246 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE  [VOL. XI 
action from time to time, as described in the previous section we arrive 
at the conclusion that age is an essential factor in determining the form 
of the pit. This became more evident by the following experiments 
which were made with a view to the determination of the changes 
which take place during the growth of the pits on the clinopinacoid. 
One of the first points to be noted in the experiments to this end is 
the striking difference between the rates of corrosion on the various 
faces. The crystal first used, when corroded sufficiently to give clear 
pits upon the prism, showed an orthopinacoid almost entirely removed, 
being covered with a closely lying series of long parallel grooves. A 
similarly striking difference between the rates of corrosion on different 
crystals was also observed, for, when a crystal of spodumene and one 
of diopside were heated together in a mixture of fluorite and sulphuric 
acid over a low bunsen flame for thirty minutes, on the cleavage plane 
of the former there were well-defined pits while each face of the latter 
had been etched into hills. 
After a sixteen-minute exposure to the action of fluorite and sulphuric 
acid over a bunsen flame good pits were obtained upon the clinopinacoid. 
Many of them had a regular rhomboidal form, being enclosed by four 
triangular figure-faces (Plate XXVI, fig. 24). In some cases however the 
parallelism of the sides was not perfect, with a result that the symmetry 
of the pit was of a lower grade than was to be expected on the clinopin- 
acoid of a monoclinic crystal (Plate XXVI, fig. 25). A few even showed the 
replacement of one of the angles by a fifth face (Plate X XVI, fig. 26). 
On a specimen from Zillerthal etched for 33% minutes by dilute 
hydrofluoric acid at 100°, some interesting pits were obtained whose 
orientation indicated the occurrence of repeated twinning. The posi- 
tion of the several individuals of the twin was shown by pits lying side 
by side in reversed position (Plate X XVI, fig. 27). Between these were pits 
which were bilaterally symmetrical showing the position of a plane 
of symmetry. These evidently lay over the edge of the twinning face. 
Apparently the point of origin of the pit was on this edge, resulting in a 
kind of pseudosymmetry. 
In the experiments here described, pits were obtained which corre- 
spond with all those mentioned in the papers to which reference has 
been made. Most of them are of Pelikan’s first type which is about the 
same as the third of Baumhauer, and the first of Greim. A number were 
like the second of Pelikan in which the two shorter sides are not parallel. 
Several of the experiments resulted in pits in which the absence of 
parallelism was characteristic of the long faces and in some cases neither 
pair was parallel. The additional pair of figure-faces mentioned by 
Pelikan was frequently observed, but did not seem always to charac- 
