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Proceedings of the Royal Irkh Academt/. 



details by means of which such a line could be established or disproved 

 south of Murrisk. 



6. While recognizing that the existence of the Croagh Patrick 

 conglomerate, considerably more important here than at Cregganbaun, 

 and containing larger blocks, suggests littoral or approximately basal 

 conditions, just as the corresponding deposit does at Lough Fee, near 

 Kylemore, yet the strike of the Wenlock beds at Leckanvy and certain 

 other points in the region, suggests that during the overfolding, best 

 seen in the newer rocks, a slip or overthrust took place, generally 

 speaking, where the two sets of rocks adjoin. Such a break, 

 whether stratigraphical or mechanical, would greatly obscure the 

 relations between the Dalradian and the Silurian rocks, as well as 

 the evidence for the age of the former in this region. 



7. What rocks formed that ancient sea-floor, and the adjoining 

 land which furnished the quartzite conglomerate, is, perhaps, one of 

 the most debated points in the geology of the British Isles. The 

 discovery that the Wenlock beds were deposited upon upturned 

 Llandeilo strata at Lough ^^'afooey, in virtue of an interruption in the 

 Silurian succession, of Llandovery date, leaves inquirers still free to 

 surmise the existence of Lower Silurian strata in the great meta- 

 morphic series of I^orth Mayo, Galway, and Donegal. The corre- 

 spondence in character between the Westport grits and black slate and 

 the rocks along the Leckanvy shore, in which the collector may indulge 

 the most sanguine hopes of finding graptolites, and those of the 

 northern part of Clare Island, which, on the published map, are 

 represented as unaltered Lower Silurian strata, may well be supposed 

 corroborative of the surmise. It is further encouraged by the 

 succession from black slate, through limestone and a boulder deposit, 

 to quartzite, as seen, say, at Achill and Donegal, which well corresponds 

 to the Lower Silurian succession from Llandeilo through Eala to 

 overlying conglomerate, or boulder-beds and grits, seen in the south- 

 east of Ireland. Even the apparent inversion of Llandeilo rocks at 

 Rossroe, which would be indicative of a region of overfolding and 

 overthrust, saving the rocks above from the metamorphism which 

 affected so strongly those below the thrust-plane — possibly of corre- 

 sponding age — might also be supposed to lend colour to the supposition. 

 What is looked for, however, is definite fossil evidence; and until this 

 is forthcoming the age of the great metamorphic series cannot be 

 regarded as a settled question. 



