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Updated: June 27, 2025
The feldspar and the mica, for example, as well as the schorl, are found variously coloured in different granites, and coloured in various proportions.
We found it difficult in the evening to stem the current, and we passed the night in a wood a little above Mendaxari; which is another granitic rock traversed by a stratum of quartz. We found in it a group of fine crystals of black schorl. On the 29th, the air was cooler. We had no zancudos, but the sky was constantly clouded, and without stars. I began to regret the Lower Orinoco.
The red granite, black sienite, and red porphyry of Egypt, which are seen at Rome in obelisks, columns, and sarcophagi, are amongst the most durable compound stones; but the grey granites of Corsica and Elba are extremely liable to undergo alteration: the feldspar contains much alkaline matter; and the mica and schorl, much protoxide of iron.
A schorl rock, of a blue colour and fine grain, composed of tourmaline and quartz, forms the bed of the Macquarie at the Cataract; and, in immediate contact with it, a mass of mica slate of alternate rose, pink, and white, was observed, which must have been covered by the waters of the river when Mr. Oxley descended it.
Talcose Granite, or Protogine of the French, is a mixture of feldspar, quartz, and talc. It abounds in the Alps, and in some parts of Cornwall, producing by its decomposition the kaolin or china clay, more than 12,000 tons of which are annually exported from that country for the potteries. The former of these is an aggregate of schorl, or tourmaline, and quartz.
From Djjdda to this place, our route, though generally between hills and mountains, had been constantly over flat The lower hills are seldom higher than four or five hundred feet. The lowest range above Djidda is calcareous; but its rocks soon change into gneiss, and a species of granite, with schorl in the place of feldspath, accompanied by predominant masses of quartz, and some mica.
Among these black schorl or tourmaline, actinolite, zircon, garnet, and fluor spar are not uncommon; but they are too sparingly dispersed to modify the general aspect of the rock. They show, nevertheless, that the ingredients were not everywhere exactly the same; and a still greater difference may be traced in the ever-varying proportions of the feldspar, quartz, and mica.
Near Valparaiso the prevailing rock is gneiss, generally including much hornblende: concretionary balls formed of feldspar, hornblende and mica, from two or three feet in diameter, are in very many places conformably enfolded by the foliated gneiss: veins of quartz and feldspar, including black schorl and well-crystallised epidote, are numerous.
I saw distinctly that, wherever they crossed each other, the veins containing mica and black schorl traversed and drove out of their direction those which contained only white quartz and feldspar. According to the theory of Werner, the black veins were consequently of a more recent formation than the white.
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