CoLK — The Prohlem of the Liffeii Valley, & 



i.e., from 650 to 780 feet above Ordnance datum, from the loops of the 

 Liffey to beyond Brittas, a distance of 85 miles. The original field-map of the 

 Geological Survey shows that the granite floor is exposed in the Liffey in the 

 north of Ballyfoyle townland at a height of 660 feet. Hence, the river must 

 be very near its floor of erosion in the adjacent Kilbride loops. The depth of 

 the drift-covering to the north near Brittas is unknown ; but the Ordovician 

 slate lies near on either hand as we pass along the broad through valley to 

 the Slade of Saggart. The rock-floor of the valley running up northward at 

 Kilbride, if we could lay it bare again, might be found to have a rise of only 

 100 feet (30 m.) from the meadows where the Liffey meanders to the water- 

 shed near Brittas ponds. (Plate II, fig. 3.) 



The Brittas River, far too small for this valley, runs down it to the 

 mountain stream of the Liffey at Ballyward bridge, the water being here 

 about 620 feet above the sea. The Liffey continues south-westward 

 to Blessington, and is joined midway by a very small stream coming 

 down the wide valley in which the main road from Dublin runs. The 

 west side of this valley is formed by the long strike-ridge of Ordovician 

 rock, which we may call after its highest townland, Slievethoul (1,308 

 feet ; 396 m.). This western valley is divided from the hollow of the Brittas 

 Eiver by the rock-ridge of Golden Hill, which includes granite, schist, and 

 slate, and which stands up as a sort of island, the two valleys meeting round 

 its northern end. The western of these valleys, that of the main road, is 

 virtually dry, but presents the appearance of having been occupied by a 

 considerable stream. It has the same old and matured aspect as the Brittas 

 River valley which unites with it. It is difficult to conceive that any rivers 

 generated on the present land-surface to the north can have eroded these large 

 valleys. Their high maturity excludes the suggestion that they are due to 

 overflow waters from the melting ice-sheet that once occupied the Irish plain. 

 They pass northward as a " through valley " into the narrower Slade of 

 Saggart, which has a rapid fall towards the limestone lowland. If we 

 adopt the suggestion that these Brittas valleys were eroded by a river that 

 had its head-waters on limestone land now lost to us,' we have still to explain 

 the Slade of Saggart. 



The Slade of Saggaet, and other IiMmature features on the Foothills 

 OF the Lbinster Chain. 

 The present meandering streamlet in the Slade of Saggart is obviously a 

 misfit or a mere remnant in its relatively majestic valley (Plate II, fig. 4). This 



'Compare E. HuU. "On the origin of 'the Scalp,'" Sci. Pioc. Rov. Dublin Soc., toL i. 

 (1878), p. 11. 



r-B2] 



