156 Proceedings of the Roi/al Irish Academy. 



the two members — really between the Upper and Lower Silurian, 

 here quite conformable. The stratification in the region to the 

 eastward is so regular and well shown that an approximate conti- 

 nuation of the boundary was drawn as represented on the present 

 map. 



Age and Structure of Croagh Patrich. 



Perhaps the most interesting result of revision- work in the West 

 of Ireland was the establishment of the age of Croagh. Patrick range 

 to be "Wenlock, though consisting of rocks which present a high 

 degree of metamorphism, so much so that they were originally 

 classed with the quartzite and associated schists of Nephin and 

 Connemara. Generally speaking, the crest of the ridge, including the 

 conical peak, and a large proportion of the northern slope, consists of 

 quartzite ; while the south flank shows crumpled schists dipping 

 northward into the hill and beneath the quartzite. These schists are 

 for the most part sericitic, but near the foot of the escarpment are 

 distinctly micaceous, the result of processes in which dynamical 

 agency operated strongly. It has already been mentioned that 

 evidences of contact metamorphism are apparent in the schists 

 bordering the granite intrusion of Corvockbrack ; and it is interest- 

 ing to find illustrations of two types of metamorphism in such close 

 proximity.^ With the exception of these cases, mineralization is not 

 manifest throughout the Silurian area, though cleavage prevails in 

 the region northward of Killary Harbour and the Erri:ff valley.- 



The intense dynamical metamorphism manifest along the south 

 flank and foot of Croagh Patrick has accompanied overfolding on a 

 vast scale, which has supplied the key to the structure of the range. 

 It is also, indeed, traversed by oblique cross-faults, wliich, however, 

 are of comparatively minor importance. The conclusions as to over- 

 folding, and as to its post-Wenlock age, find confirmation in the 

 sharply-folded condition of the unaltered Wenlock rocks, containing 

 recognizable fossils south of Lough Nafooey, represented in the 

 sketch map on p. 152. We are not, however, dependent upon inference 

 for conclusions regarding Croagh Patrick; for, happily, the overfolding 

 of the rocks which form the range leaves their original order of 

 sequence intact, though inverted, as I now proceed to show. 



Along the north flank of the range for 7 miles, from Kilgeever 



1 As mentioned in the Summary of Progress of "Work in Ireland for 1893. 

 - The micaceous schist above mentioned may conceal a not deeply buried core 

 of granite. 



