BY M. AUROUSSEAU. 301 



Zircon is plentiful in small, stout grains, while apatite is rare. 



The groundmass is fluidal in places, elsewhere imperfectly 

 spherulitic. It is greatly clouded by secondary products (limonite, 

 leucoxene, epidote, and chlorite), and is completely devitrified to 

 a fine mosaic of felspar and quartz. 



(104) Oligoclase-hiotite rhyolite. — A light, tough rock, creamy 

 to pink in colour, weathering to a yellow-brown. Scattered, 

 hexagonal flakes of biotite occur. The fracture is very irregular 

 and rough. 



Plagioclase, quartz, biotite, and magnetite occur as pheno- 

 crysts, all very sparingly. 



The groundmass is fluidal, and is completely devitrified, con- 

 sisting of a microcrystalline mosaic of quartz and felspar, with 

 imperfect felspar-spherulites. It includes numerous margarites. 

 The felspar has a refractive index sometimes greater, sometimes 

 less than that of quartz, and the arms of the extinction-crosses 

 of the spherulites are parallel to the cross-wires. It is, hence, 

 an oligoclase. The phenocrysts of plagioclase are more basic, 

 but do not exhibit twinning. The scanty quartz-phenocrysts 

 are strongly corroded. The biotite is in bent flakes, and is 

 strongly absorbtive. The pleochroism and absorbtion are, H pale 

 yellow, b black, C black; a<b=C The magnetite is passing 

 into limonite. 



{] 14:) Tuffaceous, porphyritic rhyolite. — A compact, yellow- 

 brown rock, porphyritic with fairly numerous quartzes and white 

 felspars up to 3-5 mm. in diameter. The fracture is rough, and 

 weathered surfaces are a dirty-grey. The rock has the appear- 

 ance of a typical quartz-porphyry. 



Quartz, orthoclase, plagioclase, biotite, hornblende, iron-ore, 

 and zircon are developed. 



The quartz is, in all respects, like that in (112). 



Orthoclase is next in abundance, forming stout, tabular- 

 prismatic grains, slightly resorbed. 



The plagioclase is in small, equant crystals, and is not abund- 

 ant. It is a slightly calcic oligoclase, with a maximum sym- 

 metrical extinction of 4°. Carlsbad, albite, and pericline twins 



