(_a_) ▄Form.▄[28] _Crystals bounded by planes_, _i. e._, crystals that have formed when conditions were favorable for complete development. Such crystals are often called _idiomorphic_ or _automorphic_, see Fig. 4. Crystals that are large in comparison with other accompanying crystals may be called _Phenocrysts_. Of course crystals, which are not wholly contained within the rock section, will only show the outline of the bounding planes cut by this particular section. In some cases, by a careful study of the outline of several sections of the same mineral and by a measurement of the angles, it is possible to determine the common “crystallographic forms” of the mineral. Very misleading outlines may, however, be observed; as for example the triangular outline of a section of a cube, with the corner truncated. For measuring the angles have the stage accurately centered, then bring the vertex of the angle to be measured to the intersection of the cross-wires in the eye-piece. Bring one of the sides in coincidence with one of the cross-wires. Note the reading of the graduated circle and rotate the section until the other side is in coincidence with the same wire. Take the reading again and the difference will be the angle required. _Crystals without bounding planes_, whose surfaces are more or less determined by those of adjacent crystals. Such crystals are frequently called _allotriomorphic_ or _xenomorphic_,[29] see Fig. 5. This allotriomorphic form must not be confounded with the worn or rounded boundaries of the component grains in clastic rocks. [Illustration: FIG. 6.—Corroded sanidine crystal in perlite. ] _Corroded Crystals_,[30] whose fretted outline is undoubtedly due to corrosive action of the magma, see Fig. 6. _Broken or Strained Crystals._ In some cases what were formerly larger individuals have been broken or shattered by dynamic action into much smaller fragments, see Fig. 7, showing crushed rim of fragments surrounding feldspar “auge.” In other cases an actual bending or distortion of the crystal has taken place, and again sometimes the effect of mechanical stress is only shown by the so-called “wavy” extinction. See p. 31 and Fig. 7, showing a carlsbad twin of feldspar that has been bent and therefore shows marked “wavy” extinction. [Illustration: FIG. 7.—Orthoclase “auge” (Carlsbad twin), showing bending and “wavy” extinction, surrounded by crushed rim of mineral fragments. As seen with crossed nicols. “Augen-gneiss,” Bedford, N. Y.—B. 13. ] [Illustration: FIG. 8.—Crystallites and Microlites _a_. Skeleton Forms _b_. ] _Crystallites_, in general those incipient forms of crystals which have not yet reached a stage of development sufficient to show double refraction, see Fig. 8, _a_. Quite a number of names are used to describe the different forms that occur.