244 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE _ [VOL. XI 
tinued, the quadrilateral pit became less clearly defined and finally 
assumed the form of a shallow depression, in some cases suggesting the 
more primitive first stage. 
Under similar conditions pits of the same type were produced upon 
the prism faces of other examples not only from the same locality, but 
also from Zillerthal. In one case, however, while the pits on one side 
were like those already described, the adjacent prism exhibited pits of 
a new form. They were of an unusually deep, almost square shape, 
composed of four triangular figure-faces meeting at an eccentric point. 
Experiments of a similar nature had already been made by Greim.*® 
In his description reference is made to two types of pit, enclosed, one 
by three, the other by four, triangular figure-faces. It would seem pro- 
bable from the preceding experiments, that the two types of Greim are 
merely pits in two of the stages of that development which is charac- 
terized by a successive interpolation of additional figure-faces, as has 
been brought to light by the method of intermittent etching. 
(b) Puts on the clinopinacoid. 
Pelikan’ experimenting with Nordmark diopsides etched by hydro- 
fluoric acid, produced pits on the clinopinacoid which he described as 
of two kinds as follows: 
(1) Shallow, of rhomboidal outline, the longer pair of sides being 
parallel with the prism edge, the other pair inclined in the direction of 
the edge between o10 and oo1. Frequently the sharp angles are trun- 
cated by a pair of small faces making the pit six-sided. 
It will be noted that such a pit would represent the grade of symmetry 
to be expected on the clinopinacoid of a monoclinic crystal, a face 
possessed of a binary axis of symmetry. His second type, which did 
not indicate a symmetry of so high a grade, was described as: 
(2) Apparently deeper than (1), usually of somewhat rhomboidal 
outline, but with the upper edge parallel to the edge between the basal 
and clinopinacoids. 
This is a pit without an axis of symmetry. 
These two forms sometimes occurred in correlated position such as 
might be expected upon the face of a crystal twinned parallel to the 
orthopinacoid, but as they were not found in vertical rows, Pelikan 
concluded that this position was not the result of twinning but of a 
difference in the molecular structure of the crystal. This peculiarity 
would seem to be inherent in the diopside molecule, for he found these 
two types in all diopsides examined, and in augites of a chemical com- 
Sop: Cit. 
7 Min. u. petr. Mitth., 16: 1. 1896. 
