558 Notices of Memoirs — W. A. Traill — 



as to the general character of them — in this one the greater part of 

 the shell is preserved ; (h) shows only a cast — nearly all the shell 

 having chipped away. The drawing is full size. 



The shells are extremely thin and fragile, but the structure and 

 markings are perfectly preserved and the nacreous lustre is still quite 

 brilliant. I have no doubt but a palseontologist visiting the locality, 

 and having leisure to make a careful examination, would find many 

 perfect specimens capable of determination, and probably other 

 species as well. In the mean time it is right to ]3lace the matter on 

 record — seeing that this place is, so far as I know of, the only 

 locality in the British Isles yielding lacustrine fauna of Pliocene date. 



nsroTioiES oip ^y^:K!lvcoI:E^s. 



I. — On Certain Pre- Carboniferous and Metamorphosed Trap- 

 Dykes AND THE Associated Eocks of North Mayo, Ireland.^ 

 By William A. Traill, M.A.I., F.K.G.S.I. ; H.M. Geol. Surv., 

 Ireland. 



n'lHE author first described the locality as situated in the N.W. of 

 i the Co. Mayo, between Downpatrick Head and Broad Haven ; 

 and referred to the geological map of Sir Eichard Griffith. The 

 phj'-sical features of the district presented precipitous coast sections, 

 attaining elevations of 352 ft. at Keady Point ; 640 ft. at Benwee 

 Geevraun; and 829 ft. at Benwee Head, and would compare favour- 

 ably for grandeur and boldness of scenery w4th many better known 

 localities. The geological formations composing the district belong 

 to the Carboniferous and Metamorphic rocks. The older or Meta- 

 morphic rocks lie to the w^estward, extending from Broad Haven to 

 the Glenglassera river, and consist of flaggy quartzites and micaceous 

 schists, with partings of mica-schist ; the mica is often largely crystal- 

 lized out in hexagonal plates in nests or veins. Foliation is seldom 

 developed, but the original bedding is still clearly retained, with a 

 primary dip E.N.E. at variable angles. They are often much crumpled, 

 contorted, and overlapped, more particularly in the western portions. 



The newer or Carboniferous rocks lie to the eastward, and include 

 the Carboniferous Limestones to the S.E. of the district, and the 

 Lower Carboniferous Sandstones and Shales, the bottom beds of 

 which are brought to the surface near the Glenglassera river. These 

 latter comprise white, yellow, and red sandstones, with partings of 

 red and green shales. They dip E.N.E. at from 3° to 8°. 



They rest unconformably on the Metamorphic rocks. At Foherna- 

 deevaun, at the mouth of the Glenglassera river, this most remarkable 

 example of unconformability is best seen ; it occurs at one side of a 

 fault along which the sea has worn a narrow but deep chasm ; the 

 flatter Carboniferous beds dipping at 5° rest on the more highly 

 inclined Metamorphic rocks dipping at 25° to 30°. 



1 Read before the Britisli Association for the Advancement of Science, Section C 

 Geology, Glasgow, September, 1876. 



