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disturbance, some little way distant. If the waves sent off
from the paddles of a steam-vessel be watched brealdng on
the sloping shore of a still river, the water will be seen first
to retire two or three feet, and then to return in little breakers,
precisely analogous to those consequent on an earthquake.
From the oblique direction in which the waves are sent oft’
from the paddles, the vessel has proceeded a long way ahead,
before the undulation reaches the shore; and hence it is at
once manifest, that this movement hears no relation to the
actual displacement of the fluid from the bulk of the vessel.
Indeed, it seems a general circumstance, that iu all cases
where the equilibrium of an undulation is thus destroyed,
the water is drawn from the resisting surface to form the
advancing breaker.* Considering then a wave produced hy
an earthquake as an ordinary undulation proceeding from
some point or line in the offing, we can see the cause, first
of its occurrence some time after the shock; secondly, of
its affecting the shores of the mainland and of outlying islets
in a uniform manner—namely, the water retiring first, and
then returning in a mountainous breaker; and lastly, of its
size being modified (as appears to be the case) by the
form of the neighbouring coast. For instance Talcuhano
and Callao are situated at the head of great shoaling hays,
and they have always suffered from this phenomenon;
whereas, the town of Valparaiso, which is seated close on the
border of a profound ocean, though shaken by the severest
earthquakes, has never been overwhelmed by one of these
terrific deluges. On this view, we have only to imagine, in
the case of Concepcion, a point of disturbance in the bottom
of the sea in a south-west direction, whence the wave was
seen to travel, and where the land was elevated to a greater
height than any other part,—and the whole phenomenon will
be explained.
It is probable that near every coast, the chief line of dis•
I am in d e b te d to M r. W h ew e ll fo r e x p la in in g to m e tb e p ro b ab le
m o v em e n ts o n th e s h o re , o f a n u n d u la tio n o f w h ich th e e q u ilib rium has
b e e n d e s tro y e d .
turbance would be situated at that distance in the offing,
where the fluid whioh was most agitated, from overlying the
shallow bottom near the land, joined on to that part which
covered the depths (but slightly moved) of the ocean. In
all distant parts of the coast the small oscillations of the
sea, both at the moment of the great shock, and during the
lesser following ones, would be confounded with the undulation
propagated from the focus of disturbance, and hence
the series of movements would be undistinguishable.
The most remarkable effect (or perhaps speaking more correctly,
cause) of this earthquake was the permanent elevation
of the land. Captain FitzRoy having twice visited the island
of Santa Maria, for the purpose of examining every circumstance
with extreme accuracy, has brought a mass of evidence
in proof of such elevation, far more conclusive than
that on which geologists on most other occasions place implicit
faith. The phenomenon possesses an uncommon
degree of interest, from this particular part of the coast of
Chile having previously been the theatre of several earthquakes
of the worst class. It is almost certain, from the
altered soundings, together with the circumstance of the
bottom of the bay near Penco, consisting of hard stone,
that there has been an uplifting to the amount of four fathoms,
since the famous convulsion of 1751. With this additional
instance fresh before us, we may assume as probable, according
to the principles laid down by Mr. Lyell,* other small successive
elevations, and may fearlessly maintain that the problem
of the raised shells,! recorded by Ulloa, is explained.
Some of the consequences which may be deduced from the
phenomena connected with this earthquake are most important
in a geological point of v iew; but in the present work I
cannot do more than simply allude to the results. Although
it is known that earthquakes have been felt over enormous
* L y e ll’s G e o lo g y , b o o k ii., c h a p . xv i.
! I saw th e s e sh e lls in v e ry g r e a t q u a n titie s o n th e flan k s o f th e islan d
o f Q u ir iq u in a .
I ;