T. L. Walker — Examination of Triclinic Minerals. 181 



111, 111 and 111 are equal to one another and are identical in 

 form — these triangles can be placed upon one another so as_to 

 coincide without being turned over. The faces 111, 111, 111 

 and 111 bear to each the same relationship and may be made 

 to coincide by superposition without being turned over. The 

 triangles of the first group cannot be made to coincide except 

 by turning over. The triangles of these two distinct groups 

 are related to one another as right and left. Suppose that on 

 such faces we have etching figures, triangular in outline and 

 with their edges parallel to those of the triangular face upon 

 which they occur. They too must be related to one another 

 as right and left, but with this difference, that the corrosion 

 pits have three dimensions, and by turning over the triangular 

 outlines may coincide, but the pits will then project in oppo- 

 site directions and will form a closed space which will be sym- 

 metrical about the plane of contact. This is quite like the 

 relation that the valves of a clam shell bear to one another — 

 they can never be so turned as to fit into one another. So also 

 the cast of a corrosion pit on a pyramid face of one of the 

 above groups cannot be fitted into a pit on a face of the other 

 group. The one is the mirror image of the other turned about 

 180°. The figures accompanying this paper have generally 

 been turned in this way so that the image relationship may be 

 plainly seen. 



For clearness it would be well to refer to figures which 

 agree in all respects as identical — those on all the faces of 

 either of the above groups are identical y figures which have 

 the right and left relationship may be referred to as similar, 

 while all corrosion pits not belonging to either of these classes 

 may be designated as different. 



The dagger-shaped figures on the faces 111 and 111 of axi- 

 nite crystals being similar, we must conclude that the mineral 

 is centro-symmetric. 



Cyanite. — Pale blue crystals from St. Gothard were used. 

 They are well developed in the zone of the vertical axis show- 

 ing the forms (100) (010) and (110). The crystals are contained 

 in paragonite schist and are elongated in the direction of the 

 vertical axis. As the mineral is very cleavable along (100), 

 good material is easily obtained for etching. Their cleavage 

 fragments, which had been previously carefully warmed, to 

 avoid decrepitation or unnecessary shattering from suddenly 

 heating, were plunged into a red-hot mixture of potassium bi- 

 sulphate and coarsely powdered fluorspar. After one or two 

 minutes they were taken out, allowed to cool slowly and then 

 cleansed by boiling with hydrochloric acid. Very satisfactory 

 figures were obtained. Those on 100 are shown in fig. 10 



