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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 66 



close groups of thick tabular rhombohedrons of calcite, the f^ces 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 quahtative chemical tests, which showed 

 only traces of magnesium, and by measurement of the index of refraction w. 

 which was found to be l66o, essentially that of calcite. 



Fig. 29.— a calcite rosette collected by :\Irs. 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, 0112. One specimen, which is shown in 

 figure T,2, 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 



