426 Pratt — Crystallography of the Montana Sapphires. 



These growths occur most frequently on the flat crystals. The 

 thickness of the rhombohedron rarely reaches l mm and often 

 they are so thin, that they appear like striations. Figs. 11a- 

 14&, representing the same crystals as figs. 11-14, have been 

 drawn as they appear under the lens, which brings out the rela- 

 tion of the base and rhombohedron to better advantage. 



Bauer* in a recent article, entitled " Ueber das Vorkommen 

 der Rubine in Birma," has described this same style of develop- 

 ment as occurring on the Burma rubies, but it is not so gen- 

 eral as on the Montana corundums. 



Etching-figures. — The etching-figures, which were observed 

 on nearly all the crystals examined, were on the basal plane. 

 The figures are very perfect, and although showing many dif- 

 ferent forms, they all have a rhombohedral symmetry. Fig. 5 

 represents the common etching-figure, which is a rhombohe- 

 dral depression terminating in a point. The edges of the 

 depression are sharp and well-defined, as are also the intersec- 

 tions 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, 

 all the crystal but the depression to be measured was covered 

 with a thin coating of wax. Two different crystals were 

 measured, which gave for rhombohedron on rhombohedron 

 22° 30' ; this corresponds to the rhombohedron 1017, for which 

 the calculated value is 21° 50'. The same style of figures were 

 observed whose edges were parallel to those of the negative 

 rhombohedron ; these, however, are not common in isolated 

 figures. 



Another common form is represented in figs. 6 and 7, where 

 the depression is bounded by the basal plane, which at times is 

 so large that the rhombohedral plane is hardly visible. Fig. 8 

 represents etching-figures, where, on the basal plane of a shal- 

 low depression, there is another and sometimes two other etch- 

 ing-figures. These second etching-figures are like the common 

 ones shown in fig. 5. The outer rhombohedral contour of these 

 figures is generally rounded ; this is also usually the case with 

 the deeper depressions. 



Often the etching-figures are intergrown (fig. 9) and when 

 many of these occur together they have the appearance of 

 raised figures, rather than of depressions. This raised appear- 

 ance is very striking, when there is a combination of the plus 

 and minus rhombohedron in parallel position and without over- 

 lapping each other (fig. 10). 



The figures vary considerably in size, but most of them are 

 near l mm in diameter. A few were observed that were nearly 

 2 mm in diameter. 



* Neues Jahrbuch fur Min. Geol. und Pal., ii, p. 209, 1896. 



