a compact rock derived from siltstone, mudstone, or shale which has been hardened or consolidated by pressure, cementation, or heat. It does not break into thin layers, as do shale and slate. It is regarded as a product of weak metamorphism.
A compact rock, derived from mudstone or shale but more highly indurated than either of those rocks. It lacks the fissility of shale or the cleavage of slate. Argillite is a product of weak metamorphism.