The cast iron work is evident at the base of the dome and in the slender columns topped with statues along the nave.

In 1862 Baltard was invited to design a new Parisian church, St Augustin, in which, for economy, an iron framework was used for the first time in a French church. The metal structure, made into a decorative feature, is readily apparent in the interior of the church and its lofty dome, while the masonry exterior combines an eclectic mix of Byzantine, Gothic and Baroque elements. To resolve the difficulties of the wedge-shaped site, bordered by two busy streets, Baltard placed the entrance at the narrow end facing Rue de Laborde and included a series of chapels that increase in depth as they approach the choir.