260 ‘TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE [VOL. XI 
It must be borne in mind, however, that the figure-faces of the models 
are those which might be expected to be formed in etching if their 
development could go on unhindered. That is to say, the pits in the 
models are enclosed by the minimum number of simple, plane faces, 
not the indefinite and vicinal faces which we find in actual experience. 
The model representing the pit on the orthopinacoid will be ob- 
served to be similar to that obtained by etching, with the exception of 
the faces which close the lower end. These are so inclined as to leave 
an overhanging edge. This, as has been pointed out by Goldschmidt, 
in the article already referred to, will provide a point of vigorous attack 
for the solvent, until the edge is entirely removed. Thus we have 
the typical pit on the orthopinacoid. It should be remembered in this 
connection, that in a specimen that has been somewhat overetched 
one end of each pit is much less clearly defined than the other. The 
whole pit appears as a groove closed at one end and gradually becoming 
shallower, until it reaches the surface of the crystal face. The reflection 
from an orthopinacoid covered with pits of this kind shows a bright 
light-path corresponding with the prismatic figure-faces, two moderately 
bright ones corresponding with the faces at the well-defined end, and 
some faint traces, representing the other end of the pit. This was 
repeatedly observed in examining crystals with etching developed to 
this extent. All of which shows that at the point where according to 
theory we should expect the maximum amount of corrosion, this special 
rounding off of the pit edges actually takes place. 
The model representing the theoretical form of the pit on the ortho- 
dome, will be seen by comparing it with figure 31 of Plate X XVI, to beara 
striking resemblance to the pit actually obtained by etching. Similarly 
we find the same relationship between the theoretical and experimental 
results in the case of the pit on the basal pinacoid, as shown by referring to 
Plate X XVI, figure 32. In both model and drawing, the faces closing the 
deeper end of the pit are seen to be steeper than in the case of the dome. 
Since the pit on the clinopinacoid was taken as the basis from which 
the negative corrosion crystal was built up, the actual form will neces- 
sarily be the same as the theoretical. 
The pit on the prism as deduced in this theoretical way, will be seen 
to be potentially similar to that actually obtained by etching, if we 
remember, as in the case of that on the orthopinacoid, that the over- 
hanging edge left at the lower part will be the starting point of vigorous 
solution and quickly removed. 
We have, by these experiments, been able to deduce a generalization 
which may be stated as follows: The relationship between pits developed 
upon the various faces of the same crystal may be shown if a negative 
