1. (Science: geology) a rock of igneous origin, consisting of augite and triclinic feldspar, with grains of magnetic or titanic iron, and also bottle-green particles of olivine frequently disseminated.
It is usually of a greenish black colour, or of some dull brown shade, or black. It constitutes immense beds in some regions, and also occurs in veins or dikes cutting through other rocks. It has often a prismatic structure as at the giants Causeway, in Ireland, where the columns are as regular as if the work of art. It is a very tough and heavy rock, and is one of the best materials for macadamizing roads.
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... low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, and basalt. Opal is one of the mineraloids that can form or replace fossils. The resulting fossils, though not of any extra scientific interest, appeal ...
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