48 Ascension. 



PAET I. 



but in their interstices there is some white granular 

 feldspar, abundant scales of mica, a little altered horn- 

 blende, and. as I believe, no quartz. I have described 

 these fragments in detail, because it is rare l to find 

 granitic rocks ejected from volcanos with their minerals 

 unchanged, as is the case with the first specimen, and 

 partially with the second. One other large fragment, 

 found in another spot, is deserving of notice ; it is a 

 conglomerate, containing small fragments of granitic, 

 cellular, and jaspery rocks, and of hornstone porphyries, 

 embedded in a base of wacke, threaded by numerous 

 thin layers of a concretionary pitchstone passing into 

 obsidian. These layers are parallel, slightly tortuous, 

 and short ; they thin out at their ends, and resemble in 

 form the layers of quartz in gneiss. It is probable 

 that these small embedded fragments were not separately 

 ejected, but were entangled in a fluid volcanic rock, 

 allied to obsidian ; and we shall presently see that 

 several varieties of this latter series of rock assume a 

 laminated structure. 



Trachytic series of rocks. — Those occupy the more 

 elevated and central, and likewise the south-eastern, 

 parts of the island. The trachyte is generally of a pale 

 brown colour, stained with small darker patches ; it 

 contains broken and bent crystals of glassy feldspar, 



transparent, so that if this be the case, these crystals from Ascension 

 must be considered as Labrador feldspar. Prof. Miller adds, that 

 he has seen an account, in Erdmann's ' Journal fiir technische 

 Chemie,' of a mineral ejected from a volcano, which had the external 

 characters of Labrador feldspar, but differed in the analysis from 

 that given by mineralogists of this mineral : the author attributed 

 this difference to an error in the analysis of Labrador feldspar, 

 which is very old. 



1 Daubeny, in his work on Volcanos (p. 389), remarks that this 

 is the case ; and Humboldt, in his ' Personal Narrative ' (vol. i. 

 p. 236), says, ' In general, the masses of known primitive rocks, I 

 mean those which perfectly resemble our granites, gneiss, and mica- 

 slate, are very rare in lavas : the substances we generally denote by 

 the name of granite, thrown out by Vesuvius, are mixtures of 

 nepheline, mica, and pyroxene.' 



