The Igneous and Associated Sedimentary Rocks of the Glensaul District (County Galway)

Abstract

I. I ntroduction . T he district is a small one, having a maximum length from south-west to north-east of little more than 2 miles, and a width of about a mile. Its north-eastern margin lies about 3 miles south-west of Tourmakeady Lodge, the southernmost point in the area recently described by us. That portion of the district with which this paper deals consists of the valley of the Glensaul River, together with the hills lying to the south-west, which rise to a height of 925 feet above sea-level at Lettereeneen and to nearly 700 feet at Greenaun. The area of Ordovician rocks, with which this paper is concerned, is roughly quadrilateral in shape. It is bounded by peat and drift on its southern side, partly by peat and drift and partly by grit and conglomerate of ? Bala age on its northern side, while its western and its eastern sides are bounded by faults. The western fault, very clearly seen along a stream-course that runs northwards down the slope of Lettereeneen, brings a fairly coarse quartzose conglomerate, forming the actual summit of the hill, against the Ordovician rocks. This conglomerate contains large felsite pebbles and smaller ones of quartz, and of red and black chert, and is quite different from the coarse conglomerate of Arenig age, to be described below (p. 255). We have found no fossils in it, and an account of it is beyond the purpose of this paper, but we class it with the grits and conglomerates found on