BY M. AUROUSSEAU. 



295 



ritic with numerous white felspars and subordinate hornblendes 

 up to 3 mm. long; the felspars frequently appear to invest a 

 ferromagnesian kernel; the fracture is smooth and subconchoidal. 



Microscopically, the primary minerals developed are plagio- 

 clase, hornblende, biotite, rhombic pyroxene, magnetite, apatite, 

 and zircon. 



The plagioclase is an oligoclase-andesine, with a maximum 

 symmetrical extinction of 16°. It is of subequant habit, twinned 

 on the Carlsbad, albite, and pericline laws, and is appreciably 

 zoned, though the variations in composition are but slight; the 

 kernels are acidic, and sharply separated from the more basic 

 outer zones. The felspar has a peculiar spangled appearance, 

 due to the formation of resorbtion-cavities, and the development 

 of cleavage- and fracture-cracks and sericite-scales. The acidic 

 kernels are often completely replaced by pseudomorphs of a blue- 

 green chlorite, rather strongly pleochroic and weakly birefrin- 

 gent, with a fibrous, spherulitic habit. The boundaries of these 

 aggregates are quite sharp, and concentric with the zoning of 

 the felspar. Zircon is the only mineral included. 



Common hornblende is greatly subordinate in amount to pla- 

 gioclase. Its pleochroism and absorbtion are, H pale greenish- 

 yellow, b yellow-green, C green, with H<Cb<CC- It includes 

 zircon, magnetite, and apatite, and patches of exceedingly minute 

 rods lying parallel to C in the (110) cleavage-planes. It is sur- 

 rounded by reaction-rings of moderate depth, the pyroxene of 

 which has altered to a grass-green, strongly pleochroic chlorite 

 with a birefringence equal to that of quartz, while the magnetite- 

 granules are passing into limonite. 



JBiotite occurs in thin, bent flakes, much altered, and so 

 strongly resorbed as to be represented, at times, by a mere 

 string of magnetite-granules. 



Rhombic pyroxene is very scarce, and almost completely 

 altered. It is a pale, feebly pleochroic hypersthene, passing into 

 pseudomorphic aggregates of blue-green chlorite, and brown, 

 pleochroic anthophyllite. Alteration proceeded with the para- 

 morphic formation of anthophyllite inwards from the sides and 

 transverse fractures, while chlorite formed internally. 



