104 MR, GARDINER AND PROF. REYNOLDS ON THE [May I9O9, 



6. On the Igneous and Associated Sedimentary Rocks of the 



ToURMAKEADY DISTRICT (CoUNTY Mayo). By CHARLES IrYENG 



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

 M.A., F.G.S. ; with a Fal^ontological Appendix by Frederick 

 Richard Cowper Reed, M.A., F.G.S. (Read. December 16tb, 

 1908.) 



[Plates IV-VL] 



Contents. 



Page 



I. Introduction 104 



II. The Sedimentary Eocks and the Tuffs 106 



(a) The Arenig Rocks (Mount Partry Beds). 



(b) The Llandeilo Eocks. 



(& 1 ) The Shangort Beds : the Gritty and Ashy Series. 

 (b 2 ) The Tourmakeady Beds : the Calcareous Series. 



(c) List of Fossils from the Llandeilo Beds. 



(d) The (?) Bala Eocks : Conglomerates and Grits. 



III. The Field-Relations of the Crystalline Igneous Eocks ... 127 



(a) The Felsites. 



(b) The Intrusive Andesitic Eocks. 



(c) The Hornblende-Lamprophyres and Dolerites. 



IV. Petrographical Details 133 



(a) The Felsites. 



(b) The Intrusive Andesitic Eocks. 



(c) The Hornblende-Lamprophyres and Dolerites. 



(d) The Tuffs and Breccias. 



V. The Volcanic Vent or Neck 139 



VI. Summary and Conclusions 139 



VII. Paheontological Appendix 141 



I. Introduction. 



Our object in tbe present paper is to describe a series of igneous 

 rocks associated with beds containing Ordovician fossils, and occu- 

 pying 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 4| miles. 



At the southern end of the area, on its eastern side, this igneous 

 series of rocks is bounded by massive conglomerates; and rocks 

 litbologically 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 Old 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 



