Beporis and Proceedings — Geological Societg of London. 85 



Dr. Howitt, who regarded the diabase as a rock intrusive in Devoniaa 

 times. I regard it as mainly effusive in origin and probably of Lower 

 Ordovician age. "With Professor Gregory I am unable to agree in the 

 interpretation of some of the field evidence, and I differ from him 

 in regarding the cherts as altered Ordovicians and the diabase as being 

 probably Lower Ordovician in age, and in his interpretation of the 

 relation of land and sea in Lower Ordovician times." A geological 

 sketch-map illustrates the paper. 



C. 0. c. 



Geological Society of Londox. 



December 16, 1908.— Professor W. J. Sollas, LL.D., Sc.D., F.R.S., 

 President, in the Chair. 

 The following communication was read : — 



"On the Igneous and Associated Sedimentaiy Rocks of the 

 Tourmakeady District (County Mayo)." By Charles Irving Gardiner, 

 M.A., F.G.S., and Professor Sidney Hugh Reynolds, M.A., F.G.S. 

 With a Palaeontological Appendix by F. R. Cowper Reed, M.A., F.G.S. 

 The general succession of the Ordovician Rocks of the district 

 appears to be as follows : — 



(4) ? Bala Beds. — Coarse conglomerate and sandstone containing pebbles, mainly 



of granite and felsite. 

 (3) Llandeilo Beds. 



(e) Shangort Beds. — Grits and tuffs, coarse and fine, the prevalent type 

 being a calcareous gritty tuff, in which is a series of limestone 

 breccias, having a maximum thickness of about 40 feet and largely 

 formed of disrupted fragments of the underlying limestone. 

 {b) Tourmakeady Beds. — Compact pink, grey, or white limestones, some- 

 times in beds with a maximum thickness of about 30 feet, but usually 

 represented by blocks in the Shangort Beds. 

 (rt) Red felsite or rhyolite. — A series of flows varying much in thickness, 

 (2) Arenig Beds — Mount Partry Beds. 



(d) Vaiiable tuffs, grits, and cherts, the tuffs being seen only in the 



southern half of the area, 

 (c) Coarse quartzose and felspathic grits. 

 (b) Grits, graptolitic black slates, and radiolarian cherts. 

 («) Coarse conglomerates, the pebbles of which consist almost entirely 

 of grit. 

 A considerable series of graptolites, collected from the Mount 

 Partry Beds, has been examined by Miss G. L. Elles, D.Sc, and they 

 prove to be of Upper Arenig age — about the zone of Didymograptus 

 hirundo. The radiolaria from the same series of rocks have been 

 studied by Dr. G. J. Hinde, F.R.S. 



The most interesting and puzzling beds of the district are those 

 of Llandeilo age. Although the limestones (Tourmakeady Beds) 

 occur in the main as disrupted blocks in the gritty tuffs (Shangort 

 Beds), the fossils indicate that there is no material difference in the 

 age of these two deposits ; and the authors believe that, after the 

 deposition and consolidation of the limestone, but during the prevalence 

 of the same faunal types as those which characterize that deposit, the 

 limestone was broken up by volcanic explosions, and its fragments. 



