from the great variety in its examples of underground architectural
construction, and from the number of the paintings that are found upon
its walls. But its peculiar interest is, that it affords at one point a
marked example of the connection of an _arenarium_, or pit from which
_pozzolana_ was extracted, with the streets of the cemetery itself. At
this point, the bed of compact _tufa_, in which the graves are dug,
degenerates into friable and loosely compacted volcanic sand,--and it
was here, very probably, that the cemetery was begun, at a time when
every precaution had to be used by the Christians to prevent the
discovery of their burial-places. No other of the catacombs gives a
clearer exhibition of the differences in construction resulting from
the different objects of excavation. In the Acts known as those of St.
Valentine it is related, that in the time of Claudius many Christians
were condemned to work in certain sand-pits. Under cover of such
opportunities, occasions might be found in which hidden graves could
be formed in the neighboring harder soil. In digging out the sand, the
object was to take out the greatest quantity consistent with
safety, leaving only such supports as were necessary to hold up the
superincumbent earth. There are few regular paths, but wide spaces with
occasional piers,--the passages being of sufficient width to admit of
the entrance of beasts of burden, and even of carts. The soil crumbles
so easily, that no row of excavations one above another could be made in
it; for the stroke of the pick-axe brings it down in loose masses. The
whole aspect of the sand-pit contrasts strikingly with that of the
catacombs, with their three-feet wide galleries, their perpendicular
walls, and their tier on tier of graves.

The stratum of pozzolana at the Catacombs of St. Agnes overlies a
portion of the more solid stratum of tufa, and the entrance to the
sand-pit from the cemetery is by steps leading up from the end of a long