578 Scientific Proceedings, Royal Dublin Society. 
point of the ridge connecting Montpelier Hill with the main mass 
of the Dublin Mountains to the south. Small streams enter it 
near the ends, and flow east and west from it, but at its summit it 
is quite dry, and it is this absence of any adequate eroding agent 
which is the most remarkable characteristic of this and other 
similar gaps. The form of the ground at both ends of the pass 
indicates that, previous to the Glacial Period, shallow valleys had 
been excavated in the hillside on both slopes of the ridge, and 
atmospheric erosion, aided by the wash of rain, seems to have 
lowered in some degree the narrow portion of the ridge separating 
their head waters. In the shallow depression thus formed, the 
deep notch has been cut which now unites the heads of the two 
valleys. It is about 500 yards long, and has sides from 90 to 100 
feet high, sloping at angles varying from 25 to 35 degrees, composed 
in the western part of slate and in the eastern part of granite, the 
gorge traversing the junction of the two formations, and appa- 
rently having no reference to the solid structure. The highest 
point of the floor is 1035 feet above the Ordnance datum line, and 
about 100 feet higher than the tops of the lower mounds mentioned 
below as occurring at the mouth of the Piperstown Valley. ‘The 
form of the floor is so irregular, that no deduction can be made 
from its slope as to the direction in which the eroding water flowed. 
This is principally owing to the large amount of talus which has 
fallen in from the sides. The pass proper is continued in an 
easterly direction as a comparatively shallow but fairly steep- 
sided gully, partly excavated in drift and partly in granite, run- 
ning down the slope into the Killakee Valley, and forming a trench 
in one side of the shallow depression which formerly existed here. 
The stream which flows from the west end of the gap pours 
into a slightly larger stream coming from the south, the valley of 
which, at the point of junction, turns sharply to the west and 
opens out into Glenasmole. It is this valley, in which the village 
of Piperstown lies, that contains the most remarkable deposit of 
gravels in the district. From the mouth of the Piperstown Valley 
westward there stretches away the smooth gently-sloping plain of 
boulder-clay which fills up the lower part of Glenasmole. The 
gravels set in in a fairly straight line across the mouth of the 
valley and appear to lie on the boulder clay. Immediately to the 
east of this line they rise suddenly from the plain into two very 
