442 MR. B. nOBSON OS THE IGNEOUS ROCKS 



albite combined with the pericline plan ; their margins, in some 

 cases uncorroded, are in others considerably corroded by the ground- 

 mass. While some are moderately fresh, others are much altered, 

 the alteration proceeding from the margin inwards until either only 

 a small irregular central patch is left or the whole is altered. The 

 altered portions consist, to the extent of about half, of felspar and 

 about the same quantity of chlorite, which forms irregular patches. 

 The alteration is perhaps partly due to the felspar having been 

 originally honeycombed by the interstitial matter of the groundmass, 

 as occurs in sections 172 (ejected block), 150, &c. 



Fluxion-structure is very evident in the groundmass, which consists 

 largely of plagioclase lath-shaped in section. An individual slightly 

 above the average size measured '36 millim. x '055 millim. 



The space between the plagioclase crystals of the groundmass is 

 occupied by chlorite and minute grains of iron ore, probably altered 

 ilmenite. Larger patches of iron ore, often showing crystalline form 

 and aggregates -73 millim. in diameter and more, occur. Most of the 

 iron ore appears opaque-white by reflected light, and is therefore 

 changed to ieucoxene. In other parts the change has proceeded so 

 far as to form perfectly distinct crystalline aggregates of titanite 

 (sphene). Augite is not present in any of the specimens of augite- 

 porphyrite. Its original presence is inferred from the occurrence of 

 chlorite, and the structure of the rock also from its basic nature, as 

 shown by analysis and indicated by the presence of pseudomorphs 

 after olivine in some specimens. 



Section 155 may be considered an average specimen of augite- 

 porphyrite. Sections 156, 171, 172, 174, 503,517, and 500 a' (fine- 

 grained specimen from south-west side of dyke 500 a), agree with it 

 in most respects, but porphyritic plagioclase is sometimes absent, 

 and ISTo. 156 is very amygdaloidal. Pseudomorphs (consisting of car- 

 bonates and serpentine) after large idiomorphic porphyritic olivine 

 occur in 500 a' (and 500 a), 517, and, according to Mr. Kutley *, 

 in the rock of Scarlet Stack. Mr. Rutley mentions rhombic pyroxene 

 as occurring in the dyke leading from Scarlet Stack to the mainland 

 (my 500 a). Section 500 a is considerably coarser-grained than 155, 

 and sect. 522, from Scarlet Stack, is the most coarsely crystalline of 

 all, the average size of the felspars of the groundmass being 

 •75 millim. x "10 millim., and fluxion-structure being far less 

 marked than in No. 155. The porphyritic plagioclase occasionally 

 shows zone-structure. Chalcedony occurs as an alteration-product. 



Section 500. Volcanic tuff from Scarlet Point ; this is a soft 

 yellowish-green bedded tuff. 



Microscopical characters : it contains irregular fragments of the 

 following types, all of which seem to be simply one and the same 

 augite-porphyrite magma solidified under var5'ing conditions : — 



A. Pumiceous fragments crowded with vesicles which occupy 

 more space than the solid part. The vesicles are drawn out longi- 

 tudinally, indicating flow when in a viscid state, and are occupied 



* Quoted in Messrs. Dickson and Holland's paper, Proe. L'pool Geol. Soc. 

 vol. vi. pt. i. (1889) pp. 128 and 130. 



