150 Proceedings of the Royal Irish Academy. 



side of the basin, with deposition of Wenlock strata against a denuded 

 shelving shore of sinking hand, while a conformable sequence from the 

 Lower to the Upper Silurian series exists, in the centre of the basin, 

 near Doolough. 



Silurian Eocks. 



It has long been known that, as just stated, a conformable passage 

 is traceable from Lower Silurian grits and slates, with double 

 graptolites at Doolough, to Upper Silurian beds 3 miles north- 

 west of this lake. At Cregganbaun here, that is, about 5 miles 

 south of Louisburgh, occurs an earthy arenaceous limestone, but 

 slightly cleaved, which contains a wealthy Upper Silurian fauna. 

 Amongst the fossils occur Pentamerus Kniglitii and P. ohlongus ; and 

 although the beds are represented as Llandovery on the published map, 

 it is stated in the Geological Survey Memoir^ that the rocks "are 

 highly charged with fossils of an Upper Silurian, prohahly Wenlocky 

 eharacter, consisting mostly of corals and Brachiopod shells." They 

 are taken here to be of Wenlock Age. About a mile and a half west 

 by north of this locality monoprionid graptolites have been found in. 

 Derrygarve Slate Quarry, where the rock is probably of Tarannan 

 Age. 



Above the earthy limestones at Cregganbaun follows a series of 

 green argillaceous grits, which spreads out through the wide area 

 extending northward to Louisburgh ; and below the limestone zone 

 occurs a thin band of quartzite, at the base of which is found a 

 notable quartzose conglomerate, with large, rounded blocks and pebbles 

 of quartzite, which overlies the Monograptus slate. 



The thin quartzite band and associated conglomerate are traceable 

 north-eastward by Corvockbrack (1287), and Knockaskeheen (1288), 

 and thence eastward by Knockfadda (957). That here we have the 

 Cregganbaun group is rendered the more certain by the occurrence of 

 earthy limestone and calcareous grit with identical fossils, adjacent to 

 the quartzite at Boheh, south of Croaghpatrick. The limestone passes 

 into a sericite schist at Knockfadda, the fossils being obliterated, and 

 it retains this deformed character eastward while traceable in its 

 usual place above the quartzite and conglomerate zone towards 

 Oughty hill (1104). Here the quartzite becomes a gray grit, and 

 further east it ceases to be recognizable, though its place is indicated 



1 Explanation of Sheets 83 and 84, p. 32. 



