The Bala Beds and Associated Igneous Rocks of Lambay Island, Co. Dublin

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Abstract
I. I ntroduction . Lambay Island lies off the East Coast of Ireland, some 10 miles north of Dublin, the nearest point on the mainland being in the district of Portraine, where there occurs an inlier of Bala rocks which we have already described. The name of Lambay is probably familiar to geologists from the occurrence there of the ‘Lambay porphyry,’ a very handsome and striking rock, described in detail by Von Lasaulx and mentioned in many geological and petrological works. It was with the intention of investigating the occurrence of this rock in the field, and at the same time of seeing whether the other rocks exposed on the island were of a similar nature to those at Portraine, that we visited the island. The only detailed account of the geology of Lambay appears to be in the explanatory memoir to Sheet 102 of the Geological Map of Ireland, published in 1861. Homogeneous and porphyritie trap-rocks are mentioned, catching up in places masses of slates and grits which they have baked and hardened. Ashes were also found, while limestone and graptolitic shales gave evidence that some of the rocks of the island were of Bala age. On looking at the map of the island it will be seen that a large part of it is drift-covered, but from the exposures which occur it is obvious that the greater part of Lambay is formed of igneous rocks, the sedimentaries occurring in detached masses of no very great extent, and, except in the case of

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