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 rejette_) 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 crate