162 G. H. Kinahan — Irish and Canadian Hocks, Compared. 



the gneissic granite generally graduates into gneiss, yet in some 

 places, there is a hard bovnidary between them. 



If the rocks, however, are followed westward along the strike of 

 the rocks, the change is not so gradual, as there are rather sudden 

 jumps from granitic to schistose rocks ; while in the latter are 

 found outliers, either having a nucleus of porphyritic oligoclasic 

 granite, or being entirely gneissic granite. These usually are 

 suddenly replaced by schists, as if they were small tracts which had 

 been subjected to an extra action, prior to the time of the regional 

 metamorphism. 



But if we go southward from the mass of the porphyritic oligo- 

 clasic granite, there is a con)plete change in the character of the. 

 metamorphism ; as here in places margining it are gneissic granites 

 often orthoclasic and fine-grained, but having associated, and alternat- 

 ing with them, coarse-grained and oligoclasic beds. While to the 

 S.E., at Galway town, there is a complete change of rocks, 

 principally hornblendites, that come in against the granite, and in a 

 few small patches on it. This small area of schist apparently in 

 such close proximity to the mass of the oligoclasic porphyritic 

 granite seems peculiar. This, however, might perhaps be explained 

 by phenomena that can be studied to the westward near Roundstone 

 and Slyne Head ; but as it would take some time, and the rocks are 

 not of interest in the present inquiry, it appears sufficient to have 

 recorded their presence. 



In the Co. Galway the oligoclasic porphyritic gneissic granite, 

 especially across the strike of the rocks, is a portion of the general 

 graduation from the oligoclasic porphyritic granite into the schists 

 and unaltered rocks : but in the Castlebar district, Co. Maj'o, there 

 are courses and masses of it that evidently were intruded into their 

 present positions. These Mayo rocks are very interesting, because 

 among the Laurentians of Canada we find somewhat similar gneissic 

 granite as intruded masses (called Labradorians and Norians by the 

 Canadian geologists and marked La on Selwyn's map). 



From the published writing in connection with the oligoclasic 

 granite and associated rocks of the Co. Donegal, it is evident that 

 the sequences are somewhat similar to those in the Counties Galway 

 and Wexford ; there being granite that in places graduates through 

 gneiss into schist; while in other places, as at the south of the 

 Galway, porphyritic granite, different groups of gneissic rocks, comes 

 in. At the same time, however, as a general rule, the margin of 

 the granitoid rocks is here more marked than in either Galway or 

 Wexford. 



It is unnecessary'^ to further individualize the Irish localities ; but 

 to certain traits of metamorphism I would draw attention. Years 

 ago I pointed out^ that in the West Galway and Mayo districts, rocks 

 in the " Second Stage " ^ of metamorphism followed the planes of 

 the most conspicuous structures in the original rocks ; let this struc- 

 ture be lamination, cleavage, fine jointing, oblique lamination, 

 spheroidal structure, conci'etionary structure, or any of the wavy 

 Geol. Mag. 1871, Vol. VIII. pp. 263-268. ^ Geology of Ireland, chapter x. 



