 Characteristic of this cathedral is the contrast between the rich decoration, including, in front of the altar, slabs of rare marble from the Roman Forum, a gift from Pope Pius IX, and the sombre granite of the structure.Between 1803 and 1844, because the cathedral was unusable, the abbey church of Our Lady in Saint Melaine of Rennes ( "de Rennes") served as a pro-cathedral (i.e.a substitute).