180 The East Anglian Earthquake.
Palaeozoic rocks have been proved by boring also at Calais.84
It may be safely admitted therefore that the Secondary and
Tertiary formations of East Anglia rest on beds of harder
materials, which are extensions of the older rocks of the West
of England and of the Continent of Europe.
The action of the older rocks in spreading the shock is
apparently shown by the circumstance that all the extreme
stations are either on Palaeozoic formations or very near the
outcrop of these beds. This statement holds good in the
case of Altrincham, the Derbyshire stations, Birmingham,
Wolverhampton, Sidbury, Worcester, the Leicestershire
stations,"6 Bristol, Street, and Exeter. It cannot of course
be decided whether the vibrations at each of these stations
were primary or secondary, i. e., whether the place was
shaken by the direct shock travelling along the primary rocks
or by the waves transmitted through the Lower Secondary
formations, or, again, by the vibrations communicated to the
latter by the underlying Palaeozoic bed. In places where a
double shock was noticed it is possible, as has already been
stated (p. 39), that both sets of vibrations may have made
themselves felt in succession.
It may be useful, in dealing with this part of the subject,
to consider a question of some interest which has been raised
by geologists, viz., whether the presence of hard rocks beneath
any district can be inferred because the shock happened to be
felt there. In reply to this it may be pointed out that,
supposing we have a record from a station which is fairly
beyond the radius where the energy of the primary vibrations
may be supposed to have been spent in the softer rocks, the
84 At a depth of 1098 feet from the surface Palaeozoic rooks (probably
Coal-measures) were reached.
85 It is of great interest to notice the crowded records from Northamp-
tonshire and Leicestershire, inasmuch as these stations are directly in
the line of the Paleozoic outcrops of Warwickshire and Staffordshire.
The greatest seismic conductivity appears in fact to have been in this
direction. If any weight is to be attached to the Seismological evidence,
it might be inferred that an especially hard ridge either of Primary or
Lower Secondary rocks extends in a north-westerly direction from the
focus. The decision on this point must be left to the Stratigraphical
geologist.