﻿Yol. 66.~\ ORDOVICIAN ROCKS OF THE GLENSAUL DISTRICT. 253 



9. The Igneous and Associated Sedimentary Rocks of the 

 Glensaul District (County Galway). By Charles Irving 

 Gardiner, M.A., F.G.S., and Prof. Sidney Hugh Reynolds, 

 M.A., F.Gr.S. With a Pal^eontological Appendix by Frederick 

 Richard Cowper Reed, M.A., F.G.S. (Read January 12th, 

 1910.) 



[Plates XX-XXIL] 



Contents. 



Page 



I. Introduction ,.,. 253 



II. The Sedimentary Rocks and the Tuffs 254 



(«) The Mount Partly Beds. 



(b) The Shangort and Tourinakeady Beds. 



III. Table of Fossils from the Shangort and Tourmakeady 



Beds 265 



IV. The Field-Relations of the Crystalline Igneous Rocks 267 



V. Petrographical Details 268 



VI. Comparison between the Glensaul and Tourmakeady 



Areas 269 



VII. General Succession of the Strata, and Conclusions 270 



VIII. Palseontologioal Notes 271 



I. Introduction. 



The 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. 1 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 



1 Quart. Journ. Geo]. Soc. vol. lxv (1909) pp. 104-53. 



