The ophiolites of southern Quebec are thrust sheets. Prior to their folding with the country-rock. they were emplaced as solid masses into the thick metasedimentary and metavolcanic geosynclinal prism of Notre Dame Trough, in Early Ordovician time. The occurrence of the ophiolites is controlled by their structural and stratigraphic position. Complete ophiolites occur as stratified sheets lying structurally above the Cambrian Caldwell Group, and they are overlain by a mélange assumed to be of Early Ordovician age. Dismembered ophiolites consist of peridotite sheets and lenses which may have been attached originally to the basal peridotite of the larger stratified sheets; they are tectonically intruded into Cambrian rocks.The stratified sheets have a simple, regularly-layered structure with no well developed sheeted-dike complex. Other feature s include a thin upper unit of gabbroic and basaltic rocks with a relatively thick ultramafic cumulate at the base and a thicker lower unit of Alpine peridotite. There is apparently no transition zone between the upper and lower units. All these features suggest that the ophiolites of southern Quebec represent possible fragments of an oceanic crust formed on a rapidly spreading ridge. It is assumed that they have been emplaced on the Early Ordovician continental margin by obduction and contemporaneously with the development of a subduction zone, which may have caused the magmatism that generated the adamellitic granites intruded into the ophiolitic complexes, and the calcalkatline volcanic rocks of the Lower to Middle Ordovician Ascot and Weedon Groups.