Vol. 57.] INTRUSIVE TUFF-LIKE JftOCKS IN IRELAND. 481 



a case o{ petitio principii to assume, even if the masses occupy the 

 throats of ancient volcanoes, that the contained fragments were 

 ejected during eruption. 



Rocks of a similar character, apj)arently volcanic, though in reality 

 hypogenic, occur at Balbriggan and Bellewstown Hill, north of 

 Dublin, intrusive into Upper as well as Lower Silurian strata. 



Rocks at Blackball Head, in Kerry, have been ascribed by Jukes 

 and others to a volcanic origin, because of their fragmental nature ^ ; 

 but these rocks cross the bedding of the associated sedimentary 

 strata of the region, are therefore intrusive and not contem- 

 poraneous, and may be of much later date than that usually assigned 

 to them. Similar remarks apply to the nature and origin of some 

 of the supposed 'ashes' of the Lough Guitane district, near Killarney, 

 and some of those occurring in the Limerick area. We do not 

 intend now to refer particularly to these, but pass on to the igneous 

 rocks of the South-east of Ireland, pausing only to mention the work 

 of other inquirers in this line of study. 



It is with much gratification and encouragement that we have 

 observed the remarks of Prof. Lapworth in this Journal, vol. Ivi 

 (1900) p. 23, when commenting upon Mr. Lamplugh's paper on 

 ' Some Effects of Earth-movement on the Carboniferous Volcanic 

 Rocks of the Isle of Man.' Considering the phenomena accompany- 

 ing great movements of the crust, Prof. Lapworth conceives it 

 possible that 



' igneous matter making its way between the moving masses may consohdate 

 as sills where the pressure is great. ... As movement progressed intermittently 

 we should have the formation of subterranean agglomerates, tuffs, and breccias, 

 which would be forced locally sometimes between bedding-planes, sometimes 

 into dyke-like fissures.' 



The manner in which Mrs. Ogilvie Gordon accounts for the 

 'agglomerates' of the Groden Pass and the Buchenstein Valley 

 approaches somewhat closely the origin to which we attribute such 

 masses. That authoress terms them ' shear-and- contact breccias ' 

 associated with felsite-veins, as distinguished from Prof. Bonney's 

 ' crush-breccias.' ^ If it had been further allowed that the insertion 

 of igneous rocks from below, accompanied by partial fusion of 

 fragments detached from the broken-up masses, played a large part 

 in the phenomena of the region, Mrs. Gordon's view of the origin 

 of the agglomerates would nearly harmonize with that which we 

 adopt to explain those that we have met with. We believe that in 

 the South-east of Ireland are to be found abundant illustrations 

 of the hypothesis suggested by Prof. Lapworth, namely, the sub- 

 terranean formation of tuff-like masses. Their intrusion, however, 

 does not always seem to have been accompanied by folding- move- 

 ment of the adjacent sedimentary rocks, even if such were sometimes 

 the case. 



^ See pp. 91, 92 of McHenry & Watts's ' Guide to the Collection of Rocks & 

 Fossils Geol. Surv. Irel.' 1895. 

 2 Quart. Journ. Geol. Soc. vol. Iv (1899) pp. 567-09, 584, etc. 



