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YIII. 



THE EIYETl SHARON: ITS PKESENT COUKSE AND 

 GEOLOGICAL HISTORY. 



By J. R. KILIlOE, H. Geological Survey. 



[Plates III.-YL] 



Read January 28. Ordered for publication FEimuAiiy 13. 

 Published March 29, 1907. 



The conditions of the Shannon basin in the last section of tlie river's 

 course — from Killaloe to the estuary — came under consideration 

 during the recent examination by H. M. Geological Survey of the 

 Limerick area, in 1904; and some references to the drainage were 

 made in the Explanatory Memoir accompanying the published one-inch 

 geological map, recently issued. The narrow limits of the map did 

 not alford justification for a full treatment of the interesting questions 

 involved in the history of the river ; and it is here proposed to set 

 forth, in some detail, the facts bearing upon those questions in their 

 geological, physiographic, and economic relations. 



In 1862, Professor Jukes, m.a., f.e.s., published, in the Quarterly 

 Journal of the Geological Society, an account of the formation of 

 some of the river-valleys in the south of Ireland, in which reference 

 is made to the Shannon basin and those adjoining it. The author 

 pointed out that the Shannon could not have excavated the ravine 

 at Killaloe after the limestone ground north of Slieve Bernagh and 

 Slieve Arra had reached its present relatively low level. He argued 

 that the Shannon, Barrow, Jfore, and Suir must have begun to flow 

 upon a surface high above the present basins of those rivers ; that the 

 hypothetical surface was an uplifted plain of marine denudation; and 

 that the rivers maintained their courses while the general surface was 

 lowered by subaerial waste and river-erosion. Different kinds of rock 

 became exposed, Avhich occasioned differential lowering ; ridges were 

 thus formed, and, while assuming growing importance, were trenched 

 by the rivers. 



