4. Sudden dislocations in the rocky crust of the earth, through
pressure acting in any direction.
5. Occasionally through the recoil due to explosive effects at
volcanic foci (p. 79-80).
The first and last of these I am, through subsequent light, disposed now
to withdraw or greatly to modify.
The first, the supposed "_snap and jar_, occasioned by the sudden and
violent rupture of solid rock masses," to which Mr. Scrope, in his very
admirable work on Volcanoes, is disposed to refer the impulse of
earthquake shocks (Scrope, 2nd edit., p. 294), I believe may be proved
on acknowledged physical principles--when applied to the known
elasticities and extensibilities of rocks, and keeping in view the small
thicknesses fractured _at the same instant_--to be capable of only the
most insignificant impulsive effects; and if we also take into
consideration that strata, if so fractured, are necessarily not _free_,
but surrounded by others above and below, any such impulsive effect
emanating from fracture may be held as non-existent or impossible. In
the statement of his views which follows, and in objecting to my second
and third possible causes (p. 295-296, headed "Objections to Mallet's
Theory"), Mr. Scrope appears to me to have fallen into the error of
assuming that the nature of the _impulse_, or the cause producing it,
forms any part of "my theory of earthquake movement," or in anywise
affects it. I carefully guarded against this in the original Paper
("Transactions, Royal Irish Academy," Vol. XXI., p. 60, and again, p.
97), when I stated "it is quite immaterial to the truth of my theory of
earthquake motion what view be adopted, or what mechanism be assigned,
to account for the original impulse."
As regards the fifth conjecture suggested by me, I am now, with better
knowledge and larger observation of volcanic phenomena, not prepared to
admit any single explosion at volcanic vents of a magnitude sufficient
to produce by its recoil an earthquake wave of any importance, or
extending to any great distance in the earth's crust. The rock of 200
tons weight, said to have been projected nine miles from the crater of
Cotopaxi, which I quoted from Humboldt as an example,[E] I believe to be
as purely mythical as the rock (_bloc rejetté_) of perhaps one-sixth of
that weight which, previous to the late eruption, lay in the middle of
the Atria dell Cavallo, and which it was roundly affirmed had been
_blown_ out of the crater, but which in reality had at some time rolled
down from near the top of the cone, after having been dislodged from
some part of the upper lip of the crater walls, where, as its wonderful
hardness and texture and its enamel-like surface showed, it had been
roasted for years probably.
Nor do I believe in the _sudden_ blowing away of one-half the crater and
cone of Vesuvius, or of any other volcano, at one effort, however
affirmed.
Nothing more than conjecture as to the nature of the impulse producing
great or small Earthquakes can, I believe, as yet be produced. That
there is some one master mechanism productive of most of the impulses of
great shocks is highly probable, but that more causes than one may
produce these impulses, and that the causes operative in small and long
repeated shocks, like those of Visp-Comrie and East Haddam, differ much
from those producing great Earthquakes, is almost certain.
We shall be better prepared to assign all of these when we have admitted
a true theory of volcanic action, and so are better able to see the
intimate relations in mechanism between seismic and volcanic actions.
It is not difficult meanwhile to assign the very probable mechanism of
those comparatively petty repercussions which are experienced in close
proximity to volcanic vents when in eruption, and which, though
certainly seismic in their nature, and powerful enough, as upon the
flanks of Etna, to crack and fissure well-built church-towers, can
scarcely be termed Earthquakes.
In my First Report I stated that almost nothing was known then of the
distribution of recorded Earthquakes in time or in space over our
globe's surface, and I proposed the formation and discussion of a
complete catalogue of all recorded Earthquakes, with this in view.
This was approved by the Council of the British Association and at once
undertaken by me, with the zealous and efficient co-operation of my
eldest son, Dr. J. W. Mallet. Nearly the whole of the Second British
Association Report, of 1851, is occupied with the account of the
experiments as to the transit rate of artificially made shocks in sand
and granite, as already referred to.
The Third Report, of 1852-1854, contains the whole of this, "The
Earthquake Catalogue of the British Association" (of which, through the
liberality of that body, more than one hundred copies were distributed
freely), in which are given, in columnar form, the following
particulars, from the earliest known dates to the end of 1842: