54 CORUNDUM IN THE UNITED STATES. [bull. 180. 
Fig. 12 of PI. V represents a repeated growth, each face of which is 
entirely distinct from the faces of the main crystal. In fig. 13 of 
PL V there are represented two, and in fig. 14 of PL V a series of 
such growths, where a number of the rhombohedral faces coincide. 
These growths occur most frequently on the flat crystals. The 
thickness of the rhombohedron rarely reaches 1 mm. and often they 
are so thin that they appear like striations. 
This repeated growth is very similar to that described as occurring 
on the Cowee rubies (p. 43). Bauer, 1 in an article entitled " Ueber 
das Vorkommen der Rubine in Birma," has described this same style 
of development as occurring on the Burma rubies, but it is not so 
genera] as on the Montana eorundums. 
Etching figures. 2 - — The etching figures which were observed on 
nearly all the crystals examined were on the basal plane. The fig- 
ures are very perfect, and although showing many different forms, 
they all have a rhombohedral symmetry. Fig. 5 of PI. V represents 
the common etching figure, which is a rhombohedral depression ter- 
minating in a point. The edges of the depression are sharp and well 
defined, as are also the intersections of the rhombohedral faces of the 
depression. These rhombohedral faces were smooth and gave fair 
reflections of the signal on the reflecting goniometer. In measuring 
them the entire crystal except the depression to be measured was cov- 
ered with a thin coating of wax. Two different crystals were meas- 
ured, which gave for rhombohedron on rhombohedron 22° 30'; this 
corresponds to the rhombohedron 1017, for which the calculated value 
is 21° 50'. Figures of the same style were observed whose edges were 
parallel to those of the negative rhombohedron; these, however, are 
not common in isolated forms. 
Another common form is represented in figs. 7 and 9 of PL V, where 
the depression is bounded by the basal plane, which at times is so 
large that the rhombohedral plane is hardly visible. Fig. 6 of PL V 
represents etching figures, where, on the basal plane of a shallow 
depression, there is one additional etching figure and sometimes two. 
These second etching figures are like the common ones shown in fig. 5 
of PL V. The onler rhombohedral contour of these figures is gen- 
erally rounded. This is also usually the case with the deeper 
depressions. 
Often the etching figures are intergrown, fig. 8 of PL V, and when 
many of these occur together they have the appearance of raised 
figures rather than of depressions. This raised appearance is verj 
striking when there is a combination of the plus and minus rhombo- 
hedron in parallel position and without overlapping, fig. 10 of PL V. 
The figures vary considerably in size, but most of them are near 
1 mm. in diameter. A few were observed that were nearly 2 mm. in 
diameter. 
Neues Jahrbuch fiir Min., Geol. und Pal., Vol. XI, 1896, p. L'<H». 
Am. Jour. Sci., 4th series, Vol. IV, 1897, p. 426. 
