KiLROE — The Shannon : ifs Course and Geological History, 87 



the Shannon hasin, where the river-course happened by chance to 

 find its way through an ancient valley at Rooskey. 



The Killaloe gorge presents no indication of liaving been formed 

 to any extent in pre-Carboniferous times, though valleys on both 

 sides of the group pierced by the gorge seem to have been eroded 

 at that early date ; and the continuance of the Shannon course 

 across the group, irrespective of the previously-formed and re-filled 

 valley on either side, shows that the obliteration or possible blanketing 

 over of prominences in pre-Tertiary times must have been perfect. 

 The deflection of the river to the west, however, before it reached 

 the intensely-folded region of Cork and south Limerick, proves that 

 those Hercynian disturbances probably occasioned the existence of 

 somewhat higher or less easily eroded ground, there ^ than that which 

 lay northward, along the infant Shannon basin. It is when we look 

 closely into the form of the river-bed in the vicinity of Killaloe that 

 the most difficult and interesting questions arise. 



Until the last twelve months I had strenuously maintained that 

 the river-bed has been formed entirely by ordinary current-action, and 

 solution. When studying the Lough Derg soundings, however, 1 

 perceived that ordinary river-erosion could not produce a bed of 

 the shape indicated : reference to sections is here invited (Plate IV., 

 fig. 2, already noticed, and Plate V.). 



It will be perceived that instead of the river being shallow over 

 the unyielding Silurian slate-rock, set almost vertically, and striking 

 across the river-course, it is deeper than over the limestone of Lougli 

 Derg, and much deeper than over the comparatively easily eroded 

 Old Eed Sandstone at Killaloe. The river-bed actually drops below 

 the datum line above the town, while at the town it is 100 feet 

 above datum. Old Eed Sandstone strata are here to be seen in 

 the river-bank, and Silurian rocks in situ in its bed. A barrier is 

 thus formed, partly of Silurian, and partly of Old Eed Sandstone 

 rocks, which without the artificial impounding weir would retain 

 the waters of Lough Derg to a depth of some 104 feet opposite 

 Derrycastle — two miles above Killaloe. One might have expected 

 to find a fairly level shallow bed from Killaloe northward, a sudden 

 drop from slate-rock to the sandstone floor, and a pronounced wide, 

 well-formed valley in the limestone district southward to Limerick. 

 None of tliese elements exist ; instead, we have the formidable barrier 

 at Killaloe, naturally damming up a considerable depth of water 

 in Lougli Derg, and the river falling away southward by a series 

 of rapids which correspond with drops in the canal, south of 



