The Appalachian mountain system in Newfoundland can be subdivided broadly into three distinct geologic provinces, each characterized by a different Precambrian assemblage and/or early Paleozoic depositional and structural history. These provinces are, from west to east, the Western platform, the Central Paleozoic mobile belt, and the Avalon platform. The Precambrian rocks that underlie the platforms on opposite sides of the system have little in common. Those on the west belong to the Grenville province of the Canadian shield, whereas those on the east are younger and much less metamorphosed. The platform areas were sites of shallow-water marine deposition during the Cambrian and early Ordovician, and the platform cover rocks are thin and contrast sharply in facies with t e thick, clastic sedimentary and volcanic rocks that accumulated in the Central Paleozoic mobile belt. Cambrian and early Ordovician rocks of geosynclinal aspect were emplaced tectonically on carbonate rocks of the Western platform during the medial Ordovician; the upper parts of the transported rocks include large mafic and ultramafic intrusions in association with mafic volcanic rocks. During the Silurian Period, development of the mountain system contrasted sharply with earlier Paleozoic development. Silurian volcanic rocks and sandstone, commonly red and largely of terrestrial accumulation, overlie older platforms and mobile zones alike across the system. Devonian rocks are exiguous, but the widely separated occurrences are all of terrestrial accumulation, suggesting a continuation o conditions established during the Silurian. The climactic orogeny (Acadian) that affected the Newfoundland Appalachians occurred during the Devonian Period; it is evidenced by folding, granite intrusion, and regional metamorphism that affected most of Newfoundland and transformed the Central Paleozoic mobile belt into a stable folded zone.