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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 66 



That this curvature is not merely a superficial phenomenon, due to the 

 development of vicinal planes, as is often the case in curved crystal faces, is 

 shown by the fact that the cleavage faces are also curved, their relative posi- 

 tion with respect to the surface faces being always retained, no matter where 

 they are developed. The composite character exhibited by the external faces 

 is also shown by these cleavages, which proves that the curvature of the crys- 

 tals is only apparent, and is not due to actual deformation by pressure. This 

 is borne out by microscopic examination, for the cleavage flakes show between 



Fig. 31. — Calcite rosette, varying in form from Figs. 29 and 30. Photograph 

 by Mr. T. W. Smillie, U. S. National Museum. 



crossed nicols quite uniform extinction, without a trace of the wavy darkening 

 characteristic of crystals which have been distorted by pressure. 



The origin of these specimens is to be explained, then, as due to the starting 

 of crystallization at numerous closely crowded points, the crystals being per- 

 haps at the start quite parallel, but as they grew crowding one another out of 

 parallelism, although not enough to prevent approximately equal growth of 

 every individual. The resulting groups form unusually fine display specimens, 

 all the more interesting because of the comparative rarity of this type of 

 crvstallization in the mineral calcite. 



