150 



VOLCANIC ROCKS 



Upper 

 Gallery. 



Wall-case 2 



Thin, slightly tortuous layers of pale grey-coloured 

 felspathic stone, between which are layers of opan, 



brown sphcerulites {obsidian globules) 



base. 



a soft, pearly 



19 — Irregular nodules of obsidian (pearlstone) 

 nating with thin layers of a felspathic rock, which conTah 



alter. 



(S( 



20. 



W felsp 



Compact heavy rock, with a crystalline felspathic 

 base, mottled with a black mineral, and abounding with 

 crystals of glassy felspar. 



f See Darwin " On Volcanic Islands," pp. 56 and 57.) 



21. — Compact crystalline rock, banded 



in 



straight 



lines, with numerous, extremely thin, Avhite and grey lamina?, 

 composed chiefly of felspar, and containing numerous per- 

 feet crystals of glassy felspar, placed lengthways ; they are 

 also studded with microscopically minute amorphous black 

 specks of augiteov hornblende. 



(See Darwin " On Volcanic Islands," p. 56.) 



2 - — Tnin slightly tortuous layers of grey felspathic 

 stone, passing into pearlstone, alternating with minute 

 globules of obsidian (dark broivn opaque sphcerulites). In 

 the specimen a thin layer of the brown sphcerulites, closely 

 united, intersects a layer of similar composition, and after 

 running for a short space in a slightly curved line, again 

 intersects it and likewise a second layer, lying a little way 

 beneath that first intersected. 



(See Darwin " On Volcanic Islands," pp. 58 and 59.) 

 23. — Slightly tortuous layers of light grey pearlstone, 

 sometimes passing into pitchstone, with numerous lines of 

 minute white sphaerulites, which are dissected out on two 

 of the weathered sides of the specimen. 



ular nodules of obsidian united into thin 

 layers, which alternate with other thin felspathic layers, 

 containing crystals of glassy felspar. 



25. — Irregular layers of pearlstone, with crystals of 



24.— Irr 



/' 



r 



' V 



■a* of in 

 iial 



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