J. W. Judd, _Geol. Mag._ [3], Vol. VI, p. 243, 1889. Footnote 105: The plagioclases have rather a complex composition; but may be regarded as forming a series from the composition NaAlSi_{3}O_{8}(Ab) to the composition CaAl_{2}Si_{2}O_{8}(An), consisting for the most part of isomorphous mixtures of these types, with some replacement by KAlSi_{3}O_{8}. The compositions of only a few of the common plagioclases are given above. Footnote 106: The lath-shaped feldspars, moulding the augite, give to diabases the so-called “ophitic” structure, Fig. 12. The peculiarity of this structure is that the feldspars crystallized before the augite, which is contrary to the usual order of formation. Footnote 107: For the positions of the optic axes, bisectrices, etc., relative to the cleavage plates of the different plagioclases, see Iddings’ _Rock Minerals_, p. 222, 1911. Footnote 108: _Die Gesteinsbildenden Mineralien_ (with tables), E. Weinschenk, Freiburg, 1901. _Étude sur la Détermination des Feldspaths dans les Plaques Minces_, Michel Lévy, Paris, 1894; and _The Determination of the Feldspars_, N. H. Winchell, _Am. Geol._, Vol. XXI, No. 1, 1898. Iddings’ _Rock Minerals_, Wiley & Sons, 1911. _Étude sur la Détermination des Feldspaths_ (troisième fascicule), Michel Lévy, Paris, 1904. Footnote 109: Glass models of the feldspars (size 20 × 10 cm.) by F. Krantz, Bonn. Diagrams, showing optical orientation in the plagioclases, in _Die Gesteinsbildenden Mineralien_, E. Weinschenk, p. 133, 1901. Footnote 110: For this method of investigation little cleavage flakes or plates can often be obtained from the crushed mineral, but, on account of Albite twinning, plates are more apt to be obtained parallel to the twinning plane than to the best basal cleavage. If a fragment with only one cleavage surface is obtained, it must be cemented to a glass by this surface and ground down to a thin section with parallel sides. Footnote 111: This test is only possible when suitable sections of the given feldspar in the rock section can be found. The method, however, can be used with great accuracy with the aid of some form of apparatus for properly orienting the section. See “Klein’s Apparatus for the Orientation of Thin Sections,” _Sitzungsber. Berlin. Akad._, 1895, 1151; (also in _N. Y. Acad. Sci._, Vol. XVI, p. 51, 1897); and Von Federov’s “Universal Table,” _Zeit. für Kryst, etc._, Vol. XXV., p. 351. Footnote 112: