Believing, then, the catacombs to have been begun within a few years after the first preaching of Christianity in Rome, there is abundant evidence to prove that their construction was continued during the time when the Church was persecuted or simply tolerated, and that they were extended during a considerable time after Christianity became the established creed of the empire. Indeed, several catacombs now known were not begun until some time after Constantine's conversion.[C] They continued to be used as burial-places certainly as late as the sixth century. This use seems to have been given up at the time of the frequent desolation of the land around the walls of Rome by the incursions of barbarians, and the custom gradually discontinued was never resumed. The catacombs then fell into neglect, were lost sight of, and their very existence was almost forgotten. But during the first five hundred years of our era they were the burial-places of a smaller or greater portion of the citizens of Rome,--and as not a single church of that time remains, they are, and contain in themselves, the most important monuments that exist of the Christian history of Rome for all that long period. [Footnote C: For instance, about the middle of the fourth century, St. Julius, then Pope, is said to have begun three. See Marchi's _Momumenti delle Arti Cristiane_, p. 82.] It has been much the fashion during the last two centuries, among a certain class of critics hostile to the Roman Church, and sometimes hostile to Christianity, to endeavor to throw doubts on the fact of this immense amount of underground work having been accomplished by the Christians. It has been said that the catacombs were in part the work of the heathen, and that the Christians made use of excavations which they found ready to their hand. Such and other similar assertions have been put forward with great confidence; but there is one overwhelming and complete answer to all such doubts,--a visit to the catacombs themselves. No skepticism can stand against such arguments as are presented there. Every pathway is distinctly the work of Christian hands; the whole subterranean city is filled with a host of the Christian dead. But there are other convincing proofs of the character of their makers. These are of a curiously simple description, and are due chiefly to the investigations of late years. Nine tenths of the catacombs now known are cut through one of the volcanic rocks which abound in the neighborhood of Rome. Of the three chief varieties of volcanic rock that exist there, this is the only one which is of little use for purposes of art or trade. It could not have been quarried for profit. It would not have been quarried, therefore, by the Romans, except for the purposes of burial,--and the only inscriptions and other indications of the character of the occupants of these burial-places prove that they were Christian.[D] They are very different from the