KiNAHAN — On Canadian Archcean, and Pre- Cambrian JRocks. 361 



metamorphosed Cambrians ; in east Mayo, Sligo, and Tyrone, they 

 are, probably, Cambro-Silurians or Upper Cambrians ; that is, the 

 representation of the Arenig rocks of Wales ; while in the county 

 Antrim, in the Ballycastle district, they may be either Cambro- 

 Silurian or Cambrian age. Lithologically, these are all similar, 

 but in reality they are of very different ages. 



It should be pointed out that, as a general rule, hard, well- 

 defined boundaries are characteristic of excessively altered rocks ; 

 while in unaltered rocks hard boundaries are the exception, one 

 group usually graduating into another by alternations of the dif- 

 ferent kinds of rocks; thus limestones in general graduate into 

 shales, and the latter into grits ; while grits will first alternate with 

 shales before the latter entirely predominate. The subject of hard 

 boundaries to metamorphosed rocks I have already discussed in 

 Chapter X. of the "Greology of Ireland," where it is shown that, in 

 general, small metamorphosed tracts in large unaltered areas have 

 hard, well-marked boundaries ; while here I would point out that, 

 in large areas of granitoid rocks, such as those of Gralway and 

 Donegal, there are proofs that the action which changed the 

 rocks into their present condition invaded them long after their 

 accumulation ; as prior to their metamorphism they were up- 

 turned, contorted, displaced by faults, while the consequent 

 ruptures and breaks are now more or less sealed up. The 

 breaks in the less altered rocks are much more numerous than 

 in the granitoid rocks, those that are conspicuous in the latter 

 being common to both. 



Many of the facts above mentioned respecting the Irish rocks 

 may not bear directly on the Canadian Archaean rooks; but 

 they do so indirectly, as they show that too much reliance must' 

 not be placed on mere lithological characters. Furthermore, al- 

 though in the Canadian Archseans there are many rocks of which 

 there are no equivalents in Ireland, yet in the latter country 

 there are also many that are. It has already been shown that 

 there is a great similarity between the gneiss and the calcareous 

 rocks of the two countries, and this is even greater if we study 

 the metamorphosed basic- eruptine rocks. In the section between 

 Port Arthur and Winnipeg there is scarcely any rock of this class 

 an equivalent of which could not be found in some one or other 



SUIliNo PllOC. li.D.S.—VOL. IV. PT. Vll, 2 I 



