V.—On the Occurrence of Porphyritic Structure in Some Rocks of the Lizard District

Extract

The crystals of felspar are found to be most numerous in those rocks which lie in the closest proximity to the gabbro and serpentine; they have their long axis at various angles, and are mostly small, except at Parn Voose, Cavouga, and Green Saddle. The felspathic and hornblendic lines often circle round the crystals.

Such is a brief statement of the bare facts of our observations in this particular direction. Without discussing any theory as to the true nature and origin of the whole of the schists, we think that the porphyritic structure, so prevalent in the dark bands of the “Granulitic Group,” in many of the micaceous and other rocks, as also in the later intrusions cutting the serpentine, indicates an igneous origin for many rocks hitherto regarded as schists.