XIX AUGITE-ANDESITES 



275 



In the thin section it displays the plagioclase phenocrysts in a 

 ground mass of doleritic and semi-ophitic texture showing a plexus 

 of long felspar-lathes partly invested by small augites with a fair 

 amount of altered greenish opaque interstitial glass. . . . The 

 plagioclase phenocrysts, which give extinctions of 11° to 15° (acid 

 andesine), are traversed by a network of fine cracks and contain a 

 quantity of colourless dust-like inclusions and alteration-products. 

 They are long and rectilinear in outline and are not much affected 

 by the magma. . . . The felspar-lathes, which average -26 mm. in 

 length, are occasionally lamellar when the angle of extinction 

 indicates acid andesine. Like the phenocrysts they contain many 

 dust-like inclusions. . . . T-he augite may at times form an aggre- 

 gate phenocryst of small size ; but usually it occurs as semi-ophitic 

 masses "i mm. in diameter. 



B. NON-PORPHYRITIC SUB-GENUS. — The specimen represent- 

 ing this group is a coarse-grained greyish altered rock, sp. gr. 2-81, 

 found in blocks near Waikatakata (p. 204), showing small some- 

 what opaque plagioclase phenocrysts. In the section these 

 phenocrysts are displayed in numbers together with a few of augite. 

 The groundmass, doleritic in texture in this species, displays a 

 plexus of long stout felsparJathes with numerous semi-ophitic 

 lesser augites, chloritic pseudomorphs after pyroxene, and scanty 

 interstitial altered glass. . . . The plagioclase phenocrysts, 2 to 3 mm. 

 in size, give extinctions (22° to 28°) of basic andesine. They are 

 traversed by many fine cracks and contain an abundance of colour- 

 less dust-like inclusions, apparently altered magma products. . . . 

 The small pyroxene phenocrysts consist of aggregates of smaller 

 crystals of pale augite. The lesser augites, 'i mm. in size, partly 

 invest the felspars. . . . The broad felspar-lathes, which average 

 ■25 mm. in length and give lamellar extinctions of medium andesine 

 (15°), contain inclusions similar to those of the phenocrysts. . . . 

 The scanty interstitial glass is converted into viriditic and chloritic 

 materials. Secondary calcite also occurs here and in the chloritic 

 pseudomorphs. 



The only species represented is that with felspar-lathes -2 to 

 ■3 mm. in length. 



12. Genus of the Augite-Andesites 

 Formula. — A ug, matr, non-flu, oph, non-phen, parv. 

 Characters. — In the groundmass the felspar lathes and 

 prisms are not in flow-arrangement and the augite is ophitic or 



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