The petrographic characters of the Selborne Marble, the Byrd Group, quartz porphyry dikes, and the Beacon Group are described. It is concluded that the Selborne Marble (Amphibolite Hornfels Facies) is possibly part of an older granite‐metamorphic basement from which the Byrd Group sedimentary rocks (Greenschist Facies) were derived. The quartz‐porphyry dikes are found to be highly albitic and slightly sericitised. Regional alteration of the Beacon Group was probably due to the widespread effects of dolerite sill injection. At the immediate contact with the dolerite, the rocks have been more strongly altered to Albite‐Epidote Hornfels Facies.
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D. N. B. Skinner (1965) studied this question.