116 G. H. Kinahan — Classification of Rocks. 



canos. Even tlie circles of igneous rocks and their accompanying 

 agglomerates and tuffs that surround the Coal-measures of Bally - 

 broad, co. Limerick, possibly may mark the site of an ancient 

 volcano, as they do not exceed in dimensions some of the craters 

 recently described in the Geol. Maq. by Mr. Judd. But in the 

 Carboniferous slates of the co. Cork, and the Devonian rocks near 

 Killarney, as pointed out years ago by my colleagues Gr. Y. du 

 Noyer, M.K.I.A., J. O'Kelly, M.E.I.A., and myself, there were fis- 

 sure-irruptions ; although in one place, Blackballhead, there are 

 the remains of an ancient volcano. — In the rocks of Cambro-Silurian 

 and Silurian age, in the counties of Mayo and Galway, also in 

 those of Cambro-Silurian age in the cos. Wicklov^r, Wexford, and 

 Waterford, there are vast sheets of igneous rocks, quite different 

 from any now due to volcanos. It is therefore evident that most 

 of the Irish igneous rocks, even those of Tertiary age, were 

 irrupted under different circumstances from those accompanying 

 the eruption of lavas ; and although a better classification of the 

 rocks than that at present in use (Volcanic and Plutonic) might be 

 adopted, yet it would be incorrect to call them all volcanic. 



In some place or another there are igneous rocks of the same 

 respective ages as the different geological groups, and each of these 

 groups of igneous rocks must have had their granites, although in 

 many cases they may not be now exposed by denudation. But it 

 seems extremely questionable that " the old felstones and porphyries 

 were originally identical with the more recent trachytes." The old 

 felstones and porphyries, as shown by Jukes, are in most cases the 

 roots or deep seated accompaniments of vulcanicity ; and if volcanos 

 existed in connexion, they were subsequently removed and the 

 material scattered by denudation; while the more recent trachytes 

 are normal volcanic products. 



Mr. Judd, in his recent paper on the igneous rocks of the Hebrides 

 (quoted by Mr. Allport), shows (similarly as was proved in other 

 localities twenty years ago by Haughton) that the surface or volcanic 

 siliceous rocks, in depth, pass into granite ; he also mentions, that 

 the dolerites, in depth, pass into coarsely crystalline gabbros. This 

 leads us to the consideration of the passage of basic igneous rocks 

 into normal granite. 



In Yarconnaught or West Galway there are igneous rocks (gabbro 

 or diabase) that, as pointed out in the Memoirs of the Irish branch 

 of the Geological Survey, and elsewhere, when affected by meta- 

 morphism, are changed into the rock called by Macculloch hornblende 

 rock ; diorite (ampliibole + triclinic felspar) ; syenite (amphibole -j- 

 ortboclase) ; and rocks that, in addition to amphibole, may contain 

 both orthoclase and triclinic felspar. These rocks, as the meta- 

 morphism becomes more intense, become granitic, till eventually 

 they change into hornblendic granite. Similarly with the non-meta- 

 morphic rocks, Judd points out that the dolerites of the Hebrides, in 

 depth, pass into coarsely crystalline gabbros ; for the latter rocks 

 others have proposed the name of granitone ; as the rock, although 

 granitoid, was not considered a normal granite. If, however, we 



