296 PETKOLOGICAL KOTES, i., 



Zircon is comparatively abundant, as is magnetite, which in- 

 cludes apatite, and is passing peripherally into iimonite. Apatite 

 is scarce, but is peculiar in showing prismatic cleavage. 



The groundmass is pale brownish in colour, with a slightly 

 fluidal fabric in places, and is thickly dusted with magnetite, 

 Iimonite, and chlorite. In ordinary light, it has the appearance 

 of a typical glass. It consists, however, of a very fine-grained 

 mosaic of untwinned plagioclase, more acid than the phenocrysts. 

 The separate grains are independent of flow-lines or inclusions 

 in their orientation. The mosaic, hence, represents a devitrified 

 plagioclase-glass. Small nests of granular quartz, perhaps second- 

 ary, occur occasionally. (PI. xl., fig.3). 



This rock has been extensively quarried for use as ballast on 

 the North Coast Railway. 



Eelah. 



{WZ) Hyper sthene-andesite. — Avery dark brown, heavy rock, 

 with a rough, uneven fracture, and granular appearance; porphy- 

 ritic with numerous, yellow, glassy felspars up to 3 mm. long, 

 which weather to a brown colour. The rock, as a whole, develops 

 an earthy crust on weathered surfaces, on which the felspars 

 stand out in slight relief. 



Plagioclase, hypersthene, magnetite, apatite, and zircon are 

 developed. (Pl.xL, fig.4). 



The plagioclase is a basic labradorite, with a maximum sym- 

 metrical extinction of 35", twinned on the Carlsbad, albite, and 

 pericline laws ; habit idioraorphic, subequant, with slightly 

 rounded outlines, distinctly corroded by the groundmass in places. 

 Zoning is well marked, but the variations in composition are 

 only slight, and are not progressive. Generally, there is an even 

 basic core, followed by several oscillations, with a well marked 

 acidic outer shell. Stony inclusions tend to concentrate in the 

 zones of oscillating composition. The felspar is generally fresh 

 and unaltered, though fractured. Limonite has been deposited 

 in the cracks. Small grains of hypersthene are included. 



Hypersthene forms stout columnar crystals of rounded outline, 

 and is not very abundant. The colour is fairly deep; pleochroism 



