382 PROCEEDINGS OP THE AMERICAN ACADEMY. 



some precision a scale by means of which crystal faces may be defined 

 as to molecular cohesions. This has, in fact, been accomplished in 

 certain cases ; but, in general, etch-figures have only been used quali- 

 tatively, so to speak, and as yet there has been not enough of continuity 

 of method from one investigation to another to make possible detailed 

 comparison of species with species in this matter of facial cohesion. 

 In the particular case of the group here considered, it has seemed 

 desirable to make it possible to reproduce the conditions finally selected 

 for etching, so as to permit of the discussion of new amphiboles with 

 the aid of data already in hand. May it not be possible that the 

 contradictory results of certain observations is simply due to difference 

 in methods ? Thus, Penfield, Meyer, and Bomer found the basal 

 plane of quartz characterized by etch-hills when hydrofluoric acid was 

 used,* while with the same solvent Gill obtained pits of corrosion.f 

 For these and other reasons noticed below, I have tried to establish 

 a constant method which would give good results with all varieties 

 of the great amphibole family, and one which can be extended to the 

 pyroxenes and other silicates. That such a method be wrought out, it 

 was necessary that some preliminary experiments should be made, for 

 reasons, some very obvious, others less so, all of which I shall summarize 

 in this connection. 



1. It is generally advisable to choose prominent crystal-faces, usually 

 those of simple indices aud those parallel to cleavages. That one would 

 select such faces is to be expected, but I think this point should be espe- 

 cially in mind if comparisons are to be made as widely as possible. An 

 advantage in choosing cleavage pieces or the corresponding crystal-face 

 is evident in those groups where certain members appear only as allotrio- 

 morphic individuals in rock aggregates. Even in these cases, their etch- 

 figures may be produced on a good cleavage when figures on other planes 

 would ouly be possible on artificial faces. Fortunately, too, for the 

 discussion of rock-forming amphiboles as well as of the group as a whole, 

 the cleavage pieces give the sharpest and most regularly developed 

 figures to be obtained on any given crystal. J The prismatic cleavage of 

 amphiboles has thus a superior claim to attention, and I have accordingly 

 laid most stress on this important face in the course of the present inves- 

 tigation. Next to these, pinacoids will naturally give the most useful 



* Trans. Connecticut Academy, 1889, p. 157. Neues Jahrb. fiir Min., etc., 1891, 

 Beil. Bd. VII. p. 534. 



t Zeit. fiir Kryst., Bd. XXII. p. 111. 



t Cf. Bauiuhauer, Resultate der Aetzmethode, p. 3. 



