66 Ascension. 



PAET I. 



glassy obsidian is effected in several ways : first, angulo- 

 nodular masses of obsidian, both large and small, 

 abruptly appear disseminated in a slaty, or in an amor- 

 phous, pale-coloured feldspathic 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, alternate repeatedly with very thin layers 

 of a feldspathic rock, which is striped with the finest 

 parallel zones of colour, like an agate, and which some- 

 times 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 different shades of colour, parallel to the laminas 

 of the surrounding rock ; they likewise generally con- 

 tain minute white sphaerulites, of which half is some- 

 times embedded in a zone of one shade of colour, and 

 half in a zone of another shade. The obsidian assumes 

 its jet black colour aud 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-coloured feldspathic 

 stone, often passing into an imperfect pitchstone, alter- 

 nating with layers formed of numberless little globules 

 of two varieties of obsidian, and of two kinds of spkse- 

 rulites. embedded in a soft or in a hard pearly base. 



