Definition
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Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks
From the Middle High German verb gneist (to spark; so called because the rock glitters)
Coarse Grained Rock, Medium Grained Rock, Opaque Rock
Greywacke is defined as a dark coarse-grained sandstone rock which contains more than 15 per cent clay
From German Grauwacke, from grau grey + wacke
Fine Grained Rock, Opaque Rock