He entered the Society of Jesus in Rome 12 November 1814, shortly after the re-establishment of the order, and was professor of humanities successively in the colleges of Terni, Reggio Emilia, Modena and St. Andrew of the Quirinal.
His archæological pursuits recommended him to Gregory XVI as qualified to succeed Settele in the position of Conservatore dei sacri cimiteni di Roma (1842), charged with supervising the early Christian burial places in and around the city.
[2] About this time Marchi made the acquaintance of youthful Giovanni Battista De Rossi, who accepted him as master and thenceforth accompanied him on his visits to the Roman catacombs.
In spring 1842, Marchi conducted a tour of the Catacombs of Saint Agnes for James Roosevelt Bayley, John Bede Polding, and a number of people from the English College, Rome.
He proved the Christian origin of these ancient burial-places and, through his studies, brought about (21 March 1845) the discovery of the crypts of Saints Peter and Hyacinth in the catacomb of St.