174 The East Anglian Earthquake.
Cheshire, on the Trias.
Derbyshire, on the Trias and Carboniferous.
Leicestershire, on the Lias.
Staffordshire, near the junction of the Carboniferous and
Permian.
Warwickshire, on the Trias and Lias.
Worcestershire, on the Trias.
Northamptonshire, on the Lias and Oolite.
Gloucestershire, on the Trias and Lias.
Somersetshire, on the Lias.
Devonshire, near the junction of the Carboniferous and
Triassic.
Towards these western limits, on the view that the shock
was spread in this direction along the underlying hard rocks,
it might thus be expected that there would be more numerous
records from localities where the older rocks come to the
surface than from districts where great thicknesses of softer
formations overlie these harder rocks. The geological
structure of that portion of England over which the shock
extended favours in a perfectly legitimate manner such a
comparison as is here required, and the records thus com-
pared, although the fair estimation of the facts has been a
matter of considerable difficulty, show a large prepon-
derance from the older geological districts, as will appear
from the following analysis of the evidence :—
If the boundary of the Cretaceous formations be laid down
on a map of England (see Map, Pl. III.), an irregular line
will be shown, having a general direction N.E.—S.W., and
extending from the corner of the Wash to the Bill of Port-
land, this boundary dividing the seismic area into two
portions, one lying to the N.W. and the other to the S.E.
The formations in the latter portion consist broadly of Chalk
and the overlying soft Tertiaries, while the formations in the
N.W. area consist of Triassic, Liassic and Oolitic strata,
together with hard, crystalline, Palaeozoic rocks. The rock
materials of the N.W. area are thus on the whole harder and
more conductive than the softer chalk and clay of the S.E.
area, and it might therefore be expected that the vibrations
would die out more rapidly in a S.W. than in a N.W. direction.