Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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Werner.) Of The
Valley Of Tribisch In Saxony Is Very Remarkable; But The Latter,
Which Belong To An Extended And Metalliferous Formation Of
Porphyry, Often Contain Quartz, Which Is Wanting In The Modern
Lavas.
When the basis of the lavas of the Malpays changes from
pitchstone to obsidian, its colour is paler, and is mixed with
grey; in this case, the feldspar passes by imperceptible gradations
from the common to the vitreous.
Sometimes both varieties meet in
the same fragment, as we observed also in the trappean porphyries
of the valley of Mexico. The feldsparry lavas of the Peak, of a
much less black tinge than those of Arso in the island of Ischia,
whiten at the edge of the crater from the effect of the acid
vapours; but internally they are not found to be colourless like
that of the feldsparry lavas of the Solfatara at Naples, which
perfectly resemble the trappean porphyries at the foot of
Chimborazo. 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. These masses were the same as the
porphyrschiefer of the mountain of Bilin in Bohemia; we recognised
in them small long crystals of vitreous feldspar.
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:
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