Volcanic Islands [26]
mottled with red; it consists of well- crystallised feldspar, numerous grains of quartz, and brilliant, though small, crystals of hornblende. The feldspar and hornblende in this and the succeeding cases have been determined by the reflecting goniometer, and the quartz by its action under the blowpipe. The feldspar in these ejected fragments, like the glassy kind in the trachyte, is from its cleavage a potash-feldspar.
SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches of a decayed mineral; one minute particle of which I was able to ascertain, by its cleavage, to be hornblende.
THIRDLY, a mass of confusedly crystallised white feldspar, with little nests of a dark-coloured mineral, often carious, externally rounded, having a glossy fracture, but no distinct cleavage: from comparison with the second specimen, I have no doubt that it is fused hornblende.
FOURTHLY, a rock, which at first appears a simple aggregation of distinct and large-sized crystals of dusty-coloured Labrador feldspar (Professor Miller has been so kind as to examine this mineral. He obtained two good cleavages of 86 degrees 30 minutes and 86 degrees 50 minutes. The mean of several, which I made, was 86 degrees 30 minutes. Professor Miller states that these crystals, when reduced to a fine powder, are soluble in hydrochloric acid, leaving some undissolved silex behind; the addition of oxalate of ammonia gives a copious precipitate of lime. He further remarks, that according to Von Kobell, anorthite (a mineral occurring in the ejected fragments at Mount Somma) is always white and transparent, so that if this be the case, these crystals from Ascension must be considered as Labrador feldspar. Professor Miller adds, that he has seen an account, in Erdmann's "Journal fur tecnische 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.); but in their interstices there is some white granular feldspar, abundant scales of mica, a little altered hornblende, and, as I believe, no quartz. I have described these fragments in detail, because it is rare to find granitic rocks ejected from volcanoes with their MINERALS UNCHANGED, as is the case with the first specimen, and partially with the second. (Daubeny, in his work on Volcanoes page 386, remarks that this is the case; and Humboldt, in his "Personal Narrative" volume 1 page 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.") 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, grains of specular iron, and black microscopical points, which latter, from being easily fused, and then becoming magnetic, I presume are hornblende. The greater number of the hills, however, are composed of a quite white, friable stone, appearing like
SECONDLY, a brick-red mass of feldspar, quartz, and small dark patches of a decayed mineral; one minute particle of which I was able to ascertain, by its cleavage, to be hornblende.
THIRDLY, a mass of confusedly crystallised white feldspar, with little nests of a dark-coloured mineral, often carious, externally rounded, having a glossy fracture, but no distinct cleavage: from comparison with the second specimen, I have no doubt that it is fused hornblende.
FOURTHLY, a rock, which at first appears a simple aggregation of distinct and large-sized crystals of dusty-coloured Labrador feldspar (Professor Miller has been so kind as to examine this mineral. He obtained two good cleavages of 86 degrees 30 minutes and 86 degrees 50 minutes. The mean of several, which I made, was 86 degrees 30 minutes. Professor Miller states that these crystals, when reduced to a fine powder, are soluble in hydrochloric acid, leaving some undissolved silex behind; the addition of oxalate of ammonia gives a copious precipitate of lime. He further remarks, that according to Von Kobell, anorthite (a mineral occurring in the ejected fragments at Mount Somma) is always white and transparent, so that if this be the case, these crystals from Ascension must be considered as Labrador feldspar. Professor Miller adds, that he has seen an account, in Erdmann's "Journal fur tecnische 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.); but in their interstices there is some white granular feldspar, abundant scales of mica, a little altered hornblende, and, as I believe, no quartz. I have described these fragments in detail, because it is rare to find granitic rocks ejected from volcanoes with their MINERALS UNCHANGED, as is the case with the first specimen, and partially with the second. (Daubeny, in his work on Volcanoes page 386, remarks that this is the case; and Humboldt, in his "Personal Narrative" volume 1 page 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.") 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, grains of specular iron, and black microscopical points, which latter, from being easily fused, and then becoming magnetic, I presume are hornblende. The greater number of the hills, however, are composed of a quite white, friable stone, appearing like