The Differentiated Dyke of Newmains, Dumfriesshire, and its Contact and Contamination Phenomena

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Abstract
I. Introduction Within the Southern Uplands of Scotland, the highly folded Ordovician and Silurian sediments are intruded by a suite of calc-alkaline igneous rocks of Lower Old Red Sandstone age which includes a well-developed dyke swarm. The dykes trend in a north-east and south-west direction and, although individually thin, collectively they represent a considerable body of magma. The only literature dealing with the subject, however, is Teall's generalized account in the Southern Upland Memoir (1899, pp. 625–31), a paper by Read (1926) on the micalamprophyres of Wigtownshire, and a short section in the description by Gardiner and Reynolds of the Loch Doon granite (1932, pp. 19–26). The Newmains dyke, which forms the subject of the present paper, is a member of the Southern Upland swarm and crops out on the hillside immediately north and north-east of Newmains farm, some 4½ miles west of Dumfries and within Sheet 9 of the 1-inch Geological Survey Map (Fig. 1). The dyke is about 15 feet wide and runs for a little more than a mile in a general north-east and south-west direction, in conformity with the strike of the Silurian (Llandovery-Tarannon) greywackes into which it is intruded. It is a typically heterogeneous, differentiated intrusion and consists of three main rock types associated in an intimate manner. The main rock is a granophyric spessartite or markfieldite, which passes on the one hand into a basic, hornblende-rich type and on the other into a leucocratic variety poor in ferromagnesian constituents. These three rock types are

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