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The track then turned up a short, broad branch-Wady, running from south to north, and falling into the left bank of the "Dome Valley:" a few yards brought us to a halt at the ruins of El-Kubbah. We were perplexed by the shallow conical pits in the porphyritic trap, to the east and west of the "Dome Hill;" the ground is too porous for rain cisterns, and the depth is not sufficient for quarrying.
All are even grained, consisting of a mass of crystalline grains formed during one continuous stage of solidification, and no porphyritic crystals appear as in lavas. GRANITE, as we have learned already, is composed of three minerals, quartz, feldspar, and mica. According to the color of the feldspar the rock may be red, or pink, or gray.
The porphyritic lavas are affected by the action of the sulphuric acid: the alumine, magnesia, soda, and metallic oxides gradually disappear; and often nothing remains but the silex, which unites in mammillary plates, like opal. It is not easy to form an idea of the origin of these incrustations.
In the porphyritic conglomerate formation, in its lower and middle parts, there is very rarely any evidence, with the exception of the small quartz pebbles at Jajuel near Aconcagua, and of the single pebble of granite at Copiapo, of the existence of neighbouring land: in the upper parts, however, and especially in the district of Copiapo, the number of thoroughly well-rounded pebbles of compact porphyries make me believe, that, as during the prolonged accumulation of the gypseous formation the lower beds had already been locally upheaved and exposed to wear and tear, so it was with the porphyritic conglomerate.
These streams cross and re-cross its green bosom in all directions, forming by their interlacings a curious network of silvery lines, like the bright threads in the mine, or the white veins in the porphyritic slab. Observe this little flower, with its bright petals, growing by the wayside. That humble flower owes its beauty to yonder chain.
In the form of the adjacent hills I observed nothing peculiar, unless it be a contraction not very common of the lower parts of ravines. The geological structure is, as might be expected, more remarkable. Other summits of the range are porphyritic, but the hills of Wingen present a variety of rocks, within a small space.
All the varieties of porphyritic conglomerates and breccias pass into each other, and by innumerable gradations into porphyries no longer retaining the least trace of mechanical origin: the transition appears to have been effected much more easily in the finer-grained, than in the coarser-grained varieties.
Rode over in the morning to Longwood, and then proceeded to Gregory's Valley, lying between Longwood and The Barn. This valley, nearly 1700 feet in depth, appears at one period to have been the scene of great volcanic disturbances. The lowest rock I saw was a compact porphyritic one.
There are two large districts of mica-schists, passing into glossy clay-slate, and resembling the great formation in the Chonos Archipelago. In the valley of Guasco, an escarpment of porphyritic conglomerate is first seen high up the valley, about two leagues eastward of the town of Ballenar.
Guadaloupe, an active volcano, the height of which, according to Leboucher, is 799 toises; according to Amie, 850 toises. Montserrat, a solfatara; fine porphyritic lavas with large crystals of feldspar and hornblende near Galloway, according to Mr. Nugent. Nevis, a solfatara. St. Christopher's, a solfatara at Mount Misery. St.
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