Collected papers by H.G. Midgley on the mineralogy of silicate building materials, with additional papers

The Cockburn Law intrusion is situated in that part of Scotland where many intrusions of both Caledonian and Carboniferous age form the Laws of the Lammermuirs. The intrusion has been known for many years and may have been visited by Playfair and Hall during the time of the controversy between the Plutonists and the Neptunists. Cockburn Law, one of the “ Caledonian Granite ” intrusions of the Southern Uplands, lies at the junction of Sheets 33 and 34 of the Geological Survey of Scotland, about three miles north of Duns. The intrusion forms the two hills of Cockburn Law 1,066 ft. O.D. and Stoneshiel Hill 723 ft. O.D. The hills are separated by a gorge formed by a meander in the White- adder Water. The first good description is given by Stevenson (1849), he noticed the variability of the granite and even at that early date attributed the “ marginal ‘ syenites ’ (= diorites) to greywackes fused by the agency of the molten granite F. Walker in two papers (1925) and (1928) described the Cockburn Law intrusion together with other Southern Upland Intrusions. He describes the various rock types present, including a central basic hornblende - biotite - granodiorite, an intermediate ‘ monoclinic - pyroxene-diorite ” and a marginal “ quartz-hypersthene-porphyrite In one paper (1928) F. Walker gave two analyses for the granodiorite and the quartz-biotite-hypersthene-porphyrite. No detailed description of the petrography or of the field relationship of the various rock types is given. The area has been mapped on a 6 in. to 1 mile scale, the mass is not well exposed, in places there are no exposures, being covered with grassland or ploughed land. The best exposures are to be found in the river cliffs of the Whiteadder Water, where it is possible to collect specimens showing the gradual transition of rock types on passing out from the centre of the intrusion. The hybrids are well exposed north of Law Plantation and on the summit of Cockburn Law. There are also fair exposures on the west side of Stoneshiel Hill. The rest is badly exposed, there being only occasional weathered boulders surrounded by grass and heathland. It is nowhere possible to see the transitions in any one exposure. The junction between the intrusion and the Llandovery country rock is not exposed, but it is possible to obtain evidence of the nature of the contact. The intrusion can be divided into two broad groups :— 1. The uncontaminated acid intrusive rocks, adamellite, granite, and granophyric microgranite. 2. The hybrid diorites, resulting from contamination. After consolidation of the main intrusion, continuation of igneous activity produced dykes in the surrounding Silurian rocks, and aplite veins in the main intrusion itself. In this paper the method of formation of the hybrids is indicated, their age in relation to the main body of the intrusion fixed, and the source of material suggested. Variation of the uncontaminated part in relation to the cooling processes of the magma is also explained.