[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 sepulchres of the great and rich families of Rome, who lined the Appian, the Nomentan, and Flaminian Ways with their tombs, even now magnificent in ruin; very different, too, from the columbaria, or pigeon-holes, in which the ashes of the less wealthy were packed away; and still more different from the sad undistinguished ditch that received the bodies of the poor:—­

  “Hoc miserae plebi stabat commune sepulcrum.”

[Footnote D:  The volcanic rocks are the Tufa litoide, very hard, and used for paving and other such purposes; difficult to be quarried, and unfit for graves on account of this difficulty.  The Tufi granulare, a soft, friable, coarse-grained rock, easily cut,—­fitted for excavation.  It is in this that the catacombs are made.  It is used for very few purposes in Rome.  One may now and then see some coarse filling-up of walls done with it, or its square-cut blocks piled up as a fence.  The third is the Pura pozzolana,—­which is the Tufa granulare in a state of compact sand, yielding to the print of the heel, dug like sand, and used extensively in the unsurpassed mortar of the Roman buildings.]