DALY, ETCH-FIGURES ON AMPHIBOLES. 403 



in one place, with sides mutually parallel. At the same time, other 

 aggregations could be found in which the individuals were bounded by the 

 same number of sides, and with angles sensibly equal, yet the corre- 

 sponding sides were not parallel and the orientation of the figures was 

 necessarily unlike. 



For some time I was without a clue to the meaning of these myriad 

 extraordinary figures, but another of the many valuable suggestions in 

 Becke's writings afforded some light on the problem.* When galenite is 

 etched with hydrochloric acid, the resulting chloride of lead often crystal- 

 lizes out in areas of local supersaturation of the liquid, particularly in 

 regions where the jiits are most numerous. The individual crystals of 

 the chloride may be locally oriented in the same way, and will doubtless, 

 in certain cases, favor a skeleton growth. They serve as a kind of pro- 

 tection to the surface on which they lie ; the acid will thus dissolve the 

 intervening parts of the general crystal-surface not so protected, and the 

 substance of the galenite underneath the chloride crystals is left projecting 

 as residual hills on corrosion. The common orientation of these crystals 

 and their skeleton-crystallization (touching the galenite surface only 

 where the regular growth of skeleton crystals would permit) could 

 explain the accordant attitude of certain similarly arranged groups of 

 the bosses. 



An analogous explanation is believed to apply to the curious etch-hills 

 on hornblende above noted. The chemical reaction is diflerent, the me- 

 chanical cause of differential attack is the same. Instead of hydrochloric 

 acid we have here hydrofluoric acid, and in place of a single resulting 

 compound, the chloride, there are probably several salts of hydrofluoric 

 and fluosilicic acids that are produced during chemical solution of the 

 bisilicate, and in the form of crystals or of skeletal aggregates might 

 serve as the protective caps in the lithographic process. What particu- 

 lar fluorides and silicofluorides would be most likely to play such a role, 

 it is perhaps not impossible to say. From their relative insolubility in 

 warm water, the prisms of fluosilicate of magnesium, the rhombohedrons 

 of the fluosilicate of iron, and the spindle-like crystals of the fluosilicate 

 of calcium, seem to be the most favorable to such action. The more 

 soluble octahedrons of the fluosilicate of sodium and hexagonal prisms of 

 the fluosilicate of potassium might also result in an atmosphere of hydro- 

 fluoric acid gas diluted with only a small proportion of vapor of water, 



* Aetzversuche am Bleiglanz, Min. und petrog. Mittheil, 1884-85, Bd. VI. 

 p. 240. 



