3 48 Scientific Proceedings, Royal Dublin Society, 



trary, some of the facts put forward would seem to imply that 

 the gneiss is only a portion of the rock-formations of the district, 

 which has been more metamorphosed than the rest. 



Dr. Hull declares that he has carefully examined the boundary 

 between the supposed older and younger rocks. He is obliged 

 to have recourse to the supposition that there is not only an 

 unconformability, but even a " double hiatus" between them ; 

 but no substantial evidence for an unconformability is given. 

 He acknowledges that there are various lines of fault bounding 

 the more metamorphic rocks; and concealed faults often cause 

 a delusive appearance of unconformability. It is a common 

 occurrence, well known to those who have studied metamorphic 

 re ^ionSjthat the more altered rocks often lie against the less altered. 

 Rocks of both these classes will be found in places in West 

 Gal way, in the area now said to be occupied by Laurentian 

 rocks. Moreover, in the " Geology of Ireland" I have described 

 and mapped some interesting patches of such, more intensely 

 altered rock, near the Ovoca mines, county Wicklow. These 

 are patches of " baked rocks" ( paroptetic), but in the same 

 district, at Carrig, a few miles to the northward, may be seen 

 granitoid gneiss bounded by similar lines of breaks ; and the 

 published descriptions of the lines of boundaries of the Donegal 

 gneiss would be applicable to many fault boundaries in all 

 metamorphic regions. 



In America and Scotland basal conglomerates prove the uncon- 

 formity between the Laurentians and the newer rocks, but neither 

 in Donegal, nor with the rocks of Tyrone, N.E. of Omagh,* nor in 

 the Ox Mountains, nor in Slieve Gamph, nor in N.W. Mayo, 

 near Belmullet, have such conglo^merates been recorded, although 

 two great hiatuses are supposed to exist between the old and the 

 younger rocks. 



If these rocks, from their lithological characters, are to be 

 classed as Laurentians, why are others of similar characters, and 



* These and the overlying unaltered fossiliferons rocks were originallj' said to belong to 

 the one group : but after I had shown that this was im^^ossWAQQ'' Supposed upper Cambrian 

 rods in the counties Tyrone and Mayo.^' — Proc. Royal Irish Academy, 2nd Ser., Vol. Hi. 

 Science, p. 343), Dr. Hull has suggested, first, that they are the lowest portion of the 

 Cambro-Silurian, that is the Arenig group, and afterwards that they are Laurentians. In 

 either case it is stated that my supposition is probably wrong ; although my statement is ; 

 they ' ' are probably equivalents of part of the Arenig grou])''' of Wales, put by some geologists 

 among the Cambrians." And among these Geologists are Lyell and Ramsay. 



