On the so-called “Greenstones” of Central and Eastern Cornwall

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Abstract
Conclusion: In addition to the rocks which occur in Western Cornwall, some of the areas forming the subject of the present paper afford numerous examples of ancient lava-flows so interbedded with the slates and schists of the district as to lead irresistibly to the conclusion that they are contemporaneous igneous products. These lavas, notwithstanding the alteration to which they have been subjected, closely resemble those of more modern date, and contain cavities which are now filled either with a mixture of viridite and calcite or with quartz through which vermicular chlorite is distributed. They sometimes assume a distinctly schistose structure, and contain crystals of a sanidine-like felspar, many of which are broken, while others have become sphærulitic, through a rearrangement of their constituents. The crystalline greenstones of Central and Eastern Cornwall afford a more varied and instructive series than is furnished by those of the western portion of the county. Among them are typical dolerites on the one hand, as at South Petherwin, while on the other there are rocks which, as at Carkeel, are so altered as to consist only of a granular indefinite material traversed by indistinct microlitic bodies, and, in patches, stained brown by oxide of iron. The pyroxenic constituent of these rocks, wherever its crystals admit of identification, is augite ; and they are consequently dolerites. By many petrographers they will doubtless be classed either as diabases or melaphyres ; but when it is remembered that they differ from modern dolerites only in age, and by reason

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