1916] THE GROWTH OF ETCH-FIGURES 253 
or less straight lines. Pits occurring in this linear arrangement were 
described by Baumhauer, who called them “Aetzgraben”’. These 
lines are usually irregular, but examples have been frequently observed 
in which definite crystallographic directions are thus marked by rows 
of pits. For instance, BOhmer described pits on quartz crystals of con- 
centric growth, which were found to have a corresponding concentric 
arrangement parallel to the combination edge between +R and —R.% 
An even more suggestive example was that previously referred to in 
the case of diopside, in which the pits were found along the edge of a 
twinning plane. 
It is evident that these lines of selective pitting are also lines of 
weak cohesion, and we must remember that those faces which present 
the maximum cohesion, such as cleavage planes, are corroded much 
more slowly than those of a lower degree. It has also been noted” that 
when a crystal has been fused, the resulting amorphous substance has 
a lower specific gravity, indicating that the molecules are farther apart, 
and in consequence the cohesion must be less. Such material has less 
resistance to the attack of the solvent than had the original crystal. 
In other words, the stronger the cohesion the greater the resistance to 
corrosion. 
It seems evident therefore that wherever cohesion is less than the 
average for any crystal face, there is in consequence a point where the 
corrosive can begin its action. In certain particular cases, such as along 
the edges of cleavage and twinning planes, the connection between the 
location of pits and points of weak cohesion is clearly seen, and in others, 
while not quite so evident at first sight, this relationship is demon- 
strable, for there can be nothing else in common which could account 
for the similarity of the arrangement on adjacent sides of cleavage 
plates. We are now in a position to deduce the law that the point of 
origin of every pit is at a centre of weak attachments between adjacent 
crystal molecules. Since, in the process of solution, fresh surfaces are 
constantly being formed, each of which has its own crop of pits, we 
conclude that all through the mass of the crystal there are scattered 
these points of potential pit origin. 
It is interesting to note, however, that some individual pits have 
the capacity of attaining a much greater size than the rest. At first 
the pits are practically of the same dimensions, but as time goes on, 
here and there some will be found to be noticeably large. In the ex- 
 “ Resultate”’, page 6. 
16 N. Jahrb. f. Min., Beil. Bd., 7: 540. 1891. 
M1 Walker, ‘“‘Crystallography”’, N.Y., 1914, page 44. 
