178 The East Anglian Earthquake.
is impossible to determine the true limits of the disturbance
for each formation this objection is perfectly valid: it is not
possible, for instance, to say where the sensible vibrations
died out in the clay and were continued in the chalk, or where
they ceased to affect the latter and were propagated along the
subjacent harder rocks. If indeed it were possible to map out
the actual limits of the sensible shock for each of the super-
ficial formations over which it extended, we should certainly
not have a set of concentric circles, but a series of curves of
much greater complexity.
Although the position assigned to the limiting circle may
thus appear arbitrary, it was obviously necessary to exclude a
sufficiently extended area round the focus of the disturbance
to clear the records from the effects of the direct shock as
propagated horizontally along the Tertiaries and Chalk for a
certain distance round this centre, and the radius taken
appeared to fulfil this condition. It seems indeed that at about
ninety miles from the epicentrum the sensible vibrations died
out in the clays and gravels of the London Basin, and as the
conductivity of chalk is probably not very different from that
of clay the circle shown in the map may be considered to
indicate the limits to which the shock would have extended
if the whole of this country had been composed of homo-
geneous clay or chalk.
Any importance which may still be attached to the objection
now being considered will, however, disappear on further
examination of the facts. It will be seen that the radius may
be increased or diminished by many miles—-such, for instance,
as by being made to end at the extreme apex of the London
Tertiaries as an outward limit, or by terminating at Kew as
an inward limit,—and the preponderance of records from the
north-western area still remains. It has not been thought
desirable to burden this report with a detailed justification of
these statements, as their truth can easily be tested by means
of the map (Plate III.).
One other objection which may be offered is that some of
the stations in the N.W. area are not situated on rocks of any
great degree of hardness, but on softer beds of the Lower