Expeditions organized or participated in by the Smithsonian . tes that thistype of crystallization of the mineral calcite is comparatively rare. Photo-graph by Mr. T. W. Smillie, U. S. National Museum. separated, so that the effect is that of superposed layers of curved sheets,resembling the petals of the rose to such an extent that the first word todescribe them which suggests itself is rosette. 22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 66 That this curvature is not merely a superficial phenomenon, due to thedevelopment of vicinal planes, as is often the case in curved cryst
Expeditions organized or participated in by the Smithsonian . tes that thistype of crystallization of the mineral calcite is comparatively rare. Photo-graph by Mr. T. W. Smillie, U. S. National Museum. separated, so that the effect is that of superposed layers of curved sheets,resembling the petals of the rose to such an extent that the first word todescribe them which suggests itself is rosette. 22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 66 That this curvature is not merely a superficial phenomenon, due to thedevelopment of vicinal planes, as is often the case in curved crystal faces, isshown 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 wherethey are developed. The composite character exhibited by the external facesis 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. Thisis borne out by microscopic examination, for the cleavage flakes show between. Fig. 31.—Calcite rosette, varying in form from Figs. 29 and 30. Photographby Mr. T. W. SmilHe, U. S. National Museum. crossed nicols quite uniform extinction, without a trace of the wavy darkeningcharacteristic of crystals which have been distorted by pressure. The origin of these specimens is to be explained, then, as due to the startingof crystallization at numerous closely crowded points, the crystals being per-haps at the start quite parallel, but as they grew crowding one another out ofparallelism, although not enough to prevent approximately equal growth ofevery individual. The resulting groups form unusually fine display specimens,all the more interesting because of the comparative rarity of this type ofcrvstallization in the mineral calcite. NO. 3 SMITHSONIAN EXPLORATIONS, I915 22,
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectscienti, bookyear1912
