274 A NATURALIST IN THE PACIFIC chap. 



They display in the slide small plagioclase phenocrysts, often 

 abundant, in a groundmass exhibiting a loose plexus of large lathe- 

 shaped felspar prisms, together with occasional small ophitic 

 " plates " of augite and numerous smaller semi-ophitic augites, 

 whilst there is much interstitial smoky glass. . . . The plagioclase 

 phenocrysts are as a rule i to 2 mm. in size and do not exceed 

 3 mm. They often contain abundant inclusions of the magma 

 sometimes arranged schiller-fashion, and are frequently eroded. 

 Theirlamellar extinctions ( 1 5°-3O ) indicate andesine labradorite. . . 

 The stout plagioclase lathes, which in most of my specimens range 

 between '2 and *3 mm., and contain at times magma-inclusions, 

 give the rocks their doleritic texture. . . . The occasional small 

 ophitic " plates " of pale augite are not over I mm. in size and give 

 extinctions of +30° from the single cleavage-lines. The lesser 

 augites, '2 mm. in size, are several times larger than typical granular 

 pyroxenes ( , 02- , 03 mm.), and adapt their form to the interspaces 

 of the felspar-lathes which they in part invest. . . . The copious 

 interstitial glass is generally smoky and sometimes quite opaque 

 through the deposition of magnetite. It is never clear and 

 isotropic, but displays fibrous devitrification and is usually a little 

 altered. 



These rocks come near to those of the previous sub-genus in 

 several respects, but they differ conspicuously in their non-por- 

 phyritic character, in being sometimes vesicular, and in their 

 general appearance. All the four species indicated by the length 

 of the felspar-lathes are here represented ; but the doleritic types 

 with the lathes exceeding '2 mm. are the most frequent. 



10. Genus of the Augite-Andesites 



FORMULA. — Aug, matr, non-flu, opJi,phen, opac. 



Characters. — In the groundmass the felspar-lathes and 

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

 semi-ophitic. The plagioclase phenocrysts are opaque. 



This genus may be divided into two sub-genera, porphyritic 

 and non-porphyritic, according to the average size of the plagio- 

 clase phenocrysts, whether above or below 3 mm. 



A. Porphyritic Sub-genus. — This is represented by a 

 light grey porphyritic rock, with sp. gr. 275, from the lower part 

 of Mount Freeland. It comes near to the porphyrites, and 

 displays large opaque white phenocrysts of plagioclase 5 or 6 mm. 

 long. It is a somewhat altered rock. 



