346 Scientific Proceedings, Royal Dublin Society. 



Microscope : Ground-mass.-— Holocrystalline, composed of lath- 

 shaped felspars, very heterogeneous in size and appearance ; some 

 fluxional structure. Large, fairly idiomorphic crystals of felspar, 

 with lamellar twinning up to 3 mms. in length ; clear and fresh. 

 Also a greenish, fibrous mineral extinguishing longitudinally 

 when in long prisms, sometimes in irregular grains. Prisms up 

 to 2 mms. Not much of this substance, which appears to be bastite 

 or hypersthenic It is feebly dichroic, pale green to yellowish- 

 green. Augite is scarce, clear to colourless. A fair amount of 

 magnetite in dust and small crystals, as well as a little red oxide of 

 ■iron. 



VII. Geological formation, Po c i- 



A black, very fine-grained rock. Fracture only feebly 

 glistening, nearly dull. Under hammer, breaks fairly easily 

 with conchoidal fracture more or less, but shows also some 

 cleavage. Tough. After four years' wear, fairly rounded and 

 smooth. 



Microscope : Ground-mass. — Holocrystalline, of lath-shaped and 

 irregular-sized felspars. Grrain about same as IV. Phenocrysts 

 are felspars, lamellar and simple twins ; not plentiful. A faintly 

 pleochroic enstatite or hypersthene, colourless to pale pinkish. 

 Biotite, pleochroic, associated with red oxide of iron and epidote ; 

 not abundant. Augite, small grains associated with an olive- 

 green, brightly polarising alteration product. 



VIII. Geological formation, Pc 1 TS°i- 



Like VI. in appearance, but coarser-grained on the whole, as 

 a larger part of the constituent minerals is visible to eye-examina- 

 tion. Some cleavage apparent under hammer. It breaks fairly 

 easily, giving a fine smooth surface of fracture. After two and 

 a half years' wear, is fairly rounded and smooth. 



Microscope. — Ground-mass of lath felspars, triclinic and closely 

 interfitted ; no glass. The general appearance is of a rock with 

 more phenocrysts than any so far examined, These are a 

 triclinic felspar for the most part. Augite, twinned, colourless, 

 and in minute grains. Hypersthene, fibrous, pleochroic green to 

 yellow. Bastite : possibly some of the fibrous forms are bastite. 

 Olivine doubtful ; associated with much hematite, which also veins 



