G. H. Kinahan — Irish and Canadian Hocks, Compared. 167 



This confusion of lithological and petrological groups in the 

 geology of the present time appears to me to be a most important 

 consideration in the present inquiry, because, if such a classification 

 is allowed, the Canadian groups have as much right to be called 

 '•' geological divisions " as many, so called, in England or rather in 

 Europe. 



The mistakes that may occur by substituting lithology for petrology 

 I may attempt to illustrate by facts observed in Ireland. For the 

 uppermost group of the Irish Cambi'o-Silurian the term " Slate 

 series " has been suggested, as in general it consists of argillaceous 

 accumulations, also as in it all the principal veins of roofing 

 slates are found ; while for the lowest group of the same formation 

 the term " Black Shale series " has been proposed, the rocks in it 

 being also for the most part argillaceous, but at the same time car- 

 bonaceous and of a blackish colour. In both groups, however, in 

 places, thick masses of ai'enaceous rocks may, often suddenly, cut 

 out the argillaceous rocks. This is the case in the western extension 

 of the Croghan-Kinshella hills to the N.E. of Carnew, Wexford, 

 where in the " Slate series " a considerable thickness of grit suddenly 

 appears in the slates ; while in the Erriff Valley, co. Mayo, a mass of 

 lithologically identical grits appear in the " Black Shale series " 

 Now, if the rocks in these two far-apart districts were subjected to 

 exactly the same degree of metamorphism, and under exactly the 

 same conditions, we should have two groups lithologically similar, 

 but petrologically or geologically very different ; also in both 

 localities the change from one group to another would be marked, as 

 in both these places the argillaceous rocks are suddenly replaced by 

 arenaceous ; therefore, if metamorphosed, in both cases there should 

 be hard boundaries between the metamorphosed grit and meta- 

 morphosed slates, — one probably being gneiss and the other schist. 



In different places in Ireland there are tracts of gneissic rocks 

 more or less similar to the Canadian Laurentian, yet evidently they 

 belong to distinct geological groups. Of these, those in S.E. Wex- 

 ford, Carnsore district, are probably of Cambrian age, while those of 

 north Wexford in the Croghan-Kinshella range undoubtedly belong 

 to the Upper group ("Slate series") of the Carabro-Silurian ; while 

 those of the counties of Galway and Donegal may in part belong to 

 the Cambro-Silurian and in part to the Cambrian;^ in Erris, N.W. 



this however does not appear to be proved. The different ilora and fauna were 

 developed and flourished under such conditions as were most favourable ; and conse- 

 quently each different group is most conspicuous, in more or less similar groups of 

 strata ; and for this reason such strata are said to be of one age. This often is , 

 hard to disprove, on account of not being able to trace the rocks from one place to 

 another ; but when we are able to do so, as is the case with the Carboniferous rocks 

 of Ireland, it is found that the so-called divisions are only lithological. This can 

 also be seen in the Irish Devono-vSilurian ; take, for instance, the Culfin section, 

 CO. Galway, where the lowest rocks contain Llandovery and Weulock fossils, while 

 over them are strata containing Caradoc fossils, which are succeeded by rocks having 

 typical Wenlock and Ludlow fossils, while in the group above them, according to 

 Davidson, the prominent and nearly only fossil is of a Llandovery typo. 



1 In both these counties these gneissose and granitic rocks are evidently, at least 

 in part, younger than the associated rocks, which have been proved to bo, or are 

 probably, of (Jambro- Silurian age. 



