680 MR. F. E. COWPER REED ON THE [NOV. I9OO, 



talline. Generally rocks of this kind, which comprise most of 

 the bedded grey felsites near Great Newtown Head (p. 660), contain 

 large porphyrinic felspars, mostly plagioclase, with albite-twinning 

 or microperthitic structure and extinction-angles of 13° to 15°. A 

 few phenocrysts of orthoclase also occur, but none of quartz. The 

 phenocrysts have rounded angles, and are frequently corroded or 

 broken. The associated tuffs from this locality in some cases contain 

 fragments of microlithic felsites and of pumice and large broken 

 quartz-crystals, in addition to pieces of the microcrystalline felsites. 



Some of the felsites of this series and with this type of ground- 

 mass are practically devoid of phenocrysts, while others contain 

 phenocrysts of quartz, and the porphyritic felspars exhibit mostly 

 very fine albite-twinning with extinction-angles of 6° to 8°. 

 Xenoliths of slate, etc. are common in these felsites, and flow- 

 structure is frequently observable. 



In other parts of the area felsites with a microcrystalline ground- 

 mass are common : some have only phenocrysts of quartz (Morageeha 

 Strand, p. 666) ; others contain practically no porphyritic elements 

 (Black Door) or only a very few small ones ; and in a dark-green 

 felsite these are replaced by a greenish isotropic material, as are 

 also portions of the groundmass (Bunmahon Bay). 



Some felsites have parts of their microcrystalline groundmass 

 granular, or wholly composed of a very fine granular aggregate (Black 

 Door). A groundmass composed principally of grains of quartz and 

 felspar of irregular size, but with parts microcrystalline and with 

 a few plagioclase-phenocrysts, is sometimes found (pink felsite-dyke, 

 Bunmahon Head, p, 669) ; and near Kilfarrasy Island the banded 

 intrusive felsite has a clear and regular, finely granular, groundmass 

 containing a few large fresh phenocrysts of orthoclase and plagio- 

 clase, with extinction-angles of 14° to 18°, and vesicles filled with 

 secondary quartz. A ring of regularly arranged grains of the ground- 

 mass is observable round the vesicles and some of the phenocrysts. 



Flow-structure is noticeable in many microcrystalline felsites 

 from various parts, particularly in those which have the ground- 

 mass also partly cryptocrystalline (Kilfarrasy Strand); and the 

 lines of flow sweep round the xenoliths or phenocrysts, which are 

 mostly of small size, but are in some cases large and consist of 

 quartz and felspar. The banded felsites often show no trace of 

 the banding with crossed nicols. 



The microcrystalline type of groundmass passes into the crypto- 

 crystalline (eastern end of Morageeha Strand), for both are 

 frequently associated together in the same rock, and felsites of this 

 composite type may contain no phenocrysts or only a few. Well- 

 developed perlitic structure is frequently met with in rocks 

 of this type (Lady's Cove, Garrarus), and in these examples a few 

 large decomposed orthoclases are the only phenocrysts. Instances 

 of flow-structure have been mentioned above. 



The completely cryptocrystalline type (Type B) is 

 common. Frequently there are no phenocrysts present (western 



