XI. The Pitchstone Porphyry of Penrioch, Arran

Abstract

This Pitchstone, which is noted in the list of Arran pitchstones given in the Geological Survey Memoir (1903, p. 91) but which has not hitherto been described in detail, is exposed, for a few yards only, on the hillside about half a mile N.E. of Penrioch Farm, at the extreme south end of a prominent scarp marking the line of a fault. It is intruded into the ring of schistose rocks which surround the Northern Granite. The junction between the pitchstone and schist is not a sharp one the pitchstone having penetrated along joints in the schist and included blocks up to a foot in diameter. It is not clear from the field evidence whether the intrusion is a sill or a dyke, but it is probably the latter. In the hand specimen, the rock is a glossy, black pitchstone showing abundant large and conspicuous phenocrysts of felspar, smaller ones of quartz, and numerous xenoliths of a basaltic rock of aphanitic texture, hardly to be distinguished, macroscopically, from the schistose country rock. The fracture is splintery and uneven and the rock breaks up in diverse directions owing to the numerous cooling cracks which have developed. In thin section the rock is seen to consist of a base of pale yellowish brown glass, mottled a deeper brown in patches and crowded with minute microlites, which carries large phenocrysts of felspar and quartz and abundant microphenocrysts of pyroxene. The great majority of the microlites are of a ferromagnesian mineral, but occasionally felspar This 250-word extract was created in the absence of an abstract