It is difficult, if not impossible, to compute the number of graves in these vast cemeteries. Some seventy thousand have been counted, but they are a mere fraction of the whole, as only a small part of this great necropolis has been explored. From lengthened observation Father Marchi estimates the average number of graves to be ten, five on each side, for every seven feet of gallery. Upon this basis he computed the entire number in the Catacombs to be seven millions! The more accurate estimate of their extent made by Sig. Michele De Rossi would allow room for nearly four millions of graves, or, more exactly, about three million eight hundred and thirty-one thousand.[10] This seems almost incredible; but we know that for at least three hundred years, or for 22 ten generations, the entire Christian population of Rome was buried here. And that population, as we shall see, was, even at an early period, of considerable size. In the time of persecution, too, the Christians were hurried to the tomb in crowds. In this silent city of the dead we are surrounded by a “mighty cloud of witnesses,” “a multitude which no man can number,” whose names, unrecorded on earth, are written in the Book of Life. For every one who walks the streets of Rome to-day are hundreds of its former inhabitants calmly sleeping in this vast encampment of death around its walls--“each in his narrow cell forever laid.”[11] Till the archangel awake them they slumber. “It is scarcely known,” says Prudentius, “how full Rome is of buried saints--how richly her soil abounds in holy sepulchres.” These graves were once all hermetically sealed by slabs of marble, or tiles of _terra cotta_. The former were generally of one piece, which fitted into a groove or mortice cut in the rock at the grave’s mouth, and were securely cemented to their places, as, indeed, was absolutely necessary, from the open character of the galleries in which the graves were placed. Sometimes fragments of heathen tombstones or altars were used for this purpose. The tiles were generally smaller, two or three being required for an adult grave. They were arranged in panels, and were cemented with plaster, on which a name or symbol was often rudely scratched with a trowel while soft, as in the following illustration. Most of these slabs and tiles have 23 disappeared, and many of the graves have long been rifled of their contents. In others may still be seen the mouldering skeleton of what was once man in his strength, woman in her beauty, or a child in its innocence and glee. The annexed engraving exhibits two graves, one of which is partially open, exposing the skeleton which has reposed on its rocky bed for probably over fifteen centuries. [Illustration: Fig. 6.--Loculi--Open and Closed.]