76 Proceedings of the Royal Irish Academy. 



If a zigzag line be drawn along the course of the river southward 

 from the watershed line, omitting the smaller curvatures (see map), 

 the distance to Loop Head and Kerry Head at the Shannon's mouth 

 is 189 miles, or about 175 miles from its source to Kilcredaun Point 

 at Carrigaholt. The drainage-area of the river and its tributaries is 

 6400 square miles, or about one-fifth of the area of Ireland. The water- 

 shed line bounding the basin is indicated on the map, as are also those 

 of the principal adjoining drainage-areas ; and tho heights of the 

 trigonometrical points through which, or near to which, the watershed 

 passes, as set forth on the Ordnance Map, are also given. The heights 

 on the Ordnance Map, when the line traverses low ground, do not 

 usually give the lowest — though they are nearly the lowest — points 

 separating the Shannon basin from the basins adjoining it, for the 

 points selected for indication of heights in the process of levelling, 

 were some salient features of the ground, usually drift hillocks, 

 in the central plain. The approximately lowest points upon the 

 watershed, as thus shown on the accompanying map, and in tabulated 

 form, are — 



"Watekshed Levels. 



Shannon from "Woodford River (branch of Erne), 202 feet. 



,, ,, Erne, 



„ ,, Boyne, 



,, ,, Barrow, 



,, ,, jN'ore, 



,, ,, Suir, 



,, ,, Blackwater, 



,, ,, Corrib, . : 



„ Moy, 



,, ,, Drumahaire River, 



Submergence of the land to a somewhat greater extent than 

 180 feet would connect Galway Bay with the Shannon estuary, 

 along the Fergus valley by Gort and Ennis ; a lowering of a little 

 more than 200 feet would bring the waters of Donegal Bay into the 

 Shannon basin ; and a lowering of more than 260 feet would connect 

 the Irish Sea with the Atlantic, severing the northern half of the 

 island from the southern. 



312 



26P 



374 



444 



385 



287 



308 



213 



1 Recorded by Prof. Jukes, f.r.s., Quart. Journ. Geol. Soc, Nov., 1862,. 



