Abstract
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.

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