344 Scientific Proceedings, Royal Dublin Society. 



II. Geological formation, Pc x Ts c i- 



Very fine-grained, brownish-grey, almost black. Cleavage 

 faces of largest constituents visible in reflected light. 



Breaks under hammer with sharp conchoidal fracture, and 

 yields only with difficulty to the hand-lever. After two and a 

 half years' wear, rounded and smoothed. 



Under Microscope. — Ground-mass is noncrystalline, and contains 

 some lath-shaped and some square felspars with fluxion structure 

 (trachytic) ; in this occur large isolated areas of devitrified 

 spherulitic glass, which, when examined by polarised light, show a 

 black cross ; probably opal. Also crystalline outlines with black 

 resorption borders ; contents quite altered, or fallen out wholly 

 or in part. In places suggesting an altered augite by traces of 

 cleavage, and light golden-yellow polarisation tint. 



Phenocrysts of felspar ; apparently Carlsbad twins : fresh and 

 clear. 



Hematite and magnetite common — especially the latter — in 

 grains, dust, and crystals. 



III. Geological formation, Pc x Ts c i- 



Grain about same as II. Same appearance as II., rather 

 browner in colour. Under hammer appears to be less tough than 

 II. After eleven years' wear much rounded and smoothed. 



Microscope. — Ground-mass of microliths, lath-shaped felspars, 

 and a few square forms : fluxion marked : trachytic. No glass 

 or very little. In this are felspar phenocrysts, which show some 

 zoning, and little or no lamellar twinning : fresh. Magnetite in 

 less quantity than in II., and in small grains, dust, and crystals. 

 The dust apparently replaces some decomposed mineral, and shows 

 crystalline outlines to aggregates of particles. Spots of cloudy 

 red oxide of iron. 



IV. Geological Formation, Po c i- 



Very fine grained, nearly black. Very tough under hammer : 

 breaks away with difficulty in small conchoidal flakes. After 

 four years' wear, smooth and rounded. 



Microscope : Ground-mass. — Very uniformly composed of lath- 

 shaped felspars, rather larger than those entering into the ground- 

 mass of I., II., and III. Marked fluxional structure. No lamellar 



