The Earthquake in relation to Geological Structure. 181
Seismological evidence is by itself valueless unless combined
with geological and topographical considerations, and even
then much doubt must necessarily exist. Thus it cannot be
decided whether the vibrations felt at Ostend and Boulogne80
were transmitted along the Palaeozoic ridge or along the
overlying softer beds. Again, with respect to the "Wealden
area, although the Palaeozoic rocks may by faults, folds,
anticlinal ridges, or the thinning-away of superincumbent
formations, be brought nearer the surface in some districts,
and thus cause the shock to be felt, it would be very unsafe,
in the absence of geological knowledge, to predict the presence
of these rocks under such stations from the Seismological
evidence only. Thus the shock was felt, among other places
in this area, at Tunbridge and Crowborough ; but it would be
rash to assume on these grounds the existence of a Sub-
Wealden Palaeozoic ridge, inasmuch as the shock was felt all
over the Chalk in Norfolk at a distance extending even further
north of the focus than these Wealden stations are south of
this point; so that if the vibrations travelled northwards
through the Chalk as far as, let us say, Fakenham, there is
no reason why they should not have been transmitted south-
wards through the Wealden beds for an equal or nearly equal
distance.
Summary.—As the foregoing discussion of the evidence
concerning the action of the older rocks in spreading the
shock has necessarily been somewhat lengthy, it will be
advantageous here to briefly summarise the results. The
seismic area has been regarded as being constituted broadly
of a shallow basin of Palaeozoic rocks containing, in ascending
order, beds of intermediate hardness (Lower Secondaries),
overlaid in turn by the softer Upper Secondaries and Tertiaries.
The initial blow having been struck beneath the latter, the
damage was confined to the London Clay and superficial
deposits, but the vibrations were spread widely by the under-
86 The situation of these towns will probably suggest that marginal
(littoral) effects were experienced. This may have been the case, but, if
so, the above statement is not in any way affected, as it will be seen on
consideration that the effect of a free margin would be the same whether
the district were shaken by primary or secondary vibrations.