Église de Saint Augustin, Paris 8, built in 1860-1871. Designed by Victor Baltard (1805-1874), in an eclectic style combining Tuscan Gothic, Romanesque and Byzantine elements.
Wikipedia notes that "the key innovation of Baltard was the use of cast iron for the structure of the church which permitted Baltard to greatly reduce the thickness of the walls, and to eliminate the need for heavy buttresses outside".