1862.] JUKES RIYEK-VALLEYS. 385 



1. Tlie River Sliannon. — The Shannon, soon after issuing from the 

 northern hills on to the limestone plain, forms the expansion called 

 Lough Ree, from which it slowly winds over another part of the same 

 plain, through great bogs, until it forms the similar expansion called 

 Lough Derg. From the south end of Lough Derg it runs with a 

 rather more rapid current past Killaloe, between the Slieve Arra and 

 SKeve Bernagh hills, through a valley excavated out of the Lower 

 Silurian and Old Eed Sandstone rocks. Supposing a dam of 150 or 

 200 feet in height to be thrown across the narrowest part of the 

 valley at Killaloe, the Shannon, instead of overtopping this dam, and 

 thus seeking to pour down past Killaloe, would inevitably find a 

 passage for its waters round the outside of the hills over the low 

 limestone ground, either by Scariff to the Fergus, or by Nenagh 

 and the Kilmastullagh valley to O'Brien's Bridge. 



The Shannon, therefore, has certainly not excavated the valley at 

 Killaloe since the limestone"ground attained to its present low level. 

 Moreover, if these three modes of escape were all blocked up, the 

 Shannon, after forming a great lake in the centre of Ireland, including 

 Lough Derg, Lough Ree, and the adjacent country, together with a 

 large part of the great bog of Allen, would find an exit for its waters 

 down the valley of the Barrow, the Boyne, or the Liffey, or even into 

 Galway Bay, rather than down its present course. It is, therefore, 

 exceeingly difficult to understand how the ravine at Killaloe could 

 have been excavated either by the Shannon, or by any other water, 

 supposing it not to exist and the rest of the ground to have an out- 

 line at all approximating to its present form and low level. 



2. The Rivers Barrow, Nore, and Suir. — The Barrow issues from a 

 glen on the northern flank of the Slieve Bloom Hills on to the lime- 

 stone plain, where it is separated from some of the tributaries of the 

 Shannon by elevations just sufficient to turn it to the eastward. 

 Then, after winding round the northern termination of the Coal- 

 measure hills of Carlow, it flows down, between them and the Silu- 

 rian and Granitic hills of "Wicklow, over low limestone ground as far 

 as Gores Bridge. Here, however, it leaves the limestone plain, and 

 traverses the Lower Silurian and Granite hills in a deep and some- 

 times a wide valley until it flows into Waterford Harbour. I^ear a 

 little place called St. Mullins, its waters become afiected by tRe tide, 

 whilst Granite hills, exceeding 1600 feet in height, rise on each side 

 of it ; and both above and below this, it cuts across the Granite and 

 the aqueous and other rocks indiJfferently, without any regard either 

 to their " lie " or their composition. 



The Nore and the Suir both flow from the Devil's Bit range ; 

 the Suir from Borrisnoe Mountain, and the Nore from the ground two 

 miles north of it. They diverge, however, on entering the limestone 

 plain, — the Suir flowing to the south by Cahir, and then, after a 

 detour, past Clonmel to Waterford ; while the Nore, after curving to 

 the north, traverses the limestone plain to near Abbeyleix, and then 

 cuts by a deep valley through the Coal-measure hills near Ballyragget 

 and Freshford, from which it issues out on to the low limestone 

 ground of Kilkenny. Passing over this, it makes for the high 



