(4) To check identity of twinning axis:— (a) Composition-plane (010). Measure (001) or pericline composition-plane in one of the sub-individuals. ⊥ (010) = 85°–90° [001] = 26° ± [100] = 90° ⊥(001) ∧ ⊥[001]/(010) = 64° ± ⊥[100]/(010) = 0–5° (b) Composition-plane (001). Measure (010) in one of the sub-individuals. [010] = 0°–5° ⊥ [100]/(001) = 0°–5° ⊥ (010) Δ ⊥ [010]/(001) = 85°–90° [100] = 90° ⊥ (001) = 85°–90° The method used in plagioclase determinations has been described somewhat fully, since certain anomalies were observed when the positions of twinning-axes were plotted upon the Nikitin diagram. For most other rocks the writer has usually found that the plotted points lie close to the appropriate curves as given by Nikitin. However, there are often notable discrepancies in the case of plagioclases from Dunedin mugearites (as indicated in Fig. 2) and these may possibly be due to high potash-content of the feldspars in question. The composition of normative plagioclase in the analysed specimen is An37 Ab49 Or14. Since potash-feldspar was not observed as a separate mineral in this rock, it is safe to assume that it is contained in the plagioclase of the mode. If allowance is made for entry of a small amount of normative anorthite into the modal pyroxene, and for the presence of about 8% of normative nepheline in the modal plagioclase, the composition of the latter should be somewhat more sodic than the An37 Ab49 Or14 indicated by the norm. The values actually determined* Where a plotted lies off the curve in Nikitin's diagram, the composition is obtained by dropping a perpendicular to the appropriate curve. (Nos. 246, P. 5569) vary from An39–32 in a zoned phenocryst to An45 and An47 in small phenocrysts and An40 in the groundmass, with a mean composition about An42 Ab58. C. T. Barber (1936, pp. 234–249) has discussed at length the relative importance of several factors that might contribute to lack of coincidence between the poles of optically measured twinning planes or axes and standard curves such as those of Fig. 2. He concludes (loc. cit. pp. 247–249) that variation in the potash–content has little effect in this connection, but attaches more importance to variation in physical conditions of crystallisation of the feldspar in question.