Parker, John Henry: The archaeology of Rome (1,2): Illustrations to I. The primitive fortifications. II. The walls and gates of Rome. III. The historical constructions of walls (Oxford [u.a.], 1874)

Construction of Walls.

Plate VI.
Opus Incertum.
1. Irregular Work. These names apply to the surface only.
The ornamental construction was intended to be seen ; the real con-
struction in most cases is a mass of concrete or rubble, called fartura
in Latin, and a sacco in Italian. This Opus incertum is an earlier
stage of the Opus reticulatum; it is generally characteristic of the
time of the Republic, as the reticulatum is of the time of the Early
Empire. The larger blocks of tufa, looking at first sight like large
bricks, were continued in use as late as the time of Tiberius, but
are then used with Opus reticulatum of the regular kind.
2. House on the Viminal Hill. The situation is against the
cliff, near the remains of the Lavacrum of Agrippina (destroyed in
1872), and opposite to the church of S. Vitale, which is at the foot
of the Quirinal. The new grand street of modern Rome, called the
Via Nazionale, now passes between these two hills upon a high
embankment. The house has not yet been identified, but is of the
time of Sylla. The oblong blocks of tufa have even more of the
appearance of large bricks than in the Emporium; the wall that is
faced in that manner is hollow, a space of two feet wide being left
in it, probably to keep the rooms dry, as the moisture of the earth
behind would penetrate through a solid wall.