374 PROCEEDINGS OP THE AMERICAN ACADEMY. 



PAGE 



The Dome (101) . 413 



Isomorphism in the Monoclinic Amphiboles 415 



Holohedral Character of the Monoclinic Amphiboles 418 



Comparison of the Amphiboles and Pyroxenes as to Etching Properties . . 419 



Crystallographic Orientation of the Amphiboles 422 



Optical Orientation of an Amphibole Crystal or Cleavage Plate by Means of 



Etch-Pits 423 



Etch-Figures on Anthophyllite and on Gedrite. Orthorhombic and Holohe- 

 dral Character of these Minerals 424 



Etch-Figures on Aenigmatite 425 



Summary of Conclusions 426 



Introduction. 



Among the larger groups of rock-forming silicates, there is none 

 perhaps which, in the present state of our knowledge, ofTers more dif- 

 ficulties in the determination of systematic relationships than the amphi- 

 bole family. Its importance for the petrographer needs no emphasizing 

 here, yet it is he who has to meet the difficulties of classification and 

 discussion under the most disadvantageous circumstances ; in general, by 

 reason of its association, the amphibole of an eruptive rock or of a crys- 

 talline schist lacks crystal form, and, because of numerous inclusions, it 

 may often be impossible to procure a reliable chemical analysis of the 

 mineral. Thus deprived of two principal aids to diagnosis, the worker 

 in rock-forming amphiboles must make the most of the other criteria 

 which offer themselves. In so doing, he may eventually be able to repay 

 the pure mineralogist for his services to the study of the crystalline rocks 

 and present new considerations that can lead to the interpretation of the 

 mineral species as such without relation to rock genesis or rock classifica- 

 tion. Of the methods which, so fnv, have been almost completely neg- 

 lected by petrographers in the investigation of amphiboles, is that of the 

 use of etch-figures on planes of the more important zones. I propose 

 in the following pages to record briefiy certain results I have obtained 

 while breaking ground in this new field of inquiry. 



The first, and so far as I have learned, the only published reference 

 to actual experiments in etching an amphibole, occurs in Boricky's first 

 essay on microchemical methods.* Plate II. Fig. 7 of his work repre- 

 sents a hornblende etched with fluosilicic acid on the clinopinacoid. The 

 reference in the text to this drawing was occupied with the mention of 

 the chemical reaction, and especially of its products, — nothing further. 



* Archiv d. naturw. Landesdurchforschung von Bohmen, III. Prague, 1877. 



