_Example:_ In a section of granite (biotite-granite) a grain of quartz was selected, which gave the brightest color. This color was a bright yellowish white and the known double refraction of quartz is 0.009. Following down on this diagonal line until the interference color was reached it was seen that the thickness of the section was about 0.03 mm. (a very thin section). The section of the undetermined mineral, giving the highest order color, showed a bright 2° order purple-blue. Passing along the 0.03 horizontal line until this color was reached and then up along the diagonal line, it was seen that the double refraction of this mineral must be about 0.04. The table gives muscovite and ægerite as having about this double refraction. The mineral was proved to be muscovite by its absence of color and relief and by the characteristic cleavage and parallel extinction. [Illustration: FIG. 18.—Sanidine crystal _f_, showing Carlsbad twin (which, as it consists of two parts only, may be called _simple_), and quartz _q_ in rhyolite. Crossed nicols. ] Structure: (_a_) _Twinning_, generally noticed by the parts of the twin not extinguishing at the same time. It may also be observed, without crossed nicols, just as in the case of macroscopic minerals. Twinning may be described as: _simple_, Fig. 18; _polysynthetic_, due to repeated twinning after the same law, Fig. 12; and _crossed or “gridiron,”_ due to repeated twinning after two laws, Fig. 19. [Illustration: FIG. 19.—Microcline, showing crossed or “gridiron” twinning. Crossed nicols. ] (_b_) _Zonal_ structure, often only made visible by the zones extinguishing at different times. It may, however, be noticed by the zones being of slightly different color, or by the zonal distribution of inclusions. In the case of the feldspars the zonal structure may be caused either by the crystal being formed of zones of different chemical composition (the successive zones in the plagioclases growing more acid towards the exterior), or by ultra-microscopic twinning,[67] Fig. 20. [Illustration: FIG. 20.—Zonal feldspar (Carlsbad twin) in trachyte. Crossed nicols. ] (_c_) _Aggregate_ structure, being a confused mass of separate little crystals, scales or grains all extinguishing at different times, Fig. 21. [Illustration: FIG. 21.—Sphærulites in felsite. Ground mass shows aggregate structure. Crossed nicols. ] (_d_) _Sphærulitic_ structure, produced by the aggregation, in a radiate form, of crystals or crystallites. It is generally easily perceived by the dark cross, resulting from the extinguishing of the light in those crystals whose directions of vibration are parallel to the planes of vibration of the nicols. When the stage is revolved the arms of the cross do not rotate, Fig. 21. [Illustration: