152 The Eev. Maxwell H. Close, 



must have had over 2,000 feet of such strata removed in this 

 way, besides some of the underlying Carboniferous Limestone, 

 though generally not very much of this has been eroded, as in 

 this vicinity we are on the Middle and Upper Limestone. 

 Doubtless, at Fortran e the denudation has removed the whole 

 thickness of the Carboniferous formation, together with the few 

 hundred feet of the thinning out Old Red Sandstone, if it be 

 such, which once covered the Silurian at that place — probably 

 a thickness of over 4,000 feet altogether. 



It is on such cou side rations that Professor Hull has founded 

 his explanation of the origin of the Scalp, a remarkable gap, 

 three miles W. by N. from Bray, which cuts across a spur ridge 

 from the mass of the Three Rock mountain. The water falls 

 both ways from the Scalp. Professor Hull's suggestion is that 

 that gap was cut by a river which began to flow when the Upper 

 Carboniferous strata still covered the neighbourhood, including 

 the site of the ridge, and which became unable to continue pass- 

 ing through that part of its valley when the general denudation 

 had worn down the softer rocks of the upper part of its course 

 more deeply than it was able to erode its bed through the hard 

 granite and mica slate of the ridge, which was already existing, 

 though doubtless not in its present condition. 



Notwithstanding the deposition of Drift which has taken place 

 over so much of this district, and the great accumulation of it in 

 certain places, very little of the denudation of which we have 

 been speaking can have been effected during the Drift period to 

 supply the materials then spread about, since the traces of the 

 general glaciation wrought immediately before the Drift period 

 still remain visible in so many places. 



Genekal Glaciation. — The signs of the action of the general 

 ice-sheet which once covered Ireland are abundant in the vicinity 

 of Dublin. They consist (1) of rock rounding, smoothing, and 

 striation, and (2) of Boulder Clay, or Lower Boulder Clay, as we 

 may call it, without committing ourselves to the hypothesis that 

 is sometimes implied in that title. 



The rock-grinding can be seen in many places on the quartz-rock 

 of Howth, Ireland's Eye, Shankill, and Bray Head, on the Old Red 

 Sandstone conglomerate near Donabate station, on the felstone 

 there beside the railway, on that of Lambay Island, on the granite 

 (generally recently stripped of its drift covering) near Dundrum, 



