154 R. A. WEARNE AND W. G. WOOLNOUGH. 



being more lath-shaped. The cossyrite (?) is much darker 

 than in the rock from the base, probably owing to separa- 

 tion of haematite. In both rocks the amphibole is the last 

 mineral to crystallize, enclosing quite idiomorphic quartz. 



Great dyke on Mount Alford. — The rock has an extremely 

 fine grained base of lath-shaped orthoclase, considerably 

 decomposed, with a little interstitial quartz. Here and 

 there a spherulitic structure is suggested. There are 

 very occasional phenocrysts of quartz and thoroughly 

 glassy sanidine. 



Devitrifled obsidian, Mount Alford (not in situ). — An 

 extremely fine grained rock consisting of a colourless base 

 crowded with green needles. The base consists of a mosaic 

 of untwinned orthoclase, having all the appearance of 

 having been formed by the devitrification of a glass with 

 the composition of a felspar. The green fibres are long but 

 excessively thin, strongly pleochroic brownish-green to 

 opaque, and with straight extinction ; they are probably 

 segirine. Their arrangement is variolitic, with occasional 

 bunches in which the fibres are more radially arranged. 



Obsidian, Mount Alford (not in situ). — Vitrophyricrock. 

 The base consists of nearly colourless glass, very clear and 

 free from crystallites or other elementary forms. The 

 glass is slightly perlitic, but this structure is very imper- 

 fectly developed. There are abundant phenocrysts of clear 

 fresh felspar very sharply idiomorphic. Some of the sec- 

 tions are broad, suggesting a tabular habit, others are 

 rhomb shaped as if the mineral were prismatic. Mostly 

 the sections are untwinned or twinned after the Oarlsbad 

 law, but here and there very hazy moiree structure can be 

 seen. The refractive index is less than that of Canada 

 balsam. The felspar is probably anorthoclase. These 

 phenocrysts contain inclusions in the form of very striking 

 negative crystals filled with glass. 



