1916] THE GROWTH OF ETCH-FIGURES 247 
terize the asymmetric pit, but in several cases one of the corners had 
been truncated, resulting in five-sided pits. In one case, a pit similar 
to the second type of Greim was observed (Plate XXVI, fig. 28). It is 
also important to note, in view of Baumhauer’s explanation of Pelikan’s 
supposedly asymmetric pit, as being the result of twinning, that in the 
case above mentioned, where the twinning was apparent, the result was 
not to lower the grade of symmetry of the pit, but actually to raise it. 
(c) Pits on the orthopinacoid. 
It has already been pointed out that the orthopinacoid is attacked 
with much greater rapidity by acid corrosion than is the prism or the 
clinopinacoid. An exposure of three quarters of a minute to a heated 
mixture of fluorite and sulphuric acid resulted in the entire solution of 
the face. Another specimen exposed for a quarter of a minute only, 
had its orthopinacoid etched into a series of long, parallel grooves, 
which completely covered the face. Hydrofluoric acid at 100° was 
next used and a specimen exposed for one quarter of a minute still 
showed an over-etched orthopinacoid. So, for subsequent experiments 
it was diluted with an equal amount of water and was found satisfactory. 
After an exposure under these conditions, of six and one-half minutes, 
the first clear pits were obtained. They were enclosed by six sides, 
the longest pair parallei with the vertical axis of the crystal, the other 
two pairs closing the ends of the pit, one of which was considerably 
more acute than the other. This represented a figure enclosed by 
twelve faces as shown in Plate XXVI, fig. 29. 
It differs from the pit obtained by Pelikan on this face. He describes 
it? as a long deltoid, the sharp end of which points towards the base, 
the obtuse towards the orthodome. Similarly Hintze! speaks of the 
pit as a deltoid with rounded sides, the acute end directed towards the 
negative, the obtuse towards the positive pyramid. However it is 
similar to those illustrated by Daly! as obtained by etching diopside 
from Ala with hydrofluoric acid. 
In order to obtain if possible a more definite knowledge of the faces 
which enclose the pits, the Goldschmidt reflection goniometer was used. 
This is the method already followed by the designer of the instrument 
in the investigations referred to above. The crystal was set up with 
the face whose pits were to be examined normal to the axis of the instru- 
ment, and the reflections from the pit faces read in the usual way. In 
view of the clearness of the pits which had been obtained, it was natural 
9 loc. cit. 
10 “Mineralogie”, 2: 1020. 
1 Proc. Amer. Acad. Arts and Sci., 34: 430. 1889, pl. IV, 15. 
