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To show how exceedingly firm this recent conglomerate is, I may mention, that I endeavoured with a heavy geological hammer to knock out a thick bolt of iron, which was embedded a little above low-water mark, but was quite unable to succeed. FERNANDO NORONHA. Precipitous hill of phonolite. TERCEIRA. Trachytic rocks: their singular decomposition by steam of high temperature.
In one of these solutions the phonolite of the Cerro de Flores is to be regarded as the sole volcanic production of the tract; and we are forced to unite the pyroxenic amygdaloids with the rest of the grunsteins, in one single formation, that which is so common in the transition-mountains of Europe, considered hitherto as not volcanic.
Down by the river the walls are composed of black gneiss, slates, and schists, all greatly implicated and traversed by dikes of granite. Let this formation be called the black gneiss. It is usually about 800 feet in thickness. Then over the black gneiss are found 800 feet of quartzites, usually in very thin beds of many colors, but exceedingly hard, and ringing under the hammer like phonolite.
The stones have not been worked since Captain Cook's day; yet there is not a weather-stain upon them, and the air is so dry and rarified that meat will keep fresh for three months. I found a mass of crystals of the greenish volcanic glass, called olivine, imbedded in a piece of phonolite which looked as blue and fresh as if only quarried yesterday.
These tuffs of the southern shores of the Grand Canary, containing the Upper Miocene shells, appear to be about the same age as the most ancient volcanic rocks of the island, composed of slaty diabase, phonolite, and trachyte.
Turk's Cap and Prosperous Bays. Basaltic ring. Central crateriform ridge, with an internal ledge and a parapet. Cones of phonolite. Superficial beds of calcareous sandstone. Extinct land-shells. Beds of detritus. Elevation of the land. Denudation. Craters of elevation. The whole island is of volcanic origin; its circumference, according to Beatson, is about twenty-eight miles.
Obsidian is of a black or ash-grey colour, and though opaque in mass is transparent in thin edges. Among the rocks of the trachytic family, or those in which the feldspars are rich in silica, that termed Clinkstone or Phonolite is conspicuous by its fissile structure, and its tendency to lamination, which is such as sometimes to render it useful as roofing-slate.
A person would not readily believe that these rocks could have flowed as lava; yet at St. Amidst the hillocks composed of these rocks, I found in three places, smooth conical hills of phonolite, abounding with fine crystals of glassy feldspar, and with needles of hornblende.
Phonolite, as far as my observations serve, in every instance appears to be an injected rock, like those of the plutonic series; hence probably, like these latter, it has generally been cooled without repeated and violent disturbances.
It is important to geology not to confound the modern currents of lava, the heaps of basalt, green-stone, and phonolite, dispersed over the primitive and secondary formations, with those porphyroid masses having bases of compact feldspar,* which perhaps have never been perfectly liquified, but which do not less belong to the domain of volcanoes.
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