Volcanic Islands visited during the Voyage of the Beagle. S51 



are generally of extreme thinness ; they consist either of an apparently ho- 

 mogeneous, compact rock, striped with different shades of gray and brown 

 colours, or of crystalline felspathic layers in a more or less perfect state 

 of purity, and of different thicknesses, with distinct crystals of glassy fel- 

 spar placed lengthways, or of very thin layers chiefly composed of minute 

 crystals of quartz and augite, or composed of black and red specks of an 

 augitic mineral and of an oxide of iron, either not crystallized or imper- 

 fectly so. After having fully described the obsidian, I shall return to the 

 subject of the lamination of rocks of the trachytic series. 



" The passage of the foregoing beds into the strata of glassy obsidian is ef- 

 fected in several ways; first, angulo-nodular masses of obsidian, both large 

 and small, abruptly appear disseminated in a slaty, or in an amorphous, 

 pale coloured felspathic rock, with a somewhat pearly fracture. Secondly, 

 small irregular nodules of the obsidian, either standing separately, or united 

 into thin layers, seldom more than the tenth of an inch in thickness, alter- 

 nate repeatedly with very thin layers of a felspathic rock, which is striped 

 with the finest parallel zones of colour, like an agate, and which sometimes 

 passes into the nature of pitchstone ; the interstices between the nodules 

 of obsidian are generally filled by soft white matter, resembling pumiceous 

 ashes. Thirdly, the whole substance of the bounding rock suddenly passes 

 into an angulo-concretionary mass of obsidian. Such masses (as well as 

 the small nodules) of obsidian are of a pale green colour, and are generally 

 streaked with diflferent shades of colour, parallel to the laminae of the sur- 

 rounding rock ; they likewise generally contain minute white sphaerulites, 

 of which half is sometimes imbedded in a zone of one shade of colour, and 

 half in a zone of another shade. The obsidian assumes its jet black colour 

 and perfectly conchoidal fracture, only when in large masses ; but even in 

 these, on careful examination and on holding the specimens in different 

 lights, I could generally distinguish parallel streaks of different shades of 

 darkness. 



" One of the commonest transitional rocks deserves in several respects 

 a further description. It is of a very complicated nature, and consists of 

 numerous thin, slightly tortuous layers of a pale-colcured felspathic stone, 

 often passing into an imperfect pitchstone, alternating with layers formed 

 of numberless little globules of two varieties of obsidian, and of two kinds 

 of sphaerulites, imbedded in a soft or in a hard pearly base. The sphaeru- 

 lites are either white and translucent, or dark brown and opake ; the former 

 are quite spherical, of small size, and distinctly radiated from their centre. 

 The dark brown sphaerulites are less perfectly round, and vary in diameter 

 from the -^ to -^ of an inch ; when broken they exhibit towards their cen- 

 tres, which are whitish, an obscure radiating structure; two of them, when 

 united, sometimes have only one central point of radiation ; there is occa.« 

 sionally a trace of a hollow or crevice in their centres. They stand either 

 separately, or are united two or three or many together into irregular 

 groups, or more commonly into layers, parallel to the stratification of the 

 mass. This union in many cases is so perfect, that the two sides of the 

 layer thus formed are quite even ; and these layers, as they become less 

 brown and opake, cannot be distinguished from the alternating layers of 

 the pale-coloured felspathic stone. The sphaerulites, when not united^ 

 are generally compressed in the plane of the lamination of the mass ; and 

 in this same plane they are often marked internally by zones of diflferent 

 shades of colour, and externally by small ridges and furrows. In the upper 

 part of the accompanying woodcut, the sphaerulites with the parallel ridges 

 and furrows are represented on an enlarged scale, but they are not well 

 executed ; and in the lower part their usual manner of grouping is shown. 

 In another specimen, a thin layer formed of the brown sphaerulites closely 



