XIX. Notes on a Dolerite, showing Ocellar Structure, from Craigie Hill, Ayrshire

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In my paper on the intrusive dolerites found in the neighbourhood of Glasgow which I contributed to the Society last year, I drew attention to a peculiar structure which I had noticed in a thin section from the Craigie Hill dolerite, Ayrshire, and which I described as follows:—Throughout the rock occur a large number of oval or circular-shaped aggregations of crystals and glass, the largest of which do not exceed 50 mm. in diameter. These aggregations usually consist of a central nucleus of augite, lath-shaped felspar, and brown glass. I have since examined the specimens more carefully, and I am now in a position to give a more detailed account of this interesting structure, as well as other phenomena not referred to in my last paper. The Craigie Hill intrusion occurs about 4 miles to the south of Kilmarnock, while another intrusion of the same character occurs at Dundonald, about 1 mile to the west of Craigie Hill. Both of these intrusions present all the usual characteristics of these dolerite sills. It has been shown in the paper referred to that along the line of contact with the sedimentary rocks these sills have cooled more rapidly than in their more central parts, and, as a result of this, they exhibit a much more finely crystalline structure towards their edges than they do in the centre. In some rare cases, as at the Fossil Grove, Whiteinch, the sill has sent off small veinlets and threads, which have cooled so rapidly that This 250-word extract was created in the absence of an abstract