Vol. 56.] IGNEOUS ROCKS OF COUNTY WATERPOED. 687 



The predominance of the soda-felspars is suggested by the above 

 analysis, and the microscopic examination of the rock-sections con- 

 firms this view. The groundmass is microcrystalline, and is crowded 

 with large perfect phenocrysts of albite and with large corroded 

 crystals of quartz. Parts of the groundmass form a line granular 

 mosaic. Some connexion between these soda-felspar por- 

 phyries and the soda-granites in Wicklow has been suggested. 



In others quartz-phenocrysts are less abundant, and the porphy- 

 ritic felspars are of various sizes, show albite-, pericline-, and Carlsbad- 

 twinning, but give an extinction-angle of only 10°. A few small 

 flakes and phenocrysts of greenish mica are also visible, and the 

 groundmass is granular to micropoikilitic (Bunmahon Bridge). In 

 other cases there are numerous felspar-microlites in the groundmass, 

 which is micropoikilitic (east of Knockmahon). In another the 

 groundmass is microcrystalline, the corroded quartz-phenocrysts 

 are large and more numerous than the felspars, which give extinc- 

 tion-angles of 10°, and the brownish-green mica is abundant and of 

 earlier formation than the quartz (Ballyvooney Cove). The ground- 

 mass in the example on Garrarus Strand (fig. 5, p. 662) is 

 cryptocrystalline, there are no quartz-phenocrysts, but the por- 

 phyritic felspars are large, numerous, and extinguish at 15° to 20°. 

 The pink felsite-porphyry near Ballydouane has a similar ground- 

 mass, but it contains large quartz-phenocrysts and also large felspars 

 with the peculiar structure of anorthoclase. 



In a variety of the felsite-porphyry of the Long Rock, Knockane 

 Strand, the groundmass is composed of pseudospherulites of various 

 sizes scattered among large clear micropoikilitic patches. In another 

 near Knockmahon (fig. 8, p. 668) the groundmass is micropoikilitic, 

 but contains numerous short laths of felspar. There are large 

 phenocrysts of plagioclase-felspar, but none of quartz. 



e. The Augite-Porphy rites. 



The augite-porphyrite of Knockmahon (fig. 8, p. 668) is macro- 

 scopically a dark-purplish or greenish rock, often having an 

 irregularly blotched appearance. The porphyritic crystals of augite 

 are conspicuous on a freshly-broken surface. There is some variation 

 in the composition of the rock and number of phenocrysts, but its 

 general characters are as follows : — The groundmass is crypto- 

 crystalline, crowded with small laths of felspar (but not truly 

 hyalopilitic) showing frequently lines of flow by a rough parallelism ; 

 abundant grains of magnetite also occur. The phenocrysts of 

 augite are large, perfect, and fairly numerous, but are partly or 

 entirely pseudomorphed by chlorite. The felspar-phenocrysts are 

 smaller, comparatively rare, and much decomposed. 



A greenish porphyrite which occurs in the form of veins 

 piercing this augite-porphyrite at Knockmahon, and shows colum- 

 nar structure in parts, has an abundant pale-brownish crypto- 

 crystalline groundmass without very numerous small felspar-laths, 

 but with a large number of small phenocrysts of felspar scattered 

 through it and a very few pseudomorphs of chlorite after augite. 



