280 VOLCANIC ROCKS. 



rapid cooling. In some parts of these congealed streams, I could 

 trace a transition of the obsidian into pumice. In these places, the 

 obsidian contained scattered air globules, which were almost always 

 lengthened in the direction of the stream. These globules gradually 

 augmented in number until the whole substance became a light, fra- 

 gile, and frothy pumice."* Obsidian is found in the crater of Vulcano, 

 one of the iEolian islands, and may be seen forming there at the 

 present time. 



Rocks of trachyte sometimes, though rarely, have a columnar 

 structure. Owing to the facility with which trachyte breaks down, it 

 forms beds of conglomerate intermixed with scoriae and pumice. The 

 more finely comminuted parts of trachyte, intermixed with earthy 

 matter, form beds of tufa. These beds of conglomerate and tufa, 

 frequently environ trachytic mountains, and hide from the view of the 

 geologist their connection with the subjacent rocks. 



When trachyte becomes compact and hard, and acquires a laminar 

 or slatv structure, it passes into clinkstone or phonolite, so called on 

 account of its yielding a metallic sound when struck. (See Chap. 

 IX., where it is observed, that dark lava or basalt also passes into 

 clinkstone.) Thus it appears that both the light-coloured lava, or 

 trachyte, and the dark-coloured lava or basalt, according to the dif- 

 ferent degrees of heat to which they have been subjected, or the 

 different circumstances under which they have cooled, form volcan- 

 ic glass, clinkstone, or pumice ; and the only difference to be observed 

 in the minerals formed from the trachyte or the basalt, is a difference 

 of colour in the minerals themselves, or in the glass which they yield 

 when melted. Black pumice from basalt is however very rare-t 

 Basaltic dykes, and the overlying rocks of porphyry, trap, and basalt, 

 described in Chap. IX. ought, I am persuaded, to be classed with 

 ancient volcanic rocks, but their igneous origin is not yet universally 



* Memoirs of the Wernerian Society, vol. ii. p. 47. ^ , , , . , 



t According to the microscope and mechanical analysis of light-coloured and 

 dark lavas, by M.Cordier (whether compact or sconaceous), it appears that the 

 stony lavas which melt into a white glass, contam nmety per cent, of felspar. 

 Those lavas which melt into a bottle-green glass or enamel, contam only f oBi 

 hfty-fiveto seventv per cent, of felspar; such are the peenish, pyish or dart 

 coloured basalt. On a microscopic examination of dark lava or basalt, it appears 

 fo consist of minute crvstalline grains. The whitish grams belong chiefly to fel- 

 Sar bat in the lava from Vesulius, to leucite ; a small proportion of these grains 

 are clm^ilite. The yellowish or greenish grains belong to augite and hornblende 

 Those of augite are rounded and irregular, with a vitreous fracture and splendent 

 ustre The grains of hornblende are long, and assume a prismatic form ; they 

 present indications of a laminar structure, and have little l^^tre. The p rfecj^ 

 black grains are iron sand, combined with titanium; the grains of iron ore {fero^ 

 liste) may be known by yielding a red powder when pulverized. Volcanic glass 

 fo IcanTc sW, and volcanic tufa, are all composed of the same "iinerals as the 

 most compact lava; and all the most homogeneous dark volcanic rocks are compo 

 Sffr^i^ute microscopic grains, which are chiefly felspar and fUgUe with a smal 

 proport on of olivine and iron sand. M. Cordier informed the author that the 

 Soscopic examition of lava was much facilitated by steeping the piece to be 

 examined in dilute muriatic acid. 



I 



! 



