20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 66 
close groups of thick tabular rhombohedrons of calcite, the faces of which are 
deeply curved into saddle-shape forms, a structure frequently found in the 
mineral dolomite, but comparatively rare in calcite. That the latter mineral is 
represented. however, was proved by qualitative chemical tests, which showed 
only traces of magnesium, and by measurement of the index of refraction , 
which was found to be 1.660, essentially that of calcite. 
Fic. 29.—A calcite rosette collected by Mrs. Walcott and B. Stuart Walcott 
high up on the cliffs shown in Fig. 28. Photograph by Mr. T. W. Smillie, U.S. 
National Museum. 
The fundamental crystallographic form toward which all of the crystals 
tend is the negative rhombohedron, o112. One specimen, which is shown in 
figure 32, approaches this form quite definitely. The angle between two adja- 
cent faces of this crystal close to their intersection, measured with the contact 
goniometer, is about 46°, while the theoretical angle is 45° 3’. No other faces 
