1907. Seymour. — Geology of Lamb ay. 5 



contour and physiography of Lambay as of very great 

 antiquity, possibly pre-Carboniferons or Devonian, and as 

 having been protected throughout the subsequent aeons under 

 a mantle of strata, the comparatively recent denudation of 

 which reveals once more the ancient contours pretty much in 

 their original form. Seeing that the Carboniferous limestone 

 has not yet been entirely denuded off the Silurians at 

 Portrane, which belong to the same period as the Lambay 

 rocks, it seems probable that the latter have only recently 

 emerged from the covering of the former rocks. On the 

 assumption that no Carboniferous rocks covered this portion 

 of the Silurian inlier, Lambay must be regarded as but a 

 remnant of a vastly greater mass, worn away by ages of 

 denudation, and its physical features as being geologically 

 recent. 



The present physical features, in Iheir broader aspect, are 

 intimately connected with the general geological structure of 

 the island. It is noticeable, for example, that practically all 

 the rock crags face west or south-west. This is primarily due 

 to the tilt of the strata ; which dip generally, and at a fairly 

 low angle, towards the east, and thus the edges of the beds 

 appear in the scarps. This general dip can be well seen on 

 approaching the island from the south, and also in the 

 vicinity of Lambay Head, especially on its northern side. 

 The minor features depend chiefly on local differences in the 

 weather-resisting qualities of the various rocks, some of which 

 disintegrate much more readily than others. 



It is to this latter fact that we owe the presence of certain 

 prominent scarps and rounded hillocks, such as Bell Rock, 

 Flint Rock, and Raven's Rock, the two former being made up 

 of the tough weather-resisting Lambay porphyry. 



Roughly, some two-thirds of the island lies above the 150 ft. 

 contour, the greatest height being reached at Knockbane 

 (418 ft.)- More than three-quarters of the entire shore line 

 consists of steep cliffs varying from 50 up to 150 feet as at Salt- 

 pan Bay. The central back-bone of the island, forming the 

 highest ground, runs in a sinuous line from Lambay Head to 

 Raven's Rock, and is flanked on the north by the main depres- 

 sion of the island, ending at Freshwater Bay, and by a minor 

 valley on the south running down to Seal Hole. Both valleys 



