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Updated: June 24, 2025
The other is a red granite, which penetrates the dark variety everywhere in veins. Von Oeynhausen and Von Dechen. The general height of the veins is from 16 to 20 feet, but some are much higher. Granite, syenite, and those porphyries which have a granitiform structure, in short all Plutonic rocks, are frequently observed to contain metals, at or near their junction with stratified formations.
These porphyries, moreover, with the exception of the one blackish stratum, and of the one indurated, white tufaceous bed, differ from the beds composing the Uspallata range in the line of the Villa Vicencio Pass. I will now give, first, a sketch of the structure of the range, as represented in the section, and will then describe its composition and interesting history.
Pentland, 12,454 feet above the sea; and the neighbouring peaks, composed of dark purple and whitish porphyries, some obscurely stratified with a westerly dip, and others without a trace of stratification, must exceed 13,000 feet in height.
As the strata consist of black, white, greenish and brown-coloured rocks, and as the intrusive porphyries are so brightly tinted, a most extraordinary view was presented, like a coloured geological drawing. These denuded granitic points, I estimated at about nine thousand feet in height above the sea.
The great and wondrous phenomenon then went on without violence the outpouring was enormous, and the seething fused matter, ejected from the bowels of the earth, spread slowly and peacefully in the form of vast level plains, or what are called mamelons or mounds. It was at this epoch that the rocks called feldspars, syenites, and porphyries appeared.
Seeing that the interstratified porphyries are never vesicular and often not even amygdaloidal, we must conclude that the pile was formed in deep water; how then came so many fragments to be well rounded and so many to remain angular, sometimes the two kinds being equally mingled, sometimes one and sometimes the other preponderating?
Sophia, enumerates the colors, the shades, and the spots of ten or twelve marbles, jaspers, and porphyries, which nature had profusely diversified, and which were blended and contrasted as it were by a skilful painter.
These brecciated porphyries could generally be distinguished at once from the metamorphosed, porphyritic breccia- conglomerates, by all the fragments being angular and being formed of the same variety, and by the absence of every trace of aqueous deposition.
PORPHYRITIC STRUCTURE. Rocks whose ground mass has scattered through it large conspicuous crystals are said to be PORPHYRITIC, and it is especially among volcanic rocks that this structure occurs. The ground mass of porphyries either may be glassy or may consist in part of a felt of minute crystals; in either case it represents the consolidation of the rock after its outpouring upon the surface.
On the conical summit are hard quartzoze porphyries, which have apparently forced up the gneiss and slates, which dip in all directions from the top, and are full of injected veins of quartz. Below 7000 feet, mica-schist prevails, always inclined at a very high angle; and I found jasper near Namtchi, with other indications of Plutonic action.
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