A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . d basal plane, macrodomes,brachy domes, and at the nether end the pyramid. DISTORTION OF CRYSTALS. As crystal models are prepared, the symmetry of the formsstudied, in a physical sense, is perfect, but a very slight inspectionof any collection of minerals reveals a very constant apparentdyssymmetry; unequal faces, long and short similar edges, in-clined positions, and sometimes elongations, flattenings and obliquedistortions, which quii

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A popular guide to minerals : with chapters on the Bement Collection of minerals in the American Museum of Natural History, and the development of mineralogy . d basal plane, macrodomes, brachy domes, and at the nether end the pyramid. DISTORTION OF CRYSTALS. As crystal models are prepared, the symmetry of the formsstudied, in a physical sense, is perfect, but a very slight inspectionof any collection of minerals reveals a very constant apparentdyssymmetry; unequal faces, long and short similar edges, in-clined positions, and sometimes elongations, flattenings and obliquedistortions, which quiie alter the regular form, if ideally drawn.Fig. 231 shows a spinel elongated so that the nucleal octahedron, whose faces are produced, become disguised in the inclined rhom-boidal prism with truncated angles. Similarly Fig. 232 shows thedistortion of a rhombic dodecahedron of garnet and Fig. 233 theoctahedron (flattened into one plane), a remarkable result oftenseen in a flat sheet of gold of a series of parallel octahedrons. Inall such cases the angles made by adjoining faces in a crystal of amineral are the same as those determined ia the unchanged and.