XXXII. The Silurian Volcanic Rocks of the Southern Uplands of Scotland

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R eference was made at the outset to the great development of volcanic rocks of Arenig age in the Southern Uplands, where they are brought to the surface along the crests of anticlinal folds. The largest area of these rocks now exposed at the surface occurs in the neighbourhood of Ballantrae, Ayrshire, where they consist of a succession of diabase and diabase-porphyrite lavas, tuffs and agglomerates. The age of these volcanic materials is clearly defined by fossiliferous flinty mudstones interstratified with the lavas, and by black graptolitic mudstones in the tuffs and agglomerates near the top. The organisms obtained from these deposits comprise graptolites of Middle Arenig age, and hingeless brachiopods, together with a phyllopod crustacean. The lavas, tuffs and agglomerates of Arenig age, are overlain in Ayrshire and throughout the Uplands by an important zone of radiolarian chert belonging partly to Arenig, and partly to Lower Llandeilo time. Thereafter the various intrusive igneous rocks associated with the volcanic series of Ballantrae were indicated, which include masses of serpentine, gabbro, epidiorite, dolerite and granite: their relations to each other in the field being in many places obscure. It was shown that towards the close of Llandeilo time the volcanic and plutonic rocks of the Ballantrae region, together with the overlying sediments, were elevated and exposed to denudation, the materials of the Kirkland and Benan conglomerates at the base of the Girvan series having been derived from these older rocks. Thereafter attention was directed to the numerous inliers of Arenig volcanic rocks