302 PETROLOGICAL NOTES, i., 



occur. It has the spangled appearance before-mentioned, and 

 includes zircon. Some crystals are broken and recemented, 

 others are rounded. 



Biotite is scanty and much altered. It resembles that of (104). 

 The plates are sometimes bent. 



Hornblende is represented by rare chlorite-pseudomorphs. 



The magnetite, surrounded by yellowish earthy haloes, is ap- 

 parently titaniferous. Zircon is common, and is included in 

 magnetite. 



The subfiuidal groundmass is rendered yellow by limonite, 

 chlorite, and epidote particles. Suitable illumination reveals 

 innumerable little cu spate bodies, exactly like the collapsed 

 bubbles of many tuffs. The rock is not a tuff, as the groundmass 

 distinctly corroded the quartz and felspar; but these bodies un- 

 doubtedly represent an intermixture of tuffaceous material. They 

 have been filled with quartz, and their felspathic walls have 

 devitrified along with the groundmass, which is a microcrystalline 

 mosaic of quartz and felspar ; they are, hence, not apparent 

 between crossed nicols. 



(117) Porphyroid tujfi^l. xl., fig. 6). — A very tough, compact 

 rock of dark red colour, consisting of scattered grains of quartz 

 and felspar up to 2*5 mm. in diameter, in an even, aphanitic 

 matrix containing occasional, soft, brown patches, which effer- 

 vesce strongly with acids; even the harder, red part effervesces 

 to a degree. The fracture is smooth but splintery. In hand- 

 specimens, the rock would be taken for a quartz-felspar porphyry. 

 It develops a thin, brown, earthy crust on weathering. 



The individualised matter consists of grains and fragments of 

 quartz, orthoclase, calcic oligoclase, bent and ragged flakes of 

 biotite, and grains of magnetite, precisely similar to those in the 

 associated igneous rocks. 



The matrix consists of minute, close set, cuspate bodies of 

 exceedingly fantastic forms, representing the collapsed bubbles 

 of a dispersed pumice. They have been subsequently filled with 

 clear quartz, and their walls have devitrified, giving, at first, the 

 false impression that the matrix consists essentially of cuspate 



