1916] THE GROWTH OF ETCH-FIGURES 261 
crystal is formed by bringing together two pits from opposite faces. 
Then the pit on any face may be obtained by selecting a similar and 
parallel plane on the opposite side of the centre of symmetry of the 
crystal, and moving it in towards the centre, in a direction parallel with 
itself until it cuts off a small portion of the negative crystal. The 
cavity remaining in the cut off part will be the pit required. 
5. THE BEAKs. 
(a) The nature and classification of beaks. 
The formation of clawed or beaked projections from the bottoms 
of the pits was an early observed feature of the phenomena of etching. 
They may be obtained in the majority of crystals under favourable 
circumstances. Their formation is however not at all what one would 
expect. It has been shown by Becke,” that in the central, open part of 
the pit, currents produced in the process of etching would remove the 
solvent as it became saturated with the products of solution and thus 
expose a fresh supply of unneutralized corrosive. In the corners of the 
pits this would not go on so rapidly, and action would not be so vigorous. 
This would tend to make the production of beaks an impossibility, and 
so it becomes a matter of interest to devise some explanation for their 
existence. 
The foregoing experiments show the existence of three types of 
beaks, two of which are heretofore undescribed. We have, in the first 
place, that form which extends from one corner of the pit, and which 
is apparently superficial, and which may be called the ‘“‘crack beak’”’ 
(Plate XXVII, fig. 35). In the second place we have the normal beak ex- 
tending from the deepest part of the pit, and in the third the tubular 
beak, which may have no apparent connection with any pit, and which 
may extend for a considerable distance within the body of the crystal. 
(b) The crack beaks. 
The characteristics of the crack beak are described in greater detail 
in the case of colemanite, but examples were also observed in the etching 
of diopside. It appears like a crack extending in the direction of the 
production of the longer side of the pit through one of the acute angles. 
It is evident that on any crystal face parallel to the vertical axis in any 
crystallographic system, except the isometric, the direction of the most 
easy and rapid solution will be indicated by the longest side of the pit, 
and that inclined toward this and parallel to another side of the pit, 
28 Min. u. petr. Mitth., 7: 240. 1885. 
