BY M. AUROUSSEAU. 305 



The groundmass is greatly clouded with limonite, epidote, and 

 chlorite, but a fluidal fabric is still recognisable. It consists, 

 like that of (108), (103), and (105), of a fine mosaic of untNvinned 

 plagioclase, more acid than the phenocrysts, and contains small 

 fragments of a foreign trachytic rock (cf.390). 



I am indebted to Professor Woolnough for calling my atten- 

 tion to this rock. 



KXOCKFIN. 



(391) Po7'phyritic rhyolite.—A very fresh, light pinkish-brown 

 rock, porphyritic with fairly plentiful quartzes and pink felspars 

 up to 2*75 mm. in diameter. The fracture is smooth, but irregu- 

 lar and splintery, and small fragments are milky and translucent 

 on thin edges. In hand-specimens, the rock would be classed as 

 a quartz-porphyry. 



Quartz, felspar, biotite, magnetite, and zircon occur. 



The quartz is strongly corroded, and has many inclusions and 

 inlets of the groundmass. 



Two felspars are developed, but twinning is only feebly ex- 

 pressed, and suitable comparisons of refractive index were not 

 obtained. One is optically negative, and forms simple Carlsbad- 

 twins. It is, doubtless, orthoclase. The other is optically posi- 

 tive, multiply twinned, with low symmetrical extinction-angles, 

 and seems to be albite-oligoclase. It is slightly spangled. Car- 

 bonate-replacement occasionally occurs. 



Biotite is exceedingly rare. 



A few grains of zircon are present, while magnetite is scarce. 



The groundmass is fresh- looking, slightly cloudy, and strongly 

 fluidal. It has devitrified to a fine, quartz-felspar mosaic. The 

 felspar preponderates over quartz, has a refractive index lower 

 than that of quartz, and is in fibrous aggregates, sometimes 

 spherulitic, but more often forming long strings with the fibres 

 transverse to the lines of flow. The slight cloudiness of the 

 felspar reveals the fluidal fabric. In some respects, this rock 

 resembles (114). 



Review and Conclusions. — Viewed collectively, the igneous 

 rocks above-described present niany features of interest. 



