180 T. L. Walker — Examination of Triclinic Minerals. 



difference might be found between the etching upon the upper 

 and under surface of a cleavage plate. This was found to be 

 the case; cleavage plates exposed to the action of dilute hydro- 

 chloric acid showed upon both sides the characteristic etched 

 triangles figured by Penfield. These were identical in form, 

 but very different in dimensions ; the plates were invariably 

 attacked with much greater rapidity upon the lower or attached 

 end than upon the surface directed toward the unattached end 

 of the crystal."* 



I was not aware of Miers' experiments till after those on 

 tourmaline and acid dextro-tartrate of strontium were com- 

 pleted and this paper written in almost its present form. 



From the experiments above described I think the desired 

 confirmation has been given to the statement : — Crystal faces 

 oelonging to the same crystal form give identical or similar 

 etching figures, while faces oelonging to different crystal forms 

 give different etching figures even if the faces he parallel to 

 each other / or conversely, if parallel faces give different etch- 

 ing figures, the faces do not oelong to the same crystal form. 



Taking the above statement as a basis, I proceeded to exam- 

 ine some of the common triclinic minerals with a view to 

 determining which of them, are centrosymmetric and which 

 asymmetric. 



Axinite. — I was fortunate enough to obtain a few of the 

 beautiful crystals from Bourg d'Oisans in Dauphine. This 

 mineral has been crystallographically described by many 

 authors, so that several different ways of orienting the crystals 

 are in common use. The references in this note are to Kall- 

 mann's orientation, which is probably the one most generally 

 used by English and American mineralogists. 



By acting upon the crystals for five minutes with a hot mix- 

 ture of equal volumes of concentrated sulphuric and hydro- 

 fluoric acids, very sharp figures were obtained. The most 

 easily examined and at the same time most striking are those 

 on 111, which is the face designated by the letter r by Kall- 

 mann and Schrauf._ The figures contain no line of symmetry. 

 Those on 111 and 111 are illustrated in figures 8 and 9 and are 

 related to one another in the same way as the right hand is to 

 the left. This, however, is in accord with crystallographic 

 characteristics of the system, the faces being related to each 

 other in the same way, so far as outline is concerned. This 

 dissimilarity is well seen in the case of the pyramidal faces in 

 the rhombic system. The eight faces of a simple rhombic 

 unit pyramid are all scalene triangles; the triangular faces 111, 



* Min. Magazine, vol. x, p. 276, 1894. 



