The system of intrusive sheets in the Lower Carboniferous formations of the north of England, familiar to geologists throughout the world as the Great Whin Sill, has a voluminous early literature which it is unnecessary to summarize here. It will be sufficient to refer to the classical papers by Topley and Lebour on the field relationships (1) and by Teall on the detailed petrography (4), each of which contains an ample bibliography of the earlier literature. The most recent general account is by Prof. E. J. Garwood (9), who contributed a careful discussion of the evidence bearing on the period of intrusion.

Teall's great paper closely followed on one devoted to some of the dikes of the north of England (3), and in it he drew attention to the remarkable petrographical Similarity between the rocks of the Heft and High Green dikes and that of the Whin Sill (4, p. 656).

Labradorite of exceptional purity, occurring as phenocrysts in two basaltic dikes just below the lighthouse at St. John's Point, Ardglass, County Down, Northern Ireland, was described by Professor A. Hutchinson and myself in 1912. The refractive indices for sodium-light were determined on an Abbé-Pulfrich total-reflectometer. Carefully selected material was analysed by Mr. G. G. Knighton and by Mr. M. C. Burkitt, and the specific gravity of the material analysed was determined.

Some years ago the writer read a paper on the determination of the optic axial angle of a crystal by the Becke method. This proposed method was to record the trace of the optic axial plane when horizontal, the position of one axis, and the extinction-direction. From these data and the established constants of the microscope, the position of the second optic axis may be found, as the extinction bisects the angle between the centre of the pole of the section and the two optic axes. Hence, the optic axial angle may be determined.

Considerable errors arise in these determinations owing to defects in the optical system of the microscope, and also to the inability to determine the exact position when the trace of the optic axial plane is parallel to the E-W. lines in the microscope.