Geology of Inchcolm

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Abstract
In his classic paper on the igneous geology of the Forth area,1 Sir Archibald Geikie drew attention to the occurrence of picrite on the island of Inchcolm. And as a locality for picrite the island has become widely known among geologists. But, as we hope to show in the sequel, Inchcolm exhibits several other interesting features. In outline the island may be described as horse-shoe shaped. This configuration has resulted from the denudation of an anticlinal fold pitching to the north. Most of the islands in the Firth of Forth are outcrops of intrusive sills, and Inchcolm is no exception to the rule. But, while this holds in a general way, the island is especially interesting in that it bears witness to three distinct phases 2 of igneous activity. These may be stated briefly as follows:— 1. A series of bedded lavas and tuffs outcropping at the extreme west of the island, between high and low water marks. 2. An intrusive sill which forms the bulk of the island and which is of later age than the lavas. 3. A broad east and west dyke which cuts vertically through the sill. I. The Lavas and associated sediments. At the extreme north-west corner of the island there appears a rocky ridge striking north-east. This ridge is only partially uncovered even at lowest spring tides, and, consequently, our examination of its characters had to be a hurried one. It consists of several flows of basalt, very scoriaceous in

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