Vol. 56.] IGNEOUS ROCKS OP COUNTY WATERFORD. 689 



The rock forming the headland west of Stradbally (p. 670), and 

 termed a felsite in the Geological Purvey Memoir, is of somewhat 

 doubtful nature. Under the microscope, it is seen to consist mainly 

 of felspar-laths, mostly untwinned, with larger squarer felspar- 

 crystals interspersed having extinction-angles of 15° to 16°. Irre- 

 gular small patches of a pale chloritic mineral frequently mould 

 the felspars. A few large micropoikilitic patches of quartz are 

 noticeable, and grains of brightly-polarizing epidote are plentiful. 



A rock in Bunmahon Bay, consisting of large numbers of plagio- 

 clase-crystals of various sizes embedded in a micro- to cryptocrystal- 

 line groundmass containing felspar-microlites and pale-greenish 

 grains, and also porphyritic pseudomorphs of the same pale-greenish 

 isotropic material after some pyroxene (?), is a peculiar type. The 

 felspars show irregular al bite- twinning and low extinction-angles 

 of 6° to 8°. Magnetite-grains and small crystals are also present. 

 Probably the rock should be referred to the porphyrites. 



The dark-greenish compact rocks penetrating the augite-por- 

 phyrite at Knockmahon and in the adjoining coves are of a 

 peculiar type. A pale-greenish uniform groundmass, closely set 

 with small felspar-laths, is almost isotropic with crossed nicols, 

 and magnetite and secondary quartz are present. Similar rocks, 

 but with occasional phenocrysts of plagioclase, form conspicuous 

 flows in Cooneenacartan Cove, and in these much of the ground- 

 mass is found to be of fibrous hornblende giving extinction-angles 

 of about 20°. 



The rock (p. 678) forming the greater part of the western side 

 of Bunmahon Bay is of a peculiar and somewhat enigmatical 

 character. It is greenish in a hand-specimen, with the porphyritic 

 felspars distinctly visible. Under the microscope it is seen to be 

 composed of a brownish matrix, isotropic to cryptocrystalline under 

 crossed nicols, and filled with numerous short felspar-crystals, mostly 

 rectangular or square in section, showing albite-, Carlsbad-, and 

 pericline-twinning or none at all, and many of them have rounded 

 angles. The extinction-angles are mostly 10° to 12°, but some are 

 18° to 20°. In addition there are large irregular isolated patches 

 of pale-green chlorite, but some are of regular crystalline shape and 

 may be pseudomorphous after some pyroxene. Epidote is frequently 

 abundant. The iron-ore is scanty, and is represented by minute 

 grains. Patches of the groundmass are devoid of phenocrysts. The 

 composition of this rock, according to an analysis made for me in 

 the Cambridge University Laboratory, is as follows : — ■ 



Per cent. 



Si0 2 5946 



A1 2 3 18-17 



Fe 2 3 6-52 



CaO 1-95 



MgO 6-85 



K 2 5-31 



Na 2 -59 



Moisture -62 



99-47 



Q. J. G. S. No. 224. 3 a 



