516 Scientific Proceedings, Royal Dublin Society. 



by me Cambrians in the Co. Tyrone were possibly Laurentians ; 

 and this was followed by Dr. Callaway, who, in 1881, stated that 

 patches in my Cambrians, Co. Wexford, were Laurentians. Sub- 

 sequently came Dr. Hull, who seems to consider that all the tracts 

 of highly metamorphic rocks, except those in Wexford previously 

 claimed by Dr. Callaway, are of Archaean age (" Laurentian 

 Bocks in Donegal and Elsewhere in Ireland," Trans. Roy. Dub. 

 Soc, vol. i., ser. ii., p. 245). It seems remarkable that, while the 

 rocks of the Mullet, in North-west Mayo, are included in this 

 Paper, those of South-east Wexford should be left out, more espe- 

 cially as the rocks in both localities are lithologically, microscopi- 

 cally, and apparently stratigraphically similar, if not identical. 



The Wexford rocks claimed by Dr. Callaway to be Laurentians 

 are, as he has described them, " a mosaic of irregular fragments" (!) 

 protruding into a tract of undoubted Cambrian rocks, as proved 

 by their fossils. Nowhere else in the world have the Laurentian 

 rocks appeared after this fashion, and I do not believe in their 

 existence in the Co. Wexford, as the so-called Laurentians are 

 only metamorphic intrudes of Igneous rocks and their associated 

 tuffs, similar to the intrudes found elsewhere in every group of 

 Irish strata, from the Carboniferous down to the Cambrian. 



[From Dr. Callaway's Paper, " Metamorphic and Associated Eocks South of "Wex- 

 ford" (Geol. Mag., Nov., 1881), it is evident that the writer had my memoir, but, at 

 the same time, that the maps he was consulting were those published some quarter of 

 a century prior to my being in the county or my examination of the rocks. How 

 anyone could possibly imagine that my description was that of the obsolete maps is 

 hard to conceive; more especially as on these maps are printed the dates of their 

 publication and the, names of the Surveyors.] 



The Galway metamorphosed rocks that are said to be Lauren- 

 tians are undoubtedly the equivalents of the English Ordovicians, 

 as proved by the fossils in the unaltered portions. This will be 

 more fully discussed when treating of the rocks of that county, 

 while the supposed Laurentian gneiss of the Co. Donegal ought 

 now to be disposed of, if the markings exhibited by Dr. Hull at 

 the British Association Meeting, 1886 {Birmingham), are Arenig 

 types of graptolites, which there now seems to be every reason for 

 supposing; for if this be so, it unquestionably proves that the 

 gneiss of Donegal, which is part of the same series, cannot possibly 

 be more ancient than Cambrian : that is, these rocks must be the 



