1862.] JUKES — BIVER-YALLEYS^ 379 



When Sir Eoderick Murchison became the Director-General of 

 the Survey, and ordered that descriptions, or *' Explanations," to 

 accompany each sheet of the map, should be prepared, he pointed 

 to the form of ground as one of the things to be described. I had 

 often previously thought of examining this question, and was there- 

 fore not sorry to find it brought directly before me in the course of 

 my of&cial duties as the Local Director of the Irish branch of the 

 Survey, one of which duties is, of course, the editing of these " Ex- 

 planations.'* The following notes on the formation of some of the 

 river- valleys of the southern part of Ireland contain some conclu- 

 sions at which I have arrived in the course of the last few years, 

 while engaged in that duty; and they are here offered as a contribution 

 towards the solution of this problem. 



Part I. — Physical Structure oe the South of Ireland. 



Ireland may be divided into two nearly equal parts by a line 

 drawn from Dublin Bay to Galway Bay. This line would traverse a 

 broad belt of low, very nearly level ground, the immediately subja- 

 cent rock of which is almost entirely Carboniferous Limestone. It 

 would run from the basin of the Liifey into that of the Barrow, and 

 then crossing that of the Shannon between Lough Eee, the surface 

 of which is about 125 feet above the sea, and Lough Derg, which is 

 about 17 feet lower, would pass into that of Galway Bay, which, ex- 

 cepting the Corrib, receives only a few marginal streams. 



The watershed between the basin of the Liffey and that of the 

 Barrow cannot be higher in some parts than 279 feet above the sea, 

 which is the height of the summit-level of the Grand Canal near 

 Kobertstown, about six miles west of Salins. One of the trigonome- 

 trical points on the watershed, south of Kobertstown, is only 290 feet 

 above the sea. 



The watershed between the basin of the Barrow and that of the 

 Shannon passes over ground, near Phillipstown, which is in some 

 places not higher than 261 feet, the level of the Grand Canal there, 

 one of the trigonometrical points on \h.Q watershed being only 295 

 feet. 



The great Bog of Allen,> which is an iU-defined assemblage of 

 large bogs, separated from each other chiefly by gravel-mounds and 

 esker-ridges, lies on the flat country about the watershed between 

 the basins of the Barrow and the Boyne and the adjacent parts of 

 those of the Lifl'ey and the Shannon ; so that there is a broad belt of 

 land here, in the centre of Ireland, no part of which, except perhaps 

 an occasional gTavel-mound, exceeds 300 feet above the sea. 



The summit-level of the Royal Canal, which runs from Dublin to 

 the Shannon above Lough Eee, derives its water from Lough Owel, 

 the height of which is 327 feet above the sea. 



On the west of the Shannon basin, the watershed between it and 

 Galway Bay is certainly not higher in some places than 300 feet 

 above the sea ; for this is the maximum height of the Great Midland 



