The Earthquake in Relation to Geological Structure. 163
buildings on piles or concrete sunk down to the gravel this
disturbance is not experienced.63
In cases where the materials composing the soil of an
earthquake-shaken tract are of very different degrees of hard-
ness and elasticity, it might be anticipated, and experience
has shown, that differences in the amount of damage are
displayed according to the situation of the buildings,
vibrations of large amplitude and destructive "acceleration"
in soft coherent materials being eased off and rendered
harmless on reaching hard rock. To mention one or two
instances:—The destruction caused by the Lisbon earthquake
of 1755 is stated to have been confined to the Tertiary strata,
and to have been most marked on the clay, not a building on
the secondary limestone or basalt having been injured.64
Again, in 1692, the earthquake which wrecked Port Royal,
Jamaica, is said to have exerted its most violent action upon
those portions of the town built upon newly-formed sand, the
surviving buildings having been all upon the solid white
limestone.65 On the other hand, buildings situated on loose
alluvial tufas suffered less during the Ischian earthquake of
1883 than those built directly on the solid tufa.66
Now it happens, in the case of the earthquake with which
we are concerned in the present report, that there is no such
marked difference in hardness and elasticity in the surface-
formations. The whole area of damage is on London Clay,
covered here and there with occasional patches of drift sands
or gravels and alluvium, so that we have not for comparison
such heterogeneous materials as clay and basalt or sand and
limestone, but only clay and drift-formations, which, from a
Seismological point of view, differ but very little from clay in
elasticity. It seemed likely, however, in view of the dynamical
63 Mr. Royle, whose experience on the earthquake at West Ham has
already been recorded (p. 115), compared the effects upon his house with
those produced upon his laboratory at Silvertown by the passing of a
locomotive.
64 Lyell, on the authority of Mr. Sharpe, ' Principles of Geology,' vol. ii.,
p. 148.
65 Geikie, on the authority of De la Beche, ' Text Book of Geology,'
p. 268.
66 H. J. Johnston-Lavis, Brit. Assoc. Kep. Southport, 1883, p. 501.