488 MESSES. J. E. KILKOE AND A. McHENEY OX [Aug. IQOI, 



We suggested, and in this Mr. Seymour agrees with us, that the 

 fracturing occurred in connection with continued intrusion after 

 portions of the invading magma had solidified in smaller veins. The 

 occurrence of lapilli of pumice in these intrusive rocks we conceive 

 may be accounted for in a somewhat similar manner, namely, the 

 sudden opening out of fissures and subterranean chasms before the 

 invading masses, v;'hich would admit of the development of a vesi- 

 cular structure in the rapidly-cooling magma, and the subsequent 

 fracturing and mincing-up of the newly-formed rock. 



The cleavage of these masses and of the felsites in certain places 

 — subsequently to the folding of the strata which occurred prior 

 to the invasion by the igneous masses — has led to the mistaken 

 conception of the true origin of the intrusive rocks. Planes of 

 cleavage which sometimes accord with the bedding-planes, though 

 most frequently crossing the bedding, induced Jukes and Du jSToyer, 

 who examined the ground, to suppose that even the cleaved felsites 

 were volcanic ashes. Sir Archibald Geikie and Dr. Hatch, on visiting 

 the ground some years ago, perceived that this was a superimposed 

 structure.^ Mr. Kinahan seems to have observed the true disposition 

 of the fragmental tuif-like rocks, as he mentions instances of their 

 intrusive character in the Explanatory Memoir (1882) accom- 

 panying Sheets 158 & 159 of the Geological Survey Map of 

 Ireland, p. 16, but did not follow up the inquiry. The importance 

 of the subject in igneous geology will be readily admitted. That 

 there are contemporaneous igneous rocks in the South of Ireland 

 we are well aware, though the evidence for their local occurrence 

 may need reinvestigation on the lines above indicated. Igneous 

 action began in the district after the limestone of Bala age was 

 formed, and was continued with intermissions up to the epoch at 

 which Upper Old Red began to be formed, chiefly during Old Red 

 Sandstone times, the period probably in which the Leinster granite 

 and the associated felsites were intruded. To this period we venture 

 to assign most of the tuff-like rocks. 



Indeed, we are strongly inclined to believe that these intrusive 

 breccias in most cases represent the marginal phenomena of the 

 granitic eruption, since we find outlying intrusions of the granite 

 passing into the felsitic laccolites which are directly associated with 

 the intrusive fragmental rocks. This passage is to be seen at many 

 points in the South-east of Ireland, It is most apparent to the 

 south of Vinegar Hill, near Enniscorthy (Sheet 158), where the 

 fragmental felsite graduates into an elvan, and from that into a 

 granite ; also farther south-west, about 10 miles from Enniscorthy ; 

 and again still farther south-west, in the laccolite-hill of Carrickburn 

 (Sheet 169), where the granitic central core passes outward into a 

 felsitic rock showing flow- and spherulitic structures, and which is, 

 moreover, very considerably brecciated and fragmental on its outer 

 margin, the rock-mass under these latter conditions being hitherto 

 always regarded as ' tuff.' The passage from granite into felsite 

 is also well shown near Mount Druid, in County Waterford. 



^ ' Summary of Progress of Geol. Surv. U. Iv.' for 1898, p. 59. 



