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C. A. SUSSMILCH AND W. G. STONE. 



felspar. Some specimens show evidence of intense silicifi- 

 cation, the groundmass in some instances being completely 

 changed to a fine-grained compact quartz. 



Microscopic Description. — The rock is noncrystalline 

 and porphyritic with a cryptocrystalline to microcrystalline 

 groundmass. The phenocrysts, quartz and felspar occupy 

 from about 10 to 15 per cent, of the rock. They are present 

 approximately in equal quantity and generally of about the 

 same size. 



The quartz is irregular in outline and shows more or less 

 corrosion, numerous instances of a typical character being 

 noticed. Shadowy extinction is sometimes present. 



Felspar. — Both orthoclase and plagioclase are present 

 the latter predominating. The average size being from 0*5 

 to 1 mm. It is never quite fresh, being somewhat kaolin- 

 ised and of a faint cloudy appearance. The amount of 

 alteration is not so great as in the quartz-porphyrites. It 

 generally shows incipient alteration to sericite, microscopic 

 whisps of that mineral being scattered through the pheno- 

 crysts. Extinctions measured from the twin lamellae of 

 the plagioclase gave angles corresponding to albite. The 

 orthoclase only occasionally shows twinning. The pheno- 

 crysts generally show more or less rounding through cor- 

 rosion. Calcite occurs as an alteration product replacing 

 the felspar along cleavage directions and cracks, also on 

 boundaries, some of the phenocrysts being completely 

 enveloped by a border of calcite. 



Apatite is very sparingly present as needle-like crystals 

 in groundmass, and to a less extent in the felspars. 



Iron ore is scantily scattered as microscopic grains 

 through the groundmass. Here and there small aggregates 

 occur. A little iron pyrites was detected. 



