686 ME. E. B. COWPEE EEBD ON THE [NOV. I90O, 



bands of the rock break up with crossed nicols into a blurred mosaic 

 with very few microlites and crystallites, while the darker, more 

 opaque bands are cryptocrystalline or almost isotropic and contain 

 numerous felspar-microlites and small laths arranged in lines of flow. 

 The porphyritic felspars in this rock are replaced by a finely granular 

 aggregate, and there are some large phenocrysts of augite replaced 

 more or less by chlorite and some smaller crystals of enstatite. 



The glassy type of groundmass is rare, but is found in small veins 

 near Tramore village. With crossed nicols the groundmass appears 

 cryptocrystalline or nearly isotropic, and there are generally no 

 recognizable felspathic minerals. Sheaf-like or fan-shaped groups 

 of crystallites and scattered grains and rods are abundant. 



Prom the foregoing review of the petrological characters of the 

 rocks of the small intrusive veins, it will have been apparent 

 that several types of rock are present, ranging from keratophyres to 

 trachytes, rhyolites, and andesites, with intermediate varieties. An 

 analysis of one of the trachytic group above mentioned from 

 Newtown Head, Passage, made for me by Messrs. H. 0. Jones & 

 R. Robinson in the Cambridge University Laboratory, gave the 

 following results :— Per cenfc 



SiO, 64-49 



Al.,0, 16-88 



FeA 6-16 



Cat) 2-45 



MgO 3-10 



K.,0 389 



Nt^O 219 



Tio? :::::::::::::::::::::::'::::::} traces 



Moisture -55 



99-71 



d. The Felsite-Porphyries. 



Quartz-porphyries as well as felspar-porphyries are here in- 

 cluded. These rocks are usually pale-grey or blue, but some 

 are pink or reddish. The chemical composition of that below the 

 engine-house at Knockmahon (p. 676) is given by J. Arthur Phillips, 1 

 for his short description of the rock completely tallies with the 

 characters seen in a hand-specimen : — 



Per cent. 



SiO., 72-33 



AlA 902 



Fe' o 3 634 



FeO 106 



CaO 1-92 



MgO trace 



K,0 1-46 



Na„0 583 



H 2 1-83 



99-79 



Specific gravity 2'66 



1 Phil. Mag. vol. xxxix (1870) pp. 12-13 ; Geol. Mag. 1889, p. 288. 



