Vol. 60.] 



IGXEOTTS ROCKS OF POXTESFOED HILL. 



477 



Ehyolite, but a fibro-radiate structure, which is almost invisible 

 until the specimen is examined with crossed nicols, has developed : 

 in one place rouud an elongated vesicle, and in other places around 

 felspar-phenocrysts. A mosaic of secondary, colourless quartz, 

 possibly due to solfataric action, has largely replaced some of the 

 original brown glassy matter of these spherulites, but more espe- 

 cially the spaces between them, so that the spherulitic bodies appear 

 light-brown in a nearly-colourless matrix (PI. XL1, fig. 2). 



The rock, as a whole, is more basic and slaggy-looking than the 

 Northern Ehyolite, contains little or no visible primary quartz, and 

 the felspars have more generally the albite-twinning. Its per- 

 centage of silica is 75*78, and its specific gravity 2-63. 



(6) Summary of the Bedded Rocks. 



Table showing Silica-Percextages and Specific Gravities. 



Bock-specimens. 



Northern Ehyolite (15) 



Andesite-Group. 



(a) Eed-and-green grits | L^J *'"'""'"' 



f (Agg. Crag) 



(h) Palagonite-tuffx (green, flaggy) 



[(coarse agg.-breccia). 



(c) Green halleflinta 



(d) Green grit (512) 



((516 c) 



I (528) 



(e) Audesite-lava •{ (57 a) 



I (513) 



l v (204) 



Ehyolite-grits arid breccias 



South-Eastern Ehyolite 



Percent. Sp. 

 of silica, gravity. 



81-93 2-610 



55-58 2-694 \ 

 57-07 2-570 / 

 53-45 2-743^ 



2-837 I 

 2-700J- 



2-670 I 



2790; 



50-67 2-760 ^ 



2-800 I 



2-820}. 



2-800 I 



2-830; 



74-83 2-640 



75-78 2-630 



Average 

 sp. gr. 



2-61 



2-63 



2-75 



2-80 



264 

 2-63 



The foregoing table shows that a considerable gap in silica- 

 percentage occurs between the Northern Ehyolite and the more acid 

 of the andesite-tuffs that immediately follow. This fact, combined 

 with the discordance in strike between the banding of the Northern 

 Ehyolite and the succeeding tuffs (see map, PL XXXYIII), might 

 be taken to imply, either that a considerable break in the volcanic- 

 history here exists at the base of the tuffs, or that the junction is a 

 disturbed one. Unfortunately, the junction is largely obscured by 

 the dolerite, and where this is not the case, it is impossible to see 

 the relation of the two rock-groups. There still remains another 

 alternative, namely that the Northern Ehyolite is intrusive, as stated 

 by Mr. Blake, 1 and does not belong to the bedded volcanics of the 



1 See ante, pp. 451, 452. 



