Vol. 65.] ROCKS OF THE TOERMAEEADY DISTRICT. 133 



IV. Petrographical Details. 

 {a) The Felsites. 



(1) The main green-felsite intrusion. — This is formed of 

 a pale-green rock, almost always showing in a hand-specimen large 

 and prominent phenocrysts of quartz and sometimes of felspar. 

 The specific gravity in a specimen (71) from near the northern end 

 of the mass is 2-66, while others show 2-70, 2-73, and 2-75. 



Sections from different parts of the mass do not exhibit much 

 variation in structure. In each case the ground-mass contains 

 numerous little areas which appear to be imperfect spherules. The 

 quartz-phenocrysts show strong corrosion by the ground-mass (see 

 PI. V, fig. 1), and the orthoclase is much altered, sometimes showing 

 partial replacement by epidote. A good deal of magnetite is some- 

 times present (79, 215). Small vesicles generally occur, occupied by 

 calcite (71), or by chalcedony and chlorite (79, 126). A section 

 (194), cut from a specimen taken from the extreme northern end 

 of the long north-eastward projecting tongue, is of a rock-type 

 similar to that of the main green-felsite mass, but contains an ex- 

 ceptional amount of magnetite and much apatite. Another section 

 (214) from near the base of the tongue is of a strongly micro- 

 spherulitic rock with abundant chlorite, some of wdiich is clearly 

 pseudomorphic after biotite. 



(2) The brown-felsite intrusion. — The large felsite-mass 

 between Shangort and Derrassa is composed of the handsomest rock 

 in the district. In a hand-specimen it is brown or sometimes 

 green, showing prominent quartz-crystals and numerous dark-green 

 patches which microscopic sections show to consist of chlorite 

 (perhaps replacing pyroxene) intergrown in some places with the 

 felspar, which includes plagioclase as well as orthoclase. The 

 ground-mass is felsitic, and the specific gravity of one specimen (74) 

 is 2-70 and of another (73) 2-73. 



(3) Felsite-intrusions in the Arenig and Bala rocks 

 near Tourmakeady Lodge. — These exposures include the two 

 long intrusions already mentioned, of which only the ends appear in 

 the map, and several smaller intrusions. Microscopically the rock 

 differs from that forming the main green felsite-intrusion, in the 

 absence of any tendency to a spherulitic structure in the ground- 

 mass. No sign of augite was to be seen in any section examined ; 

 but biotite in a more or less altered state was observed at several 

 points. The specific gravity of one of these rocks, which contained 

 crystals of orthoclase upwards of an inch long, is 2-68. 



(4) Ped felsites between Tourmakeady Lodge and 

 Stream C. — These rocks are very variable, but the prevalent types 

 -are brown or reddish, and, in hand-specimens, often show dark 



