1916} THE GROWTH OF ETCH-FIGURES 241 
they had become not only more numerous but also more clearly defined, 
and by fifty minutes they were quite mature and distinct. This series 
of reactions is of interest when considered in connection with the ex- 
periments above described, in which adjacent sides of cleavage frag- 
ments were simultaneously etched. It would appear that the distribu- 
tion of pits is determined by the molecular conditions of the crystal face,’ 
and that after the whole layer has been removed by corrosion, a new 
layer with new possible centres for pitting, is brought to light. The 
significance of these facts is more fully discussed in the second part of 
this paper. 
Two slight irregularities in the form and arrangement of the pits 
remain to be noted. Greim, in describing the somewhat similar figures 
on diopside, remarked that the position of the positive and negative 
hemi-pyramids could be determined by the direction of the triangular 
pits. Now in the specimen examined, although the acute angle of most 
of the pits on any given prismatic face would, as a rule, point in the same 
direction, yet there were fairly numerous examples whose orientation 
was just the opposite. Again, it was frequently noted that opposite 
faces in the crystal were acted upon by the corrosive in quite a different 
manner. One side for instance gave a predominance of the normal 
triangular pits, with a few of the beaked variety, while the other was 
characterized by the presence of only a few ordinary pits, and those of 
a much greater size, the rest being mainly of the beaked type. There 
was also no apparent connection between the number and arrangement 
of the pits on opposite sides. On one side, the pits might be very abun- 
dant and uniformly distributed, while those on the other would be 
restricted to one or two small groups. The colemanite experiments of 
the previous section go a long way towards explaining this apparent 
anomaly which has been frequently commented upon. 
The occurrence of pits of different types on the same crystal face has 
been noted by many investigators, and suggests several interesting 
problems, some of which will be referred to later when dealing with the 
pits obtained by the corrosion of diopside. In the case under consider- 
ation at least, it would appear that the various forms are not to be 
looked upon as differing in kind, but merely in their stage of develop- 
ment. 
3. ALKALINE CORROSION OF SPODUMENE. 
The production of pits upon spodumene by alkaline corrosion is not 
so readily achieved as by the use of hydrofluoric acid. The corrosive 
5 Jahrb. Min. u. Geol., 1889, 1: 252. 
