Clare Island Survey '. 



GEOLOGY. 



By T. HALL1SSY, B.A., M.E.I.A. 

 Plates I-VI. 



Read January 26. Published July 27, 1914. 



[COMMUNICATED WITH THE PERMISSION OF THR DIRECTOR OF THE GEOLOGICAL 

 SURVEY OF IRELAND.] 



GENERAL DESCRIPTION. 



Situated off the west coast of Mayo, at the entrance to Clew Bay, between 

 the parallels 53° 47' 15" and 53° 49' 45" N. lat., and the meridians 9° 56' 25" 

 and 10° 3' 20" W. long., Clare Island forms one of the most westerly outposts 

 of the continent of Europe. It is separated on the south-east from the nearest 

 point of the mainland west of Louisburgh by a strait 3 miles wide, and is 

 3i- miles distant, on the N.N.E. side, from the mainland at the entrance to 

 Achill Sound. 



If we except the triangular promontory on the north, the island is 

 roughly of the form of a trapezoid, 4 to 4| miles long from east to west, 

 and about a mile and a half wide. The shape of the northern promontory 

 is approximately that of an equilateral triangle, each side of which measures 

 about a mile and a quarter. The entire breadth of the island from Leck- 

 nacurra to Portnakilly is 2f miles, and its total area slightly less than 

 61_ square miles. 



A marked feature of the above-mentioned promontory is the extremely 

 rugged character of its surface. Prominent hummocks of bare or scantily 

 covered rock stick out here and there, and deep valleys, often occupied by 

 peat, run east and west along the general strike of the rocks. These irre- 

 gularities of the ground have been determined by the differential weathering 

 of the rocks which enter into the formation of the area, the soft shales giving 

 rise to deep valleys and depressions, and the more resistent grits standing 

 out as well-defined knolls and ridges. 



In the trapezoidal area, the most prominent features of the landscape 

 are two considerable hills, viz., Croaghmore (Knockmore), 1520 feet in height, 

 on the western side, and Knocknaveen, towards the eastern end of the 



E.I.A. PROC, VOL. XXXI. A 7 



