The Earthquake in relation to Geological Structure. 175
The true conditions will be made plain by the following
sections:—
Fig. 14.—Imaginary section from Altrincham (b) to Peldon (a). Ver-
tical heights not to scale. Horizontal scale approximately correct.
1, Palaeozoic; 2, Oolitic-Triassic; 3, Cretaceous; 4, Tertiaries. Dip
much exaggerated.
Fig. 15.—Imaginary section from Exeter (b) to Peldon (a). Vertical
heights not to scale. Horizontal scale approximately correct. Dip
much exaggerated. The numbers have the same meaning as in the last
figure.
Before comparing the number of records from the two
areas, it has been necessary to eliminate the effects of the
direct shock, otherwise the large number of observations from
places round the epicentrum would have given an over-
whelming preponderance to the S.E. area. The disturbance
having originated beneath the Tertiaries of the London
basin, the fairest way of eliminating the records due to the
direct vibrations81 has appeared to us to take as a radius one
of the extreme stations at which the shock was felt in the
London basin, and with Peldon as a centre describing a
circle with the given radius of about 86 miles, and passing
through Beading. Outside this circle the two areas may
fairly be compared, and a glance at the map will show that
the records from the N.W. area are far more numerous. It
will be of interest in the next place to consider these records
in detail:—
81 For remarks on the vibration of basin-like deposits resting on
harder beds, see a paper by B. Mallet, " On the Dynamics of Earth-
quakes," Proc. Roy. Irish Acad., vol. xxi., part 1, 1846.