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Updated: May 24, 2025
If the base be greenstone, basalt, or pitchstone, the rock may be denominated greenstone-porphyry, pitchstone-porphyry, and so forth. The old classical type of this form of rock is the red porphyry of Egypt, or the well-known "Rosso antico." RED QUARTZIFEROUS PORPHYRY is a much more siliceous rock, containing about 70 or 80 per cent of silex, while that of Egypt has only 62 per cent.
Scrope gives as an illustration of this structure a resinous trachyte or pitchstone-porphyry in one of the Ponza islands, which rise from the Mediterranean, off the coast of Terracina and Gaeta. The whole rock is in a state of decomposition, "and when the balls," says Mr.
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