242 ‘TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE _ [VOL. XI 
acts with very much greater rapidity, and the specimen frequently 
becomes covered by a layer of an insoluble silicate which cannot be 
removed. Several corrosives were used, first a fused mixture of sodium 
and potassium carbonates, which proved to be altogether too vigorous, 
then a solution of sodium carbonate at 100° C., which after a very long 
exposure resulted in a few small and doubtful pits. Finally sodium 
hydrate was tried, and after an exposure of two minutes over a bunsen 
flame, pits were obtained of moderate distinctness. When the 
operation had been interrupted at one minute no pits were found, and 
after seven minutes’ exposure the specimen had been almost dissolved 
away. 
The pits produced differ distinctly from those obtained by acid 
corrosion. The alkaline pit on the cleavage prism is typically asym- 
metric and bounded by five sides (Plate XXVI, fig. 18). It has five figure- 
faces of unequal size and different shape. The angles in some of them 
were as follows: 
ABC 123° 48’ 
BAC 56° 18’ 
BCD gorhig’ 
It is interesting to note, however, that here as in the case of the 
prevously described acid pits, the values of the angles were found 
to vary quite considerably, in one case the angle BC D was seen to 
be 108° 18’, 
The tendency observed in the case of acid etching, for the replace- 
ment of corners by additional figure-faces, was quite noticeable here 
also,for pits were seen which had six sides (Plate X XVI fig. 19). On account 
of the difficulty in obtaining pits at all, it was not found possible to 
observe the stages in their growth, but judging from analogy to the 
results obtained from acid pits on spodumene, as well as to those to be 
described, obtained from diopside, it would appear that these extra 
faces were added as the pits attained greater maturity. This process 
appeared to be more pronounced in alkaline etching, with a resulting 
tendency to a more complex outline. 
A similar example of the occurrence of a larger number of faces in 
alkaline than in acid pits was found in some etched material kindly 
placed at my disposal by Professor Walker. Among other preparations 
of the micas, there were some of phlogopite etched by hydrofluoric acid 
and fused caustic potash respectively. In the former, most of the pits 
were of a simple triangular outline while in the latter they were mainly 
six-sided with the appearance of the face of the hexoctahedron. 
On one of the etched faces of spodumene, each one of the pits was 
provided with a funnel-shaped beak somewhat similar to that obtained 
