The surface of the Campagna, especially of that part occupied by the 39 Catacombs, is almost exclusively of volcanic origin. The most ancient and lowest stratum of this igneous formation is a compact conglomerate known as _tufa lithoide_. It was extensively quarried for building, and the massive blocks of the Cloaca Maxima and the ancient wall of Romulus attest the durability of its character. Upon this rest stratified beds of volcanic ashes, pumice, and scoria, often consolidated with water, but of a substance much less firm than that of the _tufa lithoide_, and called _tufa granolare_. In insulated beds, rarely of considerable extent, in this latter formation, occurs another material, known as _pozzolana_. It consists of volcanic ashes deposited on dry land, and still existing in an unconsolidated condition. This is the material of the celebrated Roman cement, which holds together to this day the massy structures of ancient Rome. It was conveyed for building purposes as far as Constantinople, and the pier on the Tiber from which it was shipped is still called the Porto di Pozzolana. It is in these latter deposits exclusively that the _arenaria_, or sand pits, are found. The _tufa granolare_ is too firm, and contains too large a proportion of earth, to use as sand, and is yet too friable for building purposes. Yet it is in this material, entirely worthless for any economic use, that the Catacombs are almost exclusively excavated; while the _tufa lithoide_ and the _pozzolana_ are both carefully avoided where possible, the one as too hard and the other as too soft for purposes of Christian sepulture. Sometimes, indeed, as at the cemeteries of St. Pontianus and St. Valentinus, for special reasons, Catacombs were excavated in less suitable material; but still the substance removed--a shelly marl--was economically useless, and the galleries had to be supported by solid masonry. The _tufa granolare_, on the contrary, was admirably adapted for the 40 construction of these subterranean cemeteries. It could be easily dug with a mattock, yet was firm enough to be hollowed into _loculi_ and chambers; and its porous character made the chambers dry and wholesome for purposes of assembly, which was of the utmost importance in view of the vast number of bodies interred in these recesses. The differences of structure between the quarries or _arenaria_ and the Catacombs are no less striking. To this day, the vast grottoes from which the material for the building of the Coliseum was hewn, most probably by the Jewish prisoners of Titus, may still be seen on the Coelian hill. It is said that in those gloomy vaults were kept the fierce Numidian lions and leopards whose conflicts with the Christian martyrs furnished the savage pastime of the Roman amphitheatre. But nothing can less resemble the narrow and winding passages of the Catacombs than those tremendous caverns.