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 n° 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. . . . The 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*8 r, 

 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 felspar-lathes 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. — Aug; 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 



T 2 



