ave been many
centuries before that period. The celebrated Pozzolana sand makes the
best mortar in the world, from its gritty nature. This valuable sand
is found to any extent nearly all over the Campagna of Rome, in
horizontal beds or layers between the beds of tufa; some of the tufa
itself, which is sandstone, may be scraped into this sand, but it is
easier to take it as ready provided by nature. People once accustomed
to the use of this sand can not do without it, and hundreds of carts
filled with it may be seen daily traversing the Campagna, conveying it
either to Rome, or to Ostia, or to Porto, for exportation. The
horizontal layers or beds of this sand are not usually more than six
feet thick, although they extend at a certain level over the whole
surface of the country. It is therefore excavated in horizontal
corridors, with various branches, extending for many miles,
undermining the whole surface of the soil, but not in large or deep
pits, so that the name of sand-pit is rather deceitful to American
people, who commonly imagine it to be always a large and deep pit to
which these roads lead only; this is not always the case, the roads
themselves being excavated in the layer of sand, and frequently
themselves the sand-pits. Sometimes there are different layers of sand
at different levels, and in some cases there may be two sand-pit roads
one over the other, with the bed of hard tufa between them.

We are told in the _Acta Sanctorum_ that one of the punishments
inflicted on the Christians by the Emperor Maximinus in the sixth
persecution, A.D. 35, was digging sand and stone. The martyrs,
Ciriacus and Sisinnus are especially mentioned as ordered to be
strictly guarded, and compelled to dig sand and to carry it on their
own shoulders.

Some of the catacombs were evidently made under tombs by the side of
the road, and in that of S. Calixtus there are remains of the tomb on
the surface of the ground. The burial-chapels of the fourth century
commonly found over a catacomb probably replace earlier tombs. The
church of S. Urban is now considered to have been a family tomb of the
first century, made into a church long afterwards.

[Illustration: STONE COFFIN WITH OPEN SIDE.]

Many inscriptions are preserved relating to the preservation of a tomb
with the land belonging to it in perpetuity, and they frequently
mention the number of feet along the road and in the field. Their size
varies enormously. Horace mentions