ingle
case it is recorded that a man who disappeared into the cavity was in
a moment cast forth in the rush of waters which in this, as in many
other cases, spouts forth as the walls of the opening come together.

Sometimes these rents are attended by a dislocation, which brings the
earth on one side much higher than on the other. The step thus
produced may be many miles in length, and may have a height of twenty
feet or more. It needs no argument to show that we have here the top
of a fault such as produced the shock, or it may be one of a secondary
nature, such as any earthquake is likely to bring about in the strata
which it traverses. In certain cases two faults conjoin their action,
so that a portion of the surface disappears beneath the earth,
entombing whatever may have stood on the vanished site. Thus in the
great shock known as that of Lisbon, which occurred in 1755, the stone
quay along the harbour, where many thousand people had sought refuge
from the falling buildings of the city, suddenly sank down with the
multitude, and the waters closed over it; no trace of the people or of
the structure was to be found after the shock was over. There is a
story to the effect that during the same earthquake an Arab village in
northern Africa sank down, the earth on either side closing over it,
so that no trace of the habitations remained. In both these instances
the catastrophes are best explained by the diagram.

[Illustration: Fig. 21.--Diagram showing how a portion of the earth's
surface may be sunk by faulting. Fig. A shows the original position;
B, the position after faulting; b b' and c c' the planes of the
faults; the arrows the direction of the movement.]

In the earthquake of 1811 the alluvial plains on either side of the
Mississippi at many points sank down so that arable land was converted
into lakes; the area of these depressions probably amounted to some
hundred square miles. The writer, on examining these sunken lands,
found that the subsidences had occurred where the old moats or
abandoned channels of the great river had been filled in with a
mixture of decaying timber and river silt. When violently shaken, this
loose-textured _debris_ naturally settled down, so that it formed a
basin occupied by a crescent-shaped lake. The same process of settling
plentifully goes on wherever the rocks are still in an uncemented
state. The result is often the production of changes which lead to the
expulsion of gases.