152 THE Rev. 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 Portrane 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 considerations 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. 
GENERAL 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, 
