EXTINCTION ANGLES IN SECTIONS:[114] ⟂ a, MEASURED FROM ALBITE STRIATIONS. ⟂ c, MEASURED FROM CLEAVAGE CRACKS. Albite 74° 19½° Oligoclase, Ab_{4}An_{1} 88° 5° Labradorite, Ab_{1}An_{1} 60° 22° Anorthite 55½° 48° When both extinction angles can be obtained, the determination of the plagioclase is very certain, but the result cannot be regarded as definite when only one is found; and the method becomes more difficult as the crystals become smaller. The position of the axial plane should be determined by convergent light test and not simply by the direction of extinction in parallel polarized light. (4) _Michel Lévy’s_ method[115] can be employed when twinning is present after both _Carlsbad_ and _Albite_ laws. [Illustration: FIG. 74.—Extinction Angles in the Zone normal to (010) in Carlsbad Twins of the Plagioclases. ] The section to be tested should be in the zone perpendicular to (010). Such sections show sharp boundary lines between the twin lamellæ and, between crossed nicols, the _Albite_ lamellæ show the same interference color when the trace of (010) is parallel to the cross-wires of the ocular, and also in the 45° position the lamellæ are the same color and no dividing lines show. The two parts of the _Carlsbad_ twin exhibit different interference colors in the 45° position, this difference being more marked as the composition approaches that of Anorthite. The extinction angles are measured from the trace of the _Albite_ twinning plane (010), paying regard to the + and − signs, and the _concurrent series_ of angles are to be obtained from the two parts of the _Carlsbad_ twin. The range of these angles (for four type compositions, Ab, Ab_{4}An_{1}, Ab_{1}An_{1} and An) is given in the accompanying diagram for all positions of the section in the zone normal to (010). In the curves of this diagram (Fig. 74) the vertical distances are the extinction angles for every ten degrees measured from the trace of the _Albite_ twinning plane, and the horizontal distances represent varying positions of the section in the zone normal to (010) for every ten degrees of rotation from the position ∥ to the edge (100)(010) (that is ∥ _ć_ axis), through a revolution of 180° to again ∥ to the same position. The concurrent angles in one part of a _Carlsbad_ twin are represented by a heavy line and in the other part by a broken line. It will be observed that the difference between these concurrent angles is very small in Albite (3°) and increases markedly towards Anorthite (60°).