Interpretation of orientation curves for quartz. The following generalisations are now put forward with regard to interpretation of orientation curves based on measurements of extinction angles in quartz:— (1) Substantial homogeneity of the fabric throughout the field of a hand-specimen is essential to a satisfactory correlation of maxima in ab, bc and ac curves. This condition is fulfilled only if curves constructed from measurements on parallel sections cut from different parts of the specimen are essentially similar. (2) For certain simple types of fabric dominated by point maxima as contrasted with a girdle pattern, the principal features can be deduced exactly from orientation curves. The single maximum characteristic of certain tectonites that have been affected by strong slip in a single set of planes (Sander's maximum I) should readily be detected in this way (e.g., the fabric represented by D27, 28, Sander, 1930, p. 307). Again the pair of maxima (Sander's maxima II) developed symmetrically in the ac plane of many tectonites that have undergone simultaneous slip in two sets of planes with resultant “flattening” in the plane of schistosity, should also be able to be deduced as in the first example described in this paper (Figs. 1–5). (3) The more frequent case where the pattern of the quartz fabric is dominated by a girdle of optic axes perpendicular to the b fabric axis can be only imperfectly worked out from the orientation curves. The most important feature of the fabric from the tectonic viewpoint is the direction of b, and this can usually be deduced approximately from the curves. Thus in Fig. 1 the strong maximum in the ab curve indicates the presence of either a single maximum approximately perpendicular to the lineation (megascopic b), or more than one such maximum lying in a girdle perpendicular to the lineation; the ac curve shows that the latter is the actual condition and that the b axis of the quartz fabric coincides with b of the megascopic fabric. Again in Fig. 7 the sharp strong maxima in the bc curve as contrasted with the weaker, more numerous, widely distributed maxima of the ac curve suggest that b of the quartz fabric does not depart widely from the megascopic b. It should be noted that the maximum in the ab curve does not necessarily lie exactly at 90° to the b axis of the girdle, for its position is determined partly by that of the dominating maxima within the girdle. In an earlier account of the Patearoa schists (Turner, 1938) it was assumed that the eccentric position of maximum A (Fig. 6) indicated lack of coincidence between b of the megascopic fabric and b of the quartz girdle. The fuller analysis depicted in Fig. 9 shows that the main quartz maximum is indeed somewhat eccentric with regard to the megascopic b, but the quartz girdle as a whole is approximately centred at b. When the quartz maxima lie on a double girdle with a single b axis (i.e., on a small circle of the fabric diagram projected on the plane perpendicular to b), there will be two strong maxima in both ab and bc curves symmetrically located with reference to b. This is well shown in the curves constructed for quartz veins in the Waipori schists (Fig. 7), and originally was wrongly interpreted as