Note on corroded ferromagnesian phenocrysts in a Scottish basalt

Abstract

I. Introduction. The writer has recently made a study of a large number of slices of the Lower Carboniferous porphyritic basalt lavas of North Ayrshire and adjoining areas (MacGregor, 1930,1 pp. 89-99). In these rocks phenocrysts of augite, olivine and plagioclase felspar may often be seen to have sub-rounded outlines, or to have embayments and enclosures of groundmass. One rock-slice in particular, because it contains an exceptionally good example of a honeycombed augite phenocryst, deserves a more detailed description than could be accorded to it in the North Ayrshire Memoir (MacGregor, 1930, p. 97). The rock from which the slice (20,196)2 was cut was collected by Mr J. E. Richey near Craig of Neilston, Renfrewshire, in ground surveyed by him for the Geological Survey. It is an olivine-basalt of Dunsapie type (MacGregor, 1930, p. 91), and contains large phenocrysts of olivine, augite and felspar. II. Description of the Rock. The chief feature of the slice is a large phenocryst of pale fawn-brown augite about 1 cm. across (Plate XLVII., Fig. 1). The phenocryst is traversed by ramifying passages that locally pinch out almost entirely. The material in these cavities has crystallised mainly as felspar, granules of augite, and iron-ore, and closely resembles the groundmass. This material enclosed in the augite is in fact seen to be in communication with the groundmass at the right-hand side of the photograph (Plate, XLVII., Fig. 1). There is a decided tendency for little felspar laths (traversed by numerous delicate apatite needles, and accompanied by