A structural investigation of the porphyritic basalts of Arthur’s Seat, Edinburgh

Abstract

I. Introduction. It is well known that flow can impart a regular arrangement to solid bodies immersed in a moving fluid. This regularity can be either planar or linear, or a combination of the two (R. Balk, 1937). Early crystals (phenocrysts) in a porphyritic basalt may have been oriented by the flowing magma; as the latter froze, so the flow pattern would be preserved, and can now be measured. Structure maps and profiles, based on such measurements, give details of the formation of volcanic bodies that are not otherwise obtainable. With few exceptions, the flow structures investigated on Arthur’s Seat are planar. The intersection of a flow plane with the plane of an exposure gives a linear pattern on the latter. The orientation of the flow plane can be determined if this linear pattern can be found on two neighbouring, non-parallel exposure surfaces. The accuracy is greatest when the two exposures planes are at right angles both to each other and to the flow plane. The margin of error of the measurements actually made did not exceed 15°. Where flow structures are as feebly developed as in the Arthur’s Seat basalts, a higher degree of accuracy is not possible. However, if a sufficient number of measurements be made, this degree of exactitude is sufficient. Though in many instances it may seem at first glance that no structure pattern is visible on a given surface, careful observation often reveals it. The technique of observing and measuring cannot adequately be described in