1916] THE GROWTH OF ETCH-FIGURES 239 
illustrated in Plate XXVI, fig. 13. It will be remembered that in the most 
primitive type one corner, that near the point A, was always deeper 
than the rest of the pit. In some instances this deepening was very 
pronounced, occasionally even being prolonged into a knob (Plate XXVI, 
fig. 14). The beak, whenever present, extended from this corner. Finally 
in some instances pits had been formed which appeared to be composed 
solely of the beak enormously developed. To so great an extent had 
this occurred in a few cases, that a tube had been produced which 
apparently ran along parallel with the surface, and emerged again at 
a considerable distance in another small pit. This resembles what we 
saw in the etching of colemanite, in which small pits were joined by a 
connecting tube, but in the latter case they were larger and more clearly 
defined. 
It will be seen that the pits thus developed on the cleavage face of 
spodumene are of considerable diversity. All of these however may be 
considered, from the circumstances under which they were produced, 
to be comparatively young, and in order to have examples of a more 
mature type, another fragment of the crystal was exposed to similar 
corrosion conditions for a period of thirty minutes. After this some- 
what lengthy exposure there was found a series of pits of various stages 
of development. While some of them gave evidence of a greater maturity 
there were still others which resembled that previously described as 
apparently the most primitive type. This suggested the idea, which 
further experiments showed to be the correct one, that as corrosion 
continues not only are the existing pits deepened and matured, but 
new and immature ones are constantly coming into existence. 
The most distinct of these forms consisted of a shallow, triangular 
depression, differing from that observed in the specimen etched for 
fifteen minutes in being bounded by straight lines (Plate X XVI, fig. 15). 
The outlines seemed to be those of a right-angled triangle, but on 
measurement the angle A B C proved to be 96°, while the angle B A C 
was 53° 27’. Other less definite examples of this form gave values for 
the more obtuse angle of 105° 24’ and 112° 7’ respectively. This may 
be looked upon as a modification of the form first described, but differing 
from it in its deepest and best developed part lying between the sides 
-BAand BC. Some of the figures, by their size and distinctness indi- 
cated a still greater maturity. Not only were they very much deeper, 
but two of the three sides appeared to cut perpendicularly, while a third 
sloped gradually (Plate X XVI, fig. 16). In fact, in some of the examples 
the steeper sides seemed actually to overhang. This formation of an 
overhanging edge, a peculiarity which will be referred to later, is a 
rather unexpected phenomenon. Goldschmidt draws our attention to 
