394 PROCEEDINGS OF THE AMERICAN. ACADEMY. 



Strongly attacked Zillerthal actinolite regularly exhibits normal etch- 

 hills on the prism, but, since they are not of immediate interest in 

 connection with our main j^urposes, I shall pass them over with this 

 mere mention. 



MoNOCLiNic Alujiinous Amphiboles. Etch-Figures on (110). 



As might have been expected, I have observed a considerably greater 

 variety in the aluminous amphiboles than in the relatively few non- 

 aluminous species that, so far, have yielded figures that can be discussed. 

 The pits of corrosion are, however, reducible to three types, respectively 

 characteristic of basaltic hornblende, glaucophanc, and arfvedsonite, and 

 it is by the names of these species that we shall know the types. 



The Hornblende Type {\\Q). 



To use an expressive German designation, the term " hornblende " 

 is a " Sachname," and signifies a large number of bisilicates which are as 

 yet not dignified by more specific names because of our ignorance of 

 their real nature. If the phenomena of etcliing are to have weight in 

 the discussion of the isomorphism or the orientation of cleavage pieces 

 of hornblendes, it is expedient to examine an extensive and representa- 

 tive suite of specimens from the different members of the group, and 

 determine what are the variations in the figures of corrosion along the 

 series. I have accordingly etched about one hundred crystals and 

 cleavage plates from the thirty localities mentioned in the list of mate- 

 rials used. The result was to show that all these species will give etch- 

 pits whose main characters are constant, but permit of the recognition 

 of at least four sub-tyi)es. From the localities of specimens that 

 illustrate tliem best, these may be called the Wolfsberg, the Kragero, 

 the Edenville, and the Philipstad sub-types. With the exception of 

 the last, crystal-face (110) and cleavage (110) give invariably the 

 same figure. 



The Wolfsberg Sub-type of Corrosion Pits on (110). 



The front positive prism-face on the lustrous basaltic hornblendes of 

 Norway, Bohemia, Vesuvius, etc., uniformly give an etch-pit, scalene- 

 triangular in shape, with the most acute angle (corner) pointing down- 

 wards and the next most obtuse angle (corner) pointing northeast (Fig. 2, 

 Plate I. Figs, b, 6, 7, and 8). In a mature figure (Fig. 2), the three figure- 



