In the determination of feldspar microlites it is well to remember the following facts: “Microcline is rarely, or never, seen in the condition of microlites, while the associations of labradorite and albite are so different that there is little danger of confounding them. Labradorite is the commonest product of the consolidation of the basic eruptives, and albite almost invariably results from metamorphism, frequently from the contact of igneous rocks on the calcareous clastics.” N. H. Winchell, Determination of the Feldspars, _Am. Geol._, Vol. XXI, No. 1, p. 33, 1898. Footnote 113: These methods (both 2 and 3) are often not applicable on account of the tendency of the crystals in an effusive rock to parallel orientation, which may be so marked that the rock section does not show any favorable sections of the plagioclase. Footnote 114: These extinction angles, as well as those previously given, are those of only a few type feldspars of definite composition. As the composition varies through a long series, so the extinction angle changes, one being a function of the other. For a complete list of compositions and related extinction angles, see Iddings’ _Rock Minerals_ and Lévy & Lacroix’s _Les Minéraux des Roches_. Footnote 115: Iddings’ _Rock Minerals_, p. 228, Wiley & Sons, 1911. _Étude sur la Détermination des Feldspaths_ (troisième fascicule), Michel Lévy, Paris, 1904. Footnote 116: In quartz ω is the refractive index of the ray with vibration direction ∥ a [that is the direction of vibration of the faster ray (the ordinary ray)]. Hence ω is direction ∥ a and ε ∥ c. In the feldspars; α ∥ a, γ ∥ c and β ∥ b. Footnote 117: _Am. Jour. Sci._, May, 1906. This method is specially useful in detecting presence of orthoclase, when plagioclase is the dominant feldspar. Footnote 118: These tests are only possible on pure and fresh material. The specific gravity increases with the Ca % (albite 2.62, anorthite 2.75). Footnote 119: In clear unstriated granules, which may be distinguished from quartz by biaxial interference figure in convergent light. Footnote 120: The tendency of labradorite in gabbros to twinning, after both _Albite_ and _Pericline_ laws, is to be noted. Footnote 121: Werveke’s (N. J. B., 1883, II, 97) theory is that a twin lamination may be caused by the forces producing mechanical deformations, as movement in the magma and mountain making pressure. Such lamellæ are characterized by the fact that their extent and course seem to depend on fracture lines in the crystal. Footnote 122: The derivation from pyroxene and amphibole appears to be doubtful, see Weinschenk’s _Gesteinsbildenden Mineralien_, 1901, p. 121. Footnote 123: Harker’s _Petrology for Students_, p. 63, 1895. Footnote 124: