22, 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 
Fic. 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 
crystallization in the mineral calcite. 
