Note 47 — THE FLAVIAN AMPHITHEATRE, COMMONLY CALLED THE COLOSSEUM.

The basin of Nero was possibly cut through the layer of tufa, which underlies the whole soil of Rome, down to the clay under it. In some excavations made under my direction in a cave under the Aventine, near the Marmorata, which was the mouth of the Aqua Appia, a level bed of white clay was found under the tufa rock of which the Aventine Hill consists; this would account for the walls in the central part built of concrete and brick, on this clay foundation, having been frequently damaged by earthquakes, while the great stone arcades, being built upon the tufa rock, did not suffer from the shocks. Clay is always a bad foundation to build upon, and there are always settlements in buildings that rest upon it. The objection to this theory is, that the surface of the water would be twenty-one feet below the arena and the foot of the podium.