Vol. 56.] IGNEOUS KOCKS OF COUNTY WATEEPORD. 683 



and microlites of felspar, with a few quartz-grains and some large 

 porphyritic plagioclases which have extinction-angles of 20° to 22°. 

 The groundmass may be partly macrocrystalline (nodular felsite, 

 Ballydouane West Bay) or wholly so, or cryptocrystalline (plug of 

 felsite between Knockane and Morageeha Strands, p. 666), or the 

 two kinds may be combined. The felspar-laths may be arranged 

 subparallel in lines of flow (western end of Morageeha Strand), or 

 this structure may be indistinct or absent. By an increase in the 

 number of felspar-laths and a diminution of the crypto- or micro- 

 crystalline groundmass, this type is linked to the trachytic and 

 bostonitic. 



In the trachytic type (Type H) the groundmass consists 

 principally of closely-packed, short, stout laths of felspar, mostly 

 untwinned, with interspersed, small, clear, micropoikilitic or grano- 

 phyric patches (Cooneenacartan Cove). The laths are arranged 

 here and there in lines of flow and give almost straight extinction, 

 but there are a few large plagioclase-phenocrysts with extinction- 

 angles of 12° to 16°, and a few grains of quartz. In one rock from 

 this locality the porphyritic felspars have much finer and more 

 frequent albite-twinning and give extinction-angles of 20° to 25°. 

 In others from here and Bunmahon more or less abundant inter- 

 stitial greenish material occurs, which is isotropic or nearly so 

 with crossed nicols. Needles of apatite are commonly present, and 

 some grains of epidote. A variety from Knockmahon and Bun- 

 mahon with long felspar-laths is worthy of notice. 



All the examples of the two foregoing types are characterized 

 by more or less abundant patches of micropoikilitic or granophyric 

 structure, but they pass imperceptibly into rocks (Type I) in which 

 this is almost or entirely absent, and which closely resemble 

 bostonites. The greenish interstitial material is much reduced 

 in quantity, and the rock practically consists of small felspar-laths, 

 with occasionally a few grains of magnetite and quartz, and some 

 porphyritic felspars (Ballydouane West Bay, Cooneenacartan, etc.). 

 Plow-structure is generally noticeable among the felspars. In 

 others the felspars are of all sizes, ranging from microlites to large 

 phenocrysts with Carlsbad- and albite-twinning, and set in a pale- 

 greenish matrix. In these no definite flow-structure is observable 

 (Foilnaneena and Bunmahon Bay). 



The Wicklow keratophyres l resemble some of the rocks of Type G ; 

 and it has been noticed that the chemical composition of some 

 keratophyres and bostonites is very similar. 2 The trachytes described 

 by Dr. Hatch 3 from the Lower Carboniferous of Haddingtonshire 



1 Hatch, O-eol. Mag. 1889, pp. 70 & 547. 



2 Kemp & Masters, Trans. N. Y. Acad. Sc. vol. xi (1891) p. 13; J. H. 

 Sears, Bull. Mus. Comp. Zool. vol. xvi (1890) p. 167 ; Bosenbusch, Tscherm. 

 Min. u. Petr. Mitth. vol. xi (1890) p. 144 ; and J. S.Biller, Bull. Mus. Comp. 

 Zool. vol. vii (1881) p. 165. 



3 Trans. Boy. Soc. Edinb. vol. xxxvii (1892) p. 119. 



