The gold-quartz veins are mainly in the Ordovician and Silurian rocks; but some also occur in the Devonian, and there are impregnations of gold in tufas of Devonian age. [Please select]
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_) From this second turning, the gallery continued close to the line of separation of the marls and the tufas. [Please select]
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There are also numerous other kinds: for instance, in Campania, red and black tufas; in Umbria, Picenum, and Venetia, white tufa which can be cut with a toothed saw, like wood. [Please select]
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