OP ASCENSION. 



151 









alternating with 



glassy felspar, and passing into pitchstone, 

 irregular dull- red coloured tr achy tic layers. 

 26. — Irregular layers of pitchstone and greenish-grey 



felspathic layers. 



27, 27 a, & 27 b. — Pitchstone, shewing the charac- 



teristic conchoidal fracture and sharp cutting edges. 



28. — Green pitchstone, or obsidian. 



29, 29 a to 29 g. — Black obsidian or volcanic glass, 



with a conchoidal fracture and sharp cutting edges. 



30. — Black obsidian, full of minute globular vesicles, 

 which become gradually less perfectly defined until the 

 whole passes into compact obsidian. The vesicular struc- 

 ture is owing to the expansion of included gases or aqueous 

 vapour, which were not entirely driven off during the fusion 

 of the melted mass. 



31. — Obsidian, passing into vesicular, scoriaceous lava, 

 and presenting an appearance of perfect fusion. 



31a. — Volcanic slag, scoria, or cinder, presenting an 

 appearance of partial fusion, and converted in some places 

 into layers of obsidian. 



31b. — Volcanic slag, scoria, or cinder, presenting an 

 appearance of imperfect fusion. 



31c. — Volcanic slag, scoria, or cinder, presenting an 



appearance of imperfect fusion, and covered, superficially, in 

 some places, with an iridescent lustre. The specimen con- 

 tains a few small fragments of scoriaceous lava, which have 

 become entangled with and taken up by the partially fused 



slaer. 



Irachytic series of rocks occupying the more elevated and 



central, and likewise the south-eastern, parts of the 

 Island. 



b *>2. Somewhat friable white trachyte, appearing when 



viewed in mass, like a sedimentary trachytic tufa. The spe- 



nien is earthy and in a decomposing state, passing into 



lna c * a y« It also contains some cavities, with crystals of 

 9lassu frlm„„ 



Upper 

 Galleky. 



Wall-case 2. 



