Clare Island Survey — Geology. 7 3 



More than a third of the island, consisting of a marginal platform along 

 the north-eastern, eastern, and southern coasts, lies helow the 200-foot 

 contour line, and from this level the hills of Croaghmore and Knocknaveen 

 rise with remarkable abruptness. A similar phenomenon is seen on the 

 mainland to the south of Clew Bay. Here, at about the same level, the rock- 

 platform extends from the bay inland to the foot of Croagh Patrick, and from 

 the coast near Eoonah to the Corvockbrack granite ridge, beyond which the 

 land rises abruptly as before. The form of the ground here indicated cannot 

 be attributed simply to differential weathering, but the lowland and highland 

 features must be considered rather as representing different stages of the 

 sculpturing of the land by denudation. Thus the low rock-platform of Clare 

 Island and its counterpart on the mainland, doubtless, form part of the 

 great post-Carboniferous peneplain that extends over the central plain of 

 Ireland, while the hills of Croaghmore and Knocknaveen are remnants of 

 a higher and older plateau from which the mountains of the western 

 highlands have been carved. 



SOLID GEOLOGY. 

 The original survey of the island was made in the year 1868, by the late 

 E. G. Symes of the Geological Survey, and the map showing the " solid " 

 geology of the district on the 1" scale was published in 1879 ; but, in the 

 original memoirs accompanying the maps of the country round Clew Bay, 

 only very scanty references, amounting in the aggregate to a page or two of 

 letterpress, have been made to Clare Island. In 1909 the ground was 

 re-surveyed by Mr. J. E. Kilroe, and an exhaustive description of its structure 

 has been given by him in a Survey memoir just published.' The brief account 

 of the solid geology of the district contained in the present paper is based 

 mainly on information derived from the latter source. The accompanying 

 colour-printed map, also published by the Survey, is included to illustrate the 

 geological structure of the island, as well as the distribution and extent of the 

 various superficial deposits which cover the greater part of its surface. 



Eock Formations. 



The solid rocks of Clare Island consist of at least five sedimentary groups, 

 together with a few inextensive igneous intrusions, all of which are fairly 

 comparable with the rocks of the neighbouring mainland. 



Except in the Carboniferous strata, no fossils have been found in any of the 

 groups ; hence reliance has had to be placed alone on the field relations of 



1 Memoirs of the Geological Survey of Ireland. "The Geology of Clare Island, Co. Mayo." 

 1914. 



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