1916] THE GROWTH OF ETCH-FIGURES 251 
A thirty second exposure gave pits of somewhat similar form but 
with the two curved faces replaced by two short straight ones (Plate XXVI, 
fig. 34). This suggests that in alkaline etching the same series of events 
takes place as that which we have seen in the case of acid etching, 
namely the replacement of curved lines by several shorter straight ones. 
However, it was difficult to obtain clear pits and in consequence there 
was not sufficient data upon which to generalize. Extended corrosion, 
while it seemed to make the pits upon the prisms clearer, did not enable 
one to obtain any upon the pinacoids. After 40 seconds’ exposure, there 
were no clearly defined pits upon the orthopinacoids, while the clinopina- 
coid had merely minute, ill-defined spots of roughening. 
Part II—THEORETICAL. 
1. INTRODUCTORY. 
In the first part of this paper there is described a series of experiments 
with three monoclinic minerals, together with a statement of the ob- 
served phenomena. It now remains to combine these results so as to 
see their bearing upon the general problem of the origin and growth 
of etch-figures, and as we do so the processes involved begin to take 
upon themselves a somewhat new significance. 
The new generalizations to be deduced from the data now at our 
disposal may be stated briefly as follows. In the first place the point 
of origin and the distribution of the pits depend upon the molecular 
structure of the crystal and not upon any external and adventitious 
cause. In the second place in its development a pit passes through 
a regular series of stages comprising growth, maturity and decay. In 
the third place, there is a relationship between the faces of the pits 
developed on the various faces of a crystal, and in the fourth place the 
beaks are shown to be of three kinds, and suggestions to account for 
their occurrence have been offered. 
2. THE POINT OF ORIGIN. 
As has been pointed out in the experimental part of this paper, one 
of the striking and unexpected features of etch reactions is that the pits 
are not distributed evenly over the crystal. The question why one part 
of the etched face should be free from corrosion, while adjacent parts 
are closely covered with pits, is one which has attracted the attention 
of many investigators, and in consequence many explanations have 
been offered. It is evident that in order to provide an adequate theory 
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