360 Scientific Proceedings, Royal Dublin Society. 



minence given to lithology in modern Greology seems to me to be 

 a most important consideration in the present inquiry; as what 

 has to be proved is not that the Canadian Laurentians and 

 Huronians are different lithological groups, but that they are 

 different stratigraphical groups. The former is a plain fact ; but 

 I know of no positive proof that has been brought forward in 

 favour of the latter. 



The mistakes that may occur by substituting lithology for 

 stratigraphical and palseontological geology, can be illustrated by 

 facts observed in Ireland. For the uppermost groups of the Irish 

 Oambro-Silurians, I have suggested the name " Slate Series," as, 

 in general, they consist of a great thickness of argillaceous rocks, 

 and in them are found all the principal veins that have been 

 worked for roofing- slate ; while for the lowest group of the Cam- 

 bro-Silurians I have proposed the name "Black Shale Series," it 

 also consisting in a great measure of argillaceous rocks. In the 

 north of the county Wexford, a few miles N.E. of Carnew, in the 

 " Slate Series," a great thickness of grit comes in ; while in the 

 Erriff Yalley, county Mayo, a great thickness of nearly similar grit 

 makes its appearance in the rocks of the " Black Shale Series." 

 Now if the rocks of these two distinct areas were subjected to an 

 equal amount of metamorphic action, we should have two groups 

 stratigraphically very distinct, but lithologically similar. As in 

 both localities the change from arenaceous to argillaceous is quite 

 sudden, there would be hard boundaries in both places between the 

 metamorphosed grits and the metamorphosed slates. 



In different places in Ireland there are tracts of gneissose rocks 

 more or less similar to the Canadian Laurentians, yet evidently 

 they belong to a newer geological group. Of these, those in the 

 Carnsore district, S. E. Wexford, are probably metamorphosed 

 Cambrians ; while those of north Wexford, in the Croaghan Kin- 

 shelagh range, undoubtedly belong to the upper group (" Slate 

 Series") of the Cambro-Silurians ; those of the counties of Galway 

 and Donegal in part beioug to the Cambro-Silurian, and perhaps 

 in part to the Cambrian.^ In Erris, N.W. Mayo, they may be 



' In both these counties these gneissose and granitic rocks are evidently in part 

 younger than associated rocks which are either unquestionably or probably of Cambro- 

 Silurian age. 



