G. H. Kinahan — Water Basin of Lough Derg, 489 



A fourth valley extends from Lough Cooter along the Owendal- 

 ulleegli to Marble Hill, a well-marked fault having been traced 

 nearly the entire distance. The fault has its greatest throw in the 

 Derrybrian valley, immediately south of the hamlet called Derry- 

 laur, eight miles east of Gort ; but at Chevy-chase the throw is also 

 considerable, as there the limestoiie is now in juxtaposition with the 

 Silurians or the basal conglomerate of the Old Ked Sandstone. 



A fifth valley extends across Slieve Aughta, a little north of the 

 summit of Cashlaundrumlahan, in connexion with which there is 

 also a fault, as we find in the Owennaglanna valley Lower Limestone 

 shales, whose south outcrop could not anywhere be found. These 

 rocks are probably bounded on the south by a fault, as farther west 

 in the Boleyneendorrish river- valley there is another outlier of Lower 

 Limestone shale, which has been proved to be bounded on the south 

 by a fault ; but as the country on the south of the Owennaglanna is 

 so covered by deep drift and bog, no positive proofs of a fault can there 

 be obtained. 



Another fault in connexion with a valley in Slieve Aughta was 

 proved across the low country N.E. of Dalystown, and in the eastern 

 part of the Dalystown river-valley. A little on the north of the 

 village of Tynagh, this fault brings up a small tract of Old Eed 

 Sandstone. It has been proved towards the north-east as far as 

 Hearnsbrook, and S.W. into the Dalystown river valley.^ 



Lough Derg. 



Lakes in a Carboniferous Limestone country are usually very 

 irregular in outline, and, at first sight, it might be supposed that 

 there was no connexion between their shapes and the structure of the 

 subjacent rocks. On examination, however, it is generally apparent 

 that the bays and all wide stretches across the basins conform with 

 the strike of lines of breaks or displacements in the adjoining 

 country, while tlie minor features of the coast lines are due to the 

 weathering along minor breaks, joint systems, or lines of bedding, 

 more generally one or other of the first two ; the basin of Lough 

 Derg prominently illustrates these facts. 



On examining the chart of Lough Derg,^ it is evident that by a 

 few displacements the lake basin might again be made to occupy a 

 nearly N.N.E. and S.S.W. valley; also that each displacement that has 

 shifted the primary valley out of this bearing coincides more or less 

 closely with the strike of the valleys and faults in Slieve Bernagh 

 and Slieve Aughta. {See Sketch Map, PI. XVII., compiled from the 

 Ordnance and Admiralty Majjs.) This will be apparent from the 

 following general description. Beginning at the south, the bearing 

 of the lake from Killaloe to Einnaman Point is nearly N. and 

 S. ; here a change occurs, but no fault has been proved in connexion 

 with it. North of Einnaman Point, the deeps for four miles extend 

 S.S.W. and N.N.E., where they are probably crossed by a fault which 

 changes their bearing. This fault is not marked on the Grovernment 



1 Memoirs and Maps, Geological Survey, Sheets 115, 116, 124, and 125. 



2 Admiralty Chart, No. 1552, Lough Derg, Ireland. 



