Abstract
At the conclusion of my last paper on the Cheviot andesites and porphyrites (Geol. Mag. Vol. X. p. 262), I called attention to the close resemblance between the Cheviot hypersthene andesite (or porphyrite) and a rock from Steinerne Mann, Nahe, which has been called a proterobase. Both rocks are black in colour and consist macroscopically of glassy felspars (Mikrolin) imbedded in a groundmass which has a resinous or semi-resinous lustre. Microscopically they consist of two generations of felspar, pyroxene mostly rhombic, magnetite, and a glassy base with various devitrification products.
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