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geosyncline Large, downwarp structure generally of considerable extent, which may develop along a continental margin. The term was introduced originally by James Hall in 1859, and later an elaborate terminology was developed to describe and interpret the component parts. Many of these have fallen into disuse since the replacement of geosynclinal theory by more unifying theory of plate tectonics; the terms ‘eugeocline’ and ‘miogeocline’ are used today to denote particular rock associations.