s of the earth's crust. The regions, indeed, where within the
period of human history shocks of devastating energy have occurred do
not include more than one fifteenth part of the earth's surface. There
is a common notion that these movements are most apt to happen in
volcanic regions. It is, indeed, true that sensible shocks commonly
attend the explosions from great craters, but the records clearly show
that these movements are very rarely of destructive energy. Thus in
the regions about the base of Vesuvius and of AEtna, the two volcanoes
of which most is known, the shocks have never been productive of
extensive disaster. In fact, the reiterated slight jarrings which
attend volcanic action appear to prevent the formation of those great
and slowly accumulated strains which in their discharge produce the
most violent tremblings of the earth. The greatest and most continuous
earthquake disturbances of history--that before noted in the early
days of this century, in the Mississippi Valley, where shocks of
considerable violence continued for two years--came about in a field
very far removed from active volcanoes. So, too, the disturbances
beneath the Atlantic floor which originated the shocks that led to the
destruction of Lisbon, and many other similar though less violent
movements, are developed in a field apparently remote from living
volcanoes. Eastern New England, which has been the seat of several
considerable earthquakes, is about as far away from active vents as
any place on the habitable globe. We may therefore conclude that,
while volcanoes necessarily produce shocks resulting from the
discharge of their gases and the intrusion of lava into the dikes
which are formed about them, the greater part of the important shocks
are in no wise connected with volcanic explosions.

With the exception of the earthquake in the Mississippi Valley, all
the great shocks of which we have a record have occurred in or near
regions where the rocks have been extensively disturbed by
mountain-building forces, and where the indications lead us to
believe that dislocations of strata, such as are competent to rive the
beds asunder, may still be in progress. This, taken in connection with
the fact that many of these shocks are attended by the formation of
fault planes, which appear on the surface, lead us to the conclusion
that earthquakes of the stronger kind are generally formed by the
riving of fissures, which may or may not be develope