Yol. 68.'] PALiEOZOIC ROCKS OF THE KILBEIDE PENINSULA. 75 



6. The Ordovician and Silurian Eocks of the Kilbride Peninsula 

 (Mayo). By Charles Irving Gardiner, M.A., F.G.S., and 

 Prof. Sidney Hugh Keynolds, M:A., F.G.S. (Read December 

 20th, 1911.) 



[Plates VI & VII.] 



Contents. 



Page 

 I. Introduction 75 



II. The Sedimentary and Volcanic Arenig Eocks... 77 



(a) The Sedimentary Ai-enig Rocks. 



(b) Tlie Breccias. 



(c) The SpiHtes (Pillow-Layas). 



III. The Silurian Eocks 81 



(a) The Main Outcrop. 



(b) The Barnarinnia Area. 



IV. Field-Eelations of the Intrusive Igneous Eocks. 88 



(a) The Felsites. 



(b) The Lime-Bostonite. 



(c) The Labradorite-Porphjrite. 



(d) The Dolerites. 



V. Petrographical Details 91 



(a) The Felsites. 



(b) The Lime-Bostonite. 



(c) The Labradorite-Porphyrite. 

 (/) The Dolerites. 



(e) The Spihtes (Pillow-Lavas). 

 (./') TheTufFs. 



VI, Comparison of the Eocks of the Tourmakeady, 



Glensaul, and Kilbride Areas 97 



VII. General Summary and Conclusions 98 



I. Introduction. 



The Kilbride area with which this paper deals lies some 5 miles 

 to the south of the Glensaul area described by us in 1910. Its 

 limits are very sharply defined. It forms a peninsula bounded 

 by two long inlets from the south-western corner of Lough Mask, 

 the northern one being known as Derry Eay or Derrypark Say, 

 the southern as Kilbride Bay. It has a length from east to west of 

 about 4g miles, and a breadth of about a mile and a half. It ends 

 to the west in the alluvial tract through which the Finnj^ River 

 runs. From this low ground the land rises rapidly to a ridge which 

 attains its highest point at Knock Kilbride (1230 feet). Farther 

 east is another eminence (Knocknamuck), and then the ground 

 sinks slowly through Fox Hill to the shore of Lough Mask. The 

 descent from the high ground to Derry Bay on the north is far 

 steeper than that to Kilbride Bay on the south, a fact intimately 

 associated with the geological structure of the country. 



Little has been written about the geology of this area. Some 



