Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: June 28, 2025
Notwithstanding the difference of composition in the recent lavas of the Peak, there is a certain regularity of position, which must strike the naturalist least skilled in geognosy. The great elevated plain of Retama separates the black, basaltic, and earthlike lava, from the vitreous and feldsparry lava, the basis of which is obsidian, pitch-stone, and phonolite.
Near the base of this hill, I observed beds of white tuff, intersected by numerous dikes, some of amygdaloidal basalt and others of trachyte; and beds of slaty phonolite with the planes of cleavage directed N.W. and S.E. Parts of this rock, where the crystals were scanty, closely resembled common clay-slate, altered by the contact of a trap-dike.
In the other solution of the problem, the masses of phonolite, amygdaloid, and grunstein, which are found in the south of the ravine of Piedras Azules, are separated from the grunsteins and serpentine rocks that cover the declivity of the mountains north of the ravine.
The phonolite is of a greenish-grey colour, and is full of minute acicular crystals of feldspar; in most parts it has a conchoidal fracture, and is sonorous, yet it is crenulated with minute air-cavities. In a S.W. direction from Lot, there are some other remarkable columnar pinnacles, but of a less regular shape, namely, Lot's Wife, and the Asses' Ears, composed of allied kinds of rock.
These three lower beds are covered by an unconformable mantle D of stratified sandy earth, including many pebbles of quartz, pumice and phonolite, land and sea-shells. M. d'Orbigny has been so obliging as to name for me the twenty species of Mollusca embedded in the two gravel beds: they consist of: Volutella angulata, d'Orbigny, "Voyage" Mollusq. and Pal. 2. Voluta Braziliana, Sol 3.
It is the real porphyrschiefer of Werner; and it would be difficult to distinguish, in a collection of stones, the phonolite of Parapara from that of Bilin, in Bohemia. It does not, however, here form rocks in grotesque shapes, but little hills covered with tabular blocks, large plates extremely sonorous, translucid on the edges, and wounding the hands when broken.
The whole seems to be of volcanic origin; although there is no appearance of any crater, or of any one central eminence. The most remarkable feature is a hill 1,000 feet high, of which the upper 400 feet consist of a precipitous, singularly shaped pinnacle, formed of columnar phonolite, containing numerous crystals of glassy feldspar, and a few needles of hornblende.
In the middle of the Malpays, at the height of the cavern of ice, we found among the vitreous lavas with pitch-stone and obsidian bases, blocks of real greenish-grey, or mountain-green phonolite, with a smooth fracture, and divided into thin laminae, sonorous and keen edged.
Phonolite is broken shale, a thin, sharp rock that gives forth a pleasant, metallic sound when struck, like shattered crockery. For a mile this deposit lay along the trail across the width of the pass. For the bare-footed pony there was cruelty in every step. The barrier of rock was far more formidable than the river in its flood. Van was not to be halted in his object.
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.
Word Of The Day
Others Looking