phenomena with respect 

 to the amphiboles, I shall state some of the reasons why the present 

 reseai'ch was begun. 



(1) In the first place, it has been considered a good opportunity to 

 test once more how far etch-figures are dependent on the method of 

 attack, and to devise a convenient and uniform method for the group in 

 question. (2) Will the etching process furnish any information as to 

 the attackability of amphiboles in hydrofluoric acid 1 (3) Are the etch- 

 figures variable in shape with the chemical composition of the mineral? 

 Will they throw any light on the problem of isomorphism among the 

 amphiboles ? (4) Will the amphibole figures by comparison with those 

 on the corresponding faces of pyroxene, tend to strengthen the parallel 

 between the two mineral groups ? (5) Can cleavage pieces and crystals 

 of amphibole be crystallographically oriented by means of etch-figures ? 

 (6) Will the latter give us any data on the vexed question of the best 

 standard orientation of amphiboles as a whole? Is Tschermak's or 

 Nordenskiold's recommendation better fitted to disclose the many va- 

 ried relationships of the group ? (7) Are the amphiboles holohedral ? 

 (8) Are anthophyllite and gedrite really orthorhombic ? (9) Inciden- 

 tally, in connection with the attempt to solve these problems, I have 

 compared etch-figures using hydrofluoric acid with those obtained with 

 the corrosive alkalies. 



Now, in order to pave the way for a concise description, and perhaps 

 readier understanding of the following discussion, a few lines may be 

 taken to define a certain number of terms which have been introduced 

 with more or less technical import. Several of these ai-e literal or 

 slightly modified translations of the valuable German names or phrases 

 of Becke, Baumhauer, Leydolt, and others. 



The etch-figure itself may be a cavity of corrosion, an " etch-pit " 

 ("pit of corrosion," Aetzgriibchen, Aetzvertiefung), or it may be an 

 etch-hill (Aetzhiigel), a residual boss standing up in relief above the gen- 

 eral surface of the crystal. Either etch-pit or etch-hill is bounded by 

 •' figure-faces " f (Aetzflachen), in general manifestly plane, sometimes 



* Phil. Mag., 1858, Vol. V., p. 16. 



t Molengraaf's nomenclature seems unfortunate in relegating the short, useful 

 word " Aetzflache " to the comparatively unimportant curved surface which often 

 truncates a crystal edge when exposed to corrosive agents, and compelling us to 

 speak of the figure-face of a pit as an " internal etch-face " (innere Aetzflache). — 

 Zeit. fur Kryst, 1888, Bd. XIV. p. 174. 



