Summary: Specimens from some forty different localities over the composition range from about 20 % An to about 70 % An have been examined by X-ray single-crystal methods. The majority of these specimens show the normal intermediate plagioclase pattern characterized by weak pairs of subsidiary layer-lines of variable separation. It has been found that the separation of these layer-lines is a linear function of composition over the whole composition range. Further, this range is found to extend to more soda-rich compositions than was formerly believed, whilst the limits of the range are approximately defined by the compositions at which the separations of the subsidiary layer-lines about either the a- or the b-axes become zero.

The anomalous patterns shown by some specimens are discussed, and it is shown that most of these may be interpreted in terms of the previous geological history of the specimens.

A comparison of the present results with those obtained by previous workers in this field is made, and it is shown how the principal points of difference may be resolved. An account of the phase relationships of the low-temperature plagioclase series is given, and the problems arising from the present work are discussed.