Summary: The paper describes various types of zoning in plagioclase felspar in the Calciferous Sandstone basalt lavas in one district of Scotland. The zoning is classified as (a) normal, (b) simple reverse, (c) oscillatory. Simple reverse zoning is associated with other differences in the zones which point to important time intervals between the growth of the zones. Oscillatory zoning is classified as oscillatory-normal and oscillatory-reverse and attention is drawn to the occurrence of oscillatory-zoned crystals which show no general tendency towards either more calcic or more sodic composition. Distinction is drawn between the main zones and the thin shells of alternately more and less calcic composition within the main zone. The alternating composition of the thin shells is possibly the result of lack of balance between rate of growth of the crystal and rate of diffusion from the surrounding magma. Recurrence of calcic plagioclase in the inner part of main zones is explained as the result of irruption of hot magma into the crystallizing liquid, probably consequent on eruption of lava at higher levels.
This paper is published with the permission of Sir John Flett, Director of H.M. Geological Survey.