The Earthquake in Relation to Geological Structure. 169
of the Cretaceous series, which favours the view that these
places were shaken by marginal vibrations, reinforced possibly
at some places by junctional vibrations. Thus, omitting
Norfolk and Suffolk, where the precise western outcrops of
the chalk and greensand appear to be somewhat doubtful,
the shock was felt at Ely, Cambridge, Ashwell near Baldock,
Masworth near Tring, Prince's Risborough, and Shirburn
Castle near Watlington, all situated on or near Cretaceous
outcrops.
The numerous records from places round the south and
east coasts are also suggestive of the movement having been
exaggerated at these stations, on account of their marginal
situation. Thus the shock was felt at Portsmouth, Brighton,
Eastbourne, Hastings, Dover, Westgate-on-Sea, Herne Bay,
Whitstable, Sheerness, Shoeburyness, Southend, Bradwell-on-
Sea (damage), Clacton-on-Sea, Walton-on-the-Naze, Harwich,
Aldborough, Kirkley, Lowestoft, Gorleston, Lynn, Skegness,
and other places on the Lincolnshire coast (See map,
Pl. III).74
The damage at Wivenhoe.—The large amount of damage at
this place has already been described (p. 81), and may be
further considered in connection with the geological aspects
of the earthquake. That the damage here was excessive
appears from the fact that at places to the S.W., equally
distant from the focus (between Peldon and Abberton), the
damage was comparatively slight, as at Salcot, Virley, and
the Tolleshunts. Nothing certain is known of the details of
structure of the deeper formations at Wivenhoe, and the
74 To complete the idea of the effects of a free margin upon earthquake
movement it will be instructive to take the following hypothetical case :
—Imagine a large circular island of homogeneous geological structure
with a town at the centre, a circular line of cliff round its coast, and
villages scattered all over the island from the centre to the sea. If a
seismic disturbance took place beneath the central town of just sufficient
intensity to shatter the buildings, there would be, at a certain distance all
round the epicentrum, a zone (a) of villages where the shock would be
severely felt; outside this zone the shook would be less severe (zone e) ;
and then again the villages round the coast would form a marginal zone
(c), where the shock would be more strongly felt than in b, owing to the
increased movement of the ground along the sea margin.