158 Proceeding^i of the lloyal Irish Academy. 



which forms the important mass of Leckanvy Hill, rising to 1849 feet. 

 Judging from the form of the ground, a dislocation along the direction 

 of strike has probably occurred. The conditions accountable for the 

 structure of the range would seem to point to an upthrust from the 

 north along this line ; but other considerations rather favour the view 

 of direct faulting with downthrow on that side. In any case there 

 seems to be a repetition of the peak quartzite in Leckanvy Hill, 

 otherwise there would be an excessive thickness of this rock inter- 

 vening between the sericite schist of the south flank here and the 

 conglomerate on the north side, compared to the narrow outcrop and 

 small thickness of the same member which separates the sericite schist 

 and conglomerate, only two miles to the west in Kilgeever Hill. As 

 before explained, the sericite schist along the south flank of the range 

 is identical in character with that at Knockfadda ; in botli places it is 

 highly sheared, is calcareous, and shows pittings on the weathered 

 surfaces of its thinly laminated, often puckered, and in places mica- 

 cised folia ; and its relations to the quartzite, below it at Knockfadda, 

 though above it by inversion at Croagh Patrick, show conclusively 

 that we are dealing with the same member of the Wenlock group. In 

 Croagh Patrick on the north, and in Knockfadda, Boheh, and Creggan- 

 baun on the south, we therefore have opposite limbs of a synclinal 

 trough, filled in the intermediate ground with argillaceous, green, 

 fine-grained grit of remarkable uniformity in character, which hitherto 

 has not yielded any fossils. The accompanying section across the range 

 through the fossil locality may be taken to represent the structure of 

 the range.^ 



Fig. 13. — Section, from Clew Bay southward, across Leckanvy Hill (1500;, 

 Knockfadda (957), and Knockaskebeen (1288), showing Croagh 

 Patrick (2510) to the east; Drift upon Carboniferous limestone (</), 

 and metamorphosed grits {g) — the latter thrust over mica-schist and 

 quartzose conglomerate {b) ; also quartzite {q), pebble-beds {p), and 

 fossil-bearing calcareous arenaceous mudstone (c) faulted down against 

 quartzite {(/) ; metamorphosed calcareous beds [c'), green grits {W.g), 

 quartzose conglomerate {b'), greenish gray grits {L.g), and granite [G). 



^ If, as there is good reason to think, the rocks at Old Head be Old Eed Sandstone, 



