The reader cannot expect to find in these pages a description of this class of basilicas; that of S. Peter's alone would require several volumes. I have in my modest library not less than twenty-two volumes on the subject, an insignificant fraction of the Petrine literature. And what do we know about S. Peter's? Very little in comparison with the amount of knowledge that lies yet unpublished in the volumes of Grimaldi,[69] in the archives of the Vatican, in epigraphic, historical and diplomatic documents scattered among various European libraries. The history of the building has yet to be written. Duchesne's "Liber Pontificalis" and de Rossi's second volume of the "Inscriptiones Christianæ" provide the necessary foundations for such a work. Let us hope that the Vatican will soon find its own Rohault de Fleury.[70] The following sketch of the origin of the two leading sacred edifices of Rome may answer the scope of the present chapter. But let me repeat once again the declaration that I write about the monuments of ancient Rome from a strictly archæological point of view, avoiding questions which pertain, or are supposed to pertain, to religious controversy. For the archæologist the presence and execution of SS. Peter and Paul in Rome are facts established beyond a shadow of doubt by purely monumental evidence. There was a time when persons belonging to different creeds made it almost a case of conscience to affirm or deny _a priori_ those facts, according to their acceptance or rejection of the tradition of any particular church. This state of feeling is a matter of the past, at least for those who have followed the progress of recent discoveries and of critical literature. However, if my readers think that I am assuming as proved what they still consider subject for discussion, I beg to refer them to some of the standard works published on this subject by writers who are above the suspicion of partiality. Such are Döllinger's "First Age of Christianity" (translated by Henry Nutcombe Oxenham, second edition, London, Allen, 1867); Bishop Lightfoot's "Apostolic Fathers," part ii., London, Macmillan, 1885, one of the most beautiful and conclusive works on early Christian history and literature; and de Rossi's "Bullettino di archeologia cristiana," for 1877. Bishop Lightfoot justly remarks that when Ignatius--the second apostolic father, a contemporary of Trajan--writes to the Romans "I do not command you, like Peter and Paul," the words are full of meaning, if we suppose him to be alluding to the personal relations of the two apostles with the Roman Church. In fact, the reason for his use of this language is the recognition of the visit to Rome of S. Peter as well as S. Paul, which is persistently maintained in early tradition; and thus it is a parallel to the joint mention of the two apostles in "Clement of Rome" (p. 357). Döllinger adds: "That S. Peter worked in Rome is a fact so