about two thousand years, they have here and
there been subjected to slight oscillations which are most likely
connected with the movement of volcanic matter toward the vents where
it is to find escape. The most interesting evidence of this nature is
afforded by the studies which have been made on the ruins of the
Temple of Serapis at Puzzuoli. This edifice was constructed in
pre-Christian times for the worship of the Egyptian god Serapis, whose
intervention was sought by sick people. The fact that this divinity of
the Nile found a residence in this region shows how intimate was the
relation between Rome and Egypt in this ancient day. The Serapeium was
built on the edge of the sea, just above its level. When in modern
days it began to be studied, its floor was about on its original
level, but the few standing columns of the edifice afford indubitable
evidence that this part of the shore has been lowered to the amount of
twenty feet or more and then re-elevated. The subsidence is proved by
the fact that the upper part of the columns which were not protected
by the _debris_ accumulated about them have been bored by certain
shellfish, known as _Lithodomi_, which have the habit of excavating
shelters in soft stone, such as these marble columns afford. At
present the floor on which the ruin stands appears to be gradually
sinking, though the rate of movement is very slow.

Another evidence that the ejections may travel for a great distance
underground on their way to the vent is afforded by the fact that
Vesuvius and AEtna, though near three hundred miles apart, appear to
exchange activities--that is, their periods of outbreak are not
simultaneous. Although these elements of the chronology of the two
cones may be accidental, taken with similar facts derived from other
fields, they appear to indicate that vents, though far separated from
each other, may, so to speak, be fed from a common subterranean
source. It is a singular fact in this connection that the volcano of
Stromboli, though situated between these two cones, is in a state of
almost incessant activity. This probably indicates that the last-named
vent derives its vapours from another level in the earth than the
greater cones. In this regard volcanoes probably behave like springs,
of which, indeed, they may be regarded as a group. The reader is
doubtless aware that hot and cold springs often escape very near
together, the difference in the temperature being due to the