412 PROCEEDINGS OF THE AMERICAN ACADEMY. 



may reduce the iagures and their individual characters to law, and make 

 the etch-pits of determinative value. 



Owing to lack of material, etch-figures were not obtainable on 

 the clinopinacoid of barkevikite. crossite, glaucophane, riebeckite, or 

 arfvedsonite. 



Amphibole Etch-Figures on Faces other than (110) 



AND (010). 



From the preceding sketch of the pits of corrosion on the prism and 

 clinopinacoid of the aluminous and non-aluminous groups of amphiboles, 

 respectively, it is evident that the general figure-types are modified by 

 variations in the chemical nature of the species. But the prime impor- 

 tant modifications are conditioned by the presence in the riiolecule of 

 alumina, or at least of a sesquioxide. That is, in the one class, we have 

 to do with amphiboles whose molecular constitution is similar, excepting 

 perhaps arfvedsonite ; whether it be lime, iron, magnesia, or soda, — any 

 or all of them, — that, together with alumina, compose the complex 

 silicate, the results of etching are always similar.* On the other hand, 

 so soon as the alumina (sesquioxide) molecule disappears, or is present 

 in only very small amouqts, there is a radical change in the etch-pits on 

 both faces. Tliis leads to the expectation that other planes will show 

 corresponding change. The few specimens which I have been able to 

 secure confirm that conclusion, and a brief account of the observations 

 thereupon may prove of interest. 



Before proceeding directly to them, however, I shall state the negative 

 results characterizing the examination of three other faces (130), (Oil), 

 and (111) ; their indefinite etching phenomena did not allow of compar- 

 isons by measurement. V. 42 at one minute's exposure exhibited many 

 pits on (130). They were warped triangles, with the upper acute corner 

 pitching into the crystal mucli after the manner of the analogous hair-like 

 projections described by Tschermak on siderite. (See Baumhauer, 

 Eesultate der Aetzmethode, Microgram 20.) The same crystal of V. 42 



* Mr. Walker's use of the term "similar" to denote an enantiomorphous rela- 

 tionship between etch-figures seems to me to be inadvisable since it is not needed 

 in that sense, and such a usage deprives us of a convenient designation for figures 

 that are not identical but differ from one another very slightly as in the case of 

 the hornblendes. (American Journal of Science, 1898, Vol.1, p. 181.) "Analogous" 

 might be employed in this connection rather more freely to mean similarity in 

 some one or more features, and would need supplementary statement to indicate 

 wherein the analogy consists. 



