254 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE _ [VOL. XI 
periments with colemanite it was noted that some of the pits grew 
vertically downward into the mass of the crystal. In the diopside 
experiments pits were observed on the orthopinacoid which developed 
to great size and perfection. 
These differences cannot depend upon the corrosive, as they are 
formed under various conditions. They cannot be accidental in view 
of what we have already seen as to the causes of the first pit formation. 
It seems therefore reasonable to suppose that the centres of weak co- 
hesion are more extensive in some cases than in others. 
The theory of the distribution of centres of weak cohesion through- 
out the crystal gives us an explanation for another peculiarity of etching. 
Daly has observed® the occurrence of what he calls ‘‘stepped pits”, 
which are deep forms whose sides have a laminated appearance. He 
explains these as being due to the edges of parallel cleavage planes 
being brought to light by the corrosion. Pits of this type, while few 
in number, have been observed from time to time in the course of this 
research, the most notable instance being that referred to and illustrated 
in the experiments with colemanite. If a centre of weak cohesion, and 
therefore, one of potential pitting, be of sufficient extent to lay bare, 
before the pit has reached its limit, another such centre, the result will 
be a pit within a pit, and this process, if carried still further, will result 
in a large pit of the stepped type. The steps in each case being the 
remnants of the bottoms of the pits which have been so enlarged. 
Our deduction as to the point of origin leads us a step further. Since 
pits are not distributed evenly over the etched face, it follows that the 
centres of weak cohesion are similarly irregularly placed. Hence the 
cohesional attachments of a series of crystal molecules in the face will 
not be uniform. In consequence, the cohesion of any crystal face, as 
indicated, for instance by its hardness, is not an elementary quantity, 
but is the mean of an infinite number of variables. 
3. THE GROWTH OF THE PIT. 
Von Ebner in his paper on the etch-reactions of calcite and dolomite 
concluded that some figures are instantaneous and. some retarded,” 
but Baumhauer remarks: ‘‘Instante und retardirte Aetzfiguren sind 
Endglieder Reihe von Formen, welche sich durch verschiedene Gesch- 
windigkeit der Entwickelung unterscheiden. Sie sind durch Ueber- 
gange mit einander verbunden”.” In the experiment above described 
18 op. cit., page 396. 
19 op. cit., page 775. 
20 “Resultate’’, page 4. 
