xix AUGITE-ANDESITES 269 



and contain secondary calcite, sometimes to such an extent that 

 they might be taken at first sight for impure limestones. 



The small opaque plagioclase phenocrysts (under 2 mm.), that 

 they contain, are more evident in the slide than in the hand- 

 specimen, and scarcely give a macroscopic character to the rock. 

 They give extinctions (io° to 30°) ranging from those of acid 

 andesine to acid labradorite, and are traversed by numerous cracks 

 occupied by calcitic and other alteration products. The few 

 pyroxene phenocrysts that once existed are now entirely represented 

 by chloritic pseudomorphs. The groundmass displays a doleritic 

 texture, exhibiting a plexus of long felspar-lathes, "2 to "4 mm. in 

 average length, which often present a false resemblance to a flow- 

 arrangement from their aggregation into bundles. They are often 

 clouded by secondary products, but occasionally give lamellar 

 extinctions (20 to 30 ) indicating andesine labradorite. The rest 

 of the groundmass is greatly altered, the granular augite and the 

 interstitial glass, which originally existed in fair amount, being 

 replaced by calcite, chlorite, pyrites, and occasionally epidote, so 

 that the rock mass appears largely impregnated with alteration 

 products. In addition there is much secondary magnetite, and 

 in some cases there are a few minute cavities filled with chalcedonic 

 silica and zeolites. 



Reference may here be made to a singular rock from Ruku-ruku 

 Bay, which resembles the Ndriti rocks in its propylitic alteration, 

 but the felspar-lathes of the groundmass, - 2i mm. in length, give 

 the small extinctions of oligoclase. Spec. grav. 2'6i. 



Most of the prevailing rocks of Mount Freeland belong to this 

 sub-genus. They are dark grey and show small opaque plagioclase 

 phenocrysts 1 or 2 mm. in size. They usually, however, are more 

 or less altered, the change being often of the propylitic type, calcite, 

 chloritic material, viridite, and occasionally pyrites occurring as 

 alteration products. The specific gravity of the altered rocks is 

 2*61 — 2'6o, ; that of the least affected is about 276. They all, 

 however, belong to the same genus, displaying small phenocrysts of 

 plagioclase and augite in a groundmass composed of minute stoutish 

 felspar-lathes ("03 — 'o6 mm.), augite granules, magnetite, and a 

 little residual glass. The plagioclase phenocrysts owe their opacity 

 partly to the numerous fine cracks traversing them and partly to the 

 alteration products. The pyroxene phenocrysts, which are mostly 

 of pale yellow augite, display at times intergrowths of rhombic 

 pyroxene. 



