114 CORUNDUM^ ITS OCCURRENCE AND DISTRIBUTION. 
approximated closely to 67°, which agrees very well with the calcu- 
lated value, 67° 3', for 0001 > 3032. These crystals are represented 
on PL VIII. 
The crystals are developed as shown in figs. 1, 2, and 3, of PL VIII, 
the prevailing type being like fig. 3 of PL VIII. The crystals vary 
from those where the base is very largely developed, having a diam- 
eter of 8 mm., while the rhombohedron is only 1 mm., to those that 
have the base and the rhombohedron equally developed (fig. 1 of 
PL VIII). Where the faces are more equally developed, the rhom- 
bohedral faces are generally rounded. 
The basal plane often shows characteristic striations which are 
parallel to the three intersections of the base c and the rhombohedron 
x^ as shown in fig. 4 of PL VIII. These lines are sharp and distinct, 
and on the very flat crystals can easily be measured when examined 
under the microscope. The rhombohedral faces are very roughly 
striated without showing any distinct parallel lines. 
One very common development of these crystals is a repeated 
growth on the basal plane of the rhombohedron x (3032) and the base 
c (0001), as represented in fig. 2 of PL VIII. These growths are 
exceedingly varied, as shown in figs. 11-14 of PL VIII, where they are 
drawn in basal projection. In fig. 11 of PL VIII there is but one sec- 
ondary rhombohedron and base, which has one of its rhombohedral 
faces a continuation of one of the rhombohedral faces of the crystal. 
Fig. 12 of PL VIII represents a repeated growth, each face of which 
is entirely distinct from the faces of the main crystal. In fig.' 13 of 
PL VIII there are represented two, and in fig. 14 of PL VIII a series 
of such growths, where a number of the rhombohedral faces coincide. 
These growths occur most frequently on the flat crystals. The thick- 
ness of the rhombohedrons 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. 103). Bauer ,« 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 ^-o 
general as on the Montana corundums. 
Etching fgures.^ — 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 PL VIII rej^re- 
sents the common etching figure, which is a rhombohedral depression 
terminating in a point. The edges of the depression are sharp and 
" Nenes .Tahrbuch fiir Mineral., Geol. und Pal., vol. 11, 1S0(>. p. 200. 
"Am. Jour. Sci., 4th ser., vol. 4, 1897, p. 426, 
