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Updated: June 23, 2025
The hollows in some of the fragments of vesicular lava of which the breccias and conglomerates are composed are partially filled with calc-sinter, being thus half converted into amygdaloids.
The currents of lava, which we discover on the slope of Monte Verde, between the beautiful spring of Dornajito and Caravela, are black masses, altered by decomposition, sometimes porous, and with very oblong pores. The basis of these lower lavas is rather wacke than basalt; when it is spongy, it resembles the amygdaloids* of Frankfort-on-the-Main. The crystals which Mr.
This regular disposition of lithoid basaltic lava and feldsparry vitreous lava is analogous to the phenomena of all trappean mountains; it reminds us of those phonolites lying in very ancient basalts, those close mixtures of augite and feldspar which cover the hills of wacke or porous amygdaloids: but why are the porphyritic or feldsparry lavas of the Peak found only on the summit of the volcano?
The porphyries containing vitreous feldspar and hornblende, the phonolite, the greenstone, the amygdaloids, and the basalt, have forms almost as invariable as simple crystallized substances.
On the northern point of this same island, there were various ancient submarine volcanic rocks, consisting of amygdaloids with dark bole and agate, of basalt with decomposed olivine of compact lava with glassy feldspar, and of a coarse conglomerate of red scoriae, parts being amygdaloidal with carbonate of lime.
C| TUFA AND TRAP. In Londa, the bottom of the valley E| 2500 feet. is formed of ferruginous conglomerate on the surface; N| Lake Ngami. hardened sandstone, with madrepore holes, T| banks of gravel, and occasionally trap; R| south of 12 Degrees, large patches of soft A| TUFA. calcareous tufa, with pebbles of jasper, L| agates, &c., lie on various horizontal traps, | amygdaloids with analami and mesotype, which is P| burst through by basaltic rocks forming hills, L| and showing that the bottom of the valley A| RADIATED ZEOLITE. consists of old silurian schists; T| there are also various granitic rocks E| cropping through the trap.
This hill is conspicuous from rising to the height of 6,400 feet: its summit shows a nucleus, uncovered for a height of 800 feet, of fine greenstone, including epidote and octahedral magnetic iron ore; its flanks are formed of great strata of porphyritic claystone conglomerate associated with various true porphyries and amygdaloids, alternating with thick masses of a highly feldspathic, sometimes porphyritic, pale-coloured slaty rock, with its cleavage-laminae dipping inwards at a high angle.
Lower down, towards Parapara and Ortiz, where the amygdaloids and phonolites are connected with the grunstein, everything assumes a basaltic aspect. Balls of grunstein heaped one upon another, form those rounded cones, which are found so frequently in the Mittelgebirge in Bohemia, near Bilin, the country of phonolites. The following is the result of my partial observations.
This phenomenon is the more remarkable, as the grunsteins which are called primitive almost always contain quartz in Europe. The most general dip of the slates of Piedras Azules, of the grunsteins of Parapara, and of the pyroxenic amygdaloids embedded in strata of grunstein, does not follow the slope of the ground from north to south, but is pretty regular towards the north.
These rocks possess an extremely varying character; they consist of black, brown, and grey, compact, basaltic bases, with numerous crystals of augite, hornblende, olivine, mica, and sometimes glassy feldspar. A common variety is almost entirely composed of crystals of augite with olivine. These compact lavas alternate with tuffs, amygdaloids, and wacke, and in some places with coarse conglomerate.
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