Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 7.—Orthoclase auge (Carlsbad twin), showing bending and wavyextinction, surrounded by crushed rim of mineral fragments. As seen with crossednicols. Augen-gneiss, Bedford, N. Y. — B. 13. a Carlsbad twin of feldspar that has been bent and at the sametime shows marked wavy extinction. Crystallites, in general those incipient forms of crystals, whichhave not yet reached a stage of development sufficient to show. P
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 7.—Orthoclase auge (Carlsbad twin), showing bending and wavyextinction, surrounded by crushed rim of mineral fragments. As seen with crossednicols. Augen-gneiss, Bedford, N. Y. — B. 13. a Carlsbad twin of feldspar that has been bent and at the sametime shows marked wavy extinction. Crystallites, in general those incipient forms of crystals, whichhave not yet reached a stage of development sufficient to show. Pig. 8. —Crystallites and .Microlites a. Skeleton Crystals b. double refraction, .see Fig. 8, a. Quite a number of names areused to describe the different forms that occur. Microlites, more or less completely defined microscopic crystals, IXDEX OF REFRACT/ON. 17 which usually show double refraction, but cannot be alwaysspecifically determined, see Fig. 8, a. Skeleton Crystals or cr}-stallizations which have not producedentire and complete individuals, see Fig. 8, b. {b) Color. It must be remembered that the colors observedare always due to transmitt^^d light and may be called ahsoiptiontintsr Minerals which in hand specimens are opaque, are oftencolored in sections ; and minerals which are commonly coloredmay appear colorless in sections. At times color may be given toa section simply by the presence of a great number of minuteinclusions. (r) Index of Refraction. // = sin //sin /-. This can be ap-proximately determined by the appearance of the surface and out-line of a mineral, that
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