The Earthquake in Relation to Geological Structure. 171
railway-cutting may have saved the buildings on each side
by offering a break in the continuity of the ground, and
thus preventing the free propagation of horizontal surface
vibrations.
With reference to other details of geological structure, it
has not been found possible to trace any distinct connection
between lines of faulting and the distribution of the shock.
The well-known fault which passes through Greenwich, for
example, as far as can be ascertained, appears to have had
no effect in modifying the disturbance. The relation of the
main seismic axis to the known lines of faulting, &c, will be
considered subsequently.
The connection between the main lines of jointing, the
direction of coast-lines, and the occurrence of earthquakes
is a branch of seismology to which special attention has been
directed by Prof. J. P. O'Reilly,'8 but as we are in ignorance
of the direction of the lines of jointing of the older rocks
beneath Essex, it is difficult to apply these principles in the
case of the present earthquake. It may be pointed out,
however, that the direction of the main N.E.-S.W. axis of
disturbance (p. 92) corresponds with the general direction of
the coast-line at this place, a fact which is so far in accord-
ance with the theory. In his earthquake map of the British
Islands Prof. O'Reilly places the focus of the present dis-
turbance on a great circle passing across England from the
southerly dip of the chalk it might have been expected that this forma-
tion would have been nearer the surface at Colchester than at E. Mersea ;
so that it is possible that there may be between these places a fault or
some other disturbance bringing up the chalk abruptly. Whether such
a disturbance exists, and, if so, whether it had any connection with the
earthquake, cannot at present be decided.
76 Trans. Seism. Soc. Japan, vol. iii., p. 148.
77 ' Nature,' Oct. 26th, 1882, p. 629 ; also June 4th, 1885, p. 114.
78 Proc. Roy. Irish Acad., vol. iii. (2nd ser.), pp. 295, 310, and 503;
ibid., vol. iv., p. 116 ; and ' Trans.,' vol. xxvi., pp. 611 and 641, and vol.
xxviii., p. 285. The earthquake map of the British Islands above
referred to accompanies this last memoir. Conclusions somewhat
analogous to Prof. O'Reilly's have been arrived at by Richard Owen
(Proc. Amer. Assoc. Montreal, 1882, p. 329).