168 G. H. Kinahan — Irish and Canadian Rocks, Compared. 



Mayo, tbey may be metamorphosed Cambrians ; in east Mayo, Sligo, 

 and Tyrone, they are possibly Cambro-Silnrians or Upper Cambrians, 

 that is, the representatives of the Arenig rocks of Wales ; while in 

 the CO. Antrim, in the Ballycastle district, they are either Cambro- 

 Silnrians or Cambrians. All these lithologically belong to one group, 

 while petrologically they represent different groups, belonging to 

 different geological ages. 



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

 boundaries are characteristic of excessively altered rocks ; while 

 partly or unaltered rocks usually graduate into one another by alter- 

 nations of intermediate kinds of rocks ; thus limestones in general 

 graduate into argillaceous rocks, and the latter into arenaceous 

 rocks ; while a mass of arenaceous rocks will usually at its margin 

 first alternate with argillaceous rocks, before the latter predominate. 



The subject of hard boundaries to metamorphosed rocks has been 

 discussed in Chap. x. of the Geology of Ireland, where it is illus- 

 trated that, in general, small altered tracts in large unaltered areas 

 have well-marked boundaries ; while here I would point out that in 

 large areas of granitoid rocks, like those of Galway and Donegal, there 

 are facts which indicate that the metamorphism which invaded the 

 rocks was long subsequent to their original accumulation; as prior 

 to their metamorphism they were upturned, contorted, displaced by 

 faults and denuded, while subsequently such ruptures and breaks 

 were sealed up by the metamorphic action ; as proved by the breaks 

 in the less altered rocks being much more numerous than in the 

 granitoid rocks, while all in the latter are also found in the former. 



Many of the phenomena mentioned as occurring in the Irish rocks 

 may not directly apply to the Canadian, but indirectly they do ; as 

 they go to prove that too much reliance must not be placed on 

 merely lithological characters. Furthermore, although there are 

 many varieties of the Canadian Archsean rocks for which there are 

 no equivalents in Ireland, yet there are also many that have. 

 Already the great similarity between the gneiss of the two countries 

 and of the calcareous rocks has been pointed out; while there is 

 even a greater similarity between the metamorphosed basic eruptive 

 rocks, if one set is compared with the other. It was remarkable in 

 the section between Port Arthur and Winnipeg, also in Ottawa 

 County, and other places in the Province of Quebec, the few rocks 

 I saw with which I was not previously acquainted. 



To return to the boundaries of the Ontario Laui'entians and 

 Huronians. All those I saw strongly suggested that they are more 

 ^ lithological than petrological, as in no place does there appear to be 

 a regular or well-marked unconformability or even an overlap of the 

 Huronians on the Laurentians ; while at each side of the boundary 

 there are striking similarities in the direction and dip of the beds in 

 both classes of rocks ; also all breaks, faults, and such like in the 

 supposed older rocks appear also to occur in the supposed newer 

 rocks. There not even being such marked differences in character 

 between these Laurentians and Huronians, as are common in the 

 Irish Cambro-Silurians, between those altered into schists and those 

 altered into granitoid rocks. 



