1916] THE GROWTH OF ETCH-FIGURES 243 
under special circumstances in acid etching. Here, as in the previous 
experiments, the numbers of the pits on the opposite sides of a crystal 
were not the same. 
In comparing these pits with those described for acid corrosion, no 
clear relationship was observable. These are typically quadrilateral 
and those triangular, but there is one shallow, five-sided form illus- 
trated which could apparently have been derived from a triangular 
form by that replacement which this paper shows to be so characteristic 
of the growth of etch figures (Plate XXVI, fig. 20). 
5. ACID CORROSION OF DIOPSIDE. 
The excellence of the material procurable made it possible to carry 
out the corrosion experiments with greater thoroughness in the case of 
diopside than with spodumene. The faces of the prismatic zone of the 
former were readily obtained in brilliant condition, and the base and 
clinopinacoid which are invariably rough and unsuited for crystallo- 
graphic investigation were artificially cut and polished on one specimen. 
It was thus possible to prepare a more complete series of the pits of 
diopside than had yet been published so far as the writer has been able 
to discover. It was found that not only did the pinacoids give better 
pits than the prisms, which were the only faces available in the spodu- 
menes, but that the various stages in their development were more 
clearly defined. 
(a) Pits on the prisms. 
A crystal of transparent diopside from De Kalb, N.Y., was exposed 
to the corrosive action of a mixture of powdered fluorite and sulphuric 
acid heated over a low bunsen flame for two minutes. On the prisms 
small pits of a very primitive form were developed (Plate X XVI, fig. 21). 
They were mere shallow depressions bounded by two sides, one straight 
and the other curved. One angle was slightly more obtuse than the 
other, and at this point was situated the deepest part of the pit. In 
some cases which appeared to be rather more mature, the figure was a 
steadily deepening groove, one end of which had become a dark, indis- 
tinctly bordered hollow. 
On continued exposure to the corrosive an interesting modification 
of the primitive pit was observed. Gradually the borders of the indis- 
tinct deep portion took on a definite appearance, until after four minutes 
there had been developed a quadrilateral pit, bounded by four unequal, 
triangular figure-faces (Plate XXVI, fig. 22), lying at the end of the shallow 
depression which represented the primitive pit. As the corrosion con- 
