90 EOGK-METAMORPHISM. 



Many of the bedded crystalline calcitic rocks of Connemara 

 appear to be simply mineralized metamorphics. There are 

 calcareo-siliceous bands, more or less crystalline, commencing 

 at Lough Bonn, a little north of Oughterard, and skirting, 

 apparently continuously, the mail-road on to Ballinahinch, 

 thence across the mountains to Letterfrac*. There are good 

 reasons for believing that they were also originally impure 

 limestones. Still we entertain a suspicion that the " crystalline 

 limestones " which have been noticed, by Mr. R. Glascott Symes, 

 occurring in what appear to be corresponding metamorphic 

 schists in Mayo, will turn out to be methylosed hernithrenes. 

 They are ' ' seldom following the foliation of the beds, but occur 

 along lines of great fault." " From such evidence," according to 

 Mr. Symes, "it may be assumed that this crystallized limestone 

 has been formed by infiltration or percolation of bicarbonate of 

 lime, from the once overlying Carboniferous rocks, into joints or 

 cracks in the now metamorphic series." Prof. Hull, however, 

 expresses his aversion to this view, and considers these limestones 

 " to belong to the group of strata in which they are found, just 

 as similar limestones do in the West Galway district "f. 



It is highly probable that the pure marbles (carrarite) of the 

 Apuan Alps are simply mineralized. But it rarely happens that 

 the Irish cases to which we have referred are unassociated with 

 methylosed rocks that is, beds chemically changed into dolo- 

 mites, ophites, and even into true hemithrenes. Moreover the 

 limestones in the north-east of Donegal are undoubtedly no 

 more than mineralized, and probably of Lower Silurian age ; 

 while the calcitic rocks occurring further west, filled with ido- 

 crase and other mineral silicates, and interbedded or intimately 

 associated with granites &c., may be of methylotic origin and 

 represent a much earlier geological period. 



* We are not sufficiently acquainted with the stratigraphy of the calca- 

 reous marbles and ophites of the Barna-Oran district, east of Ballinahinch, to 

 .offer a decided opinion as to whether the former are methylosed or mine- 

 ralized though, from their being associated with the latter, the probability 

 seems strong to us that they were originally silacid rocks. 



t Explan, Mem. of Sheets 41, 53, and 64, Geological Survey of Ireland, 

 p. 12. Mr. Symes mentions other cases of the kind occurring in Mayo beds 

 of micaceous limestone in mica-schist, in some places traversing the latter 

 at right angles, at others following the direction of its folia. See Expl. Mem, 

 of Sheet 75, p. 12, and Expl, Mem. Sheets 63 and 74, p. 11, 



