On the Igneous and Associated Sedimentary Rocks of the Tourmakeady District, County Mayo

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Abstract
Our object in the present paper is to describe a series of igneous rocks associated with beds containing Ordovician fossils, and occupying a tract of country, having a width of about a mile, which extends along the western shore of Lough Mask from the hamlet of Derrindaffderg in the north to the neighbourhood of Tourmakeady in the south, a distance of about four and a half miles. At the southern end of the area, on its eastern side, this igneous series of rocks is bounded by massive conglomerates; and rocks lithblogically similar, though probably of much later date, form the whole of the western boundary. Along most of the eastern border occur conglomerates and sandstones, to which we, following the Survey, shall refer as the basal beds of the Carboniferous, although they may be regarded with equal probability as representing the 01d Red Sandstone. The surface is undulating, but the physical features are not striking—the ground never rising, in the area with which we deal, to a greater height than about 400 feet, or some 350 feet above Lough Mask. A number of streams traverse the district from west to east, and afford an excellent series of sections ; but, apart from these, exposures are almost everywhere numerous, this being partly due to the fact that the district is free from drift. Considering the interesting and varied character of its geology, it is remarkable that comparatively little has hitherto been written about this area. In 1874, G. H. Kinahan,' in a paper

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