The Earthquake in relation to Geological Structure. 179
Secondary strata. It may be pointed out, however, that the
mode of treatment adopted is a purely statistical one, the
whole of the sub-Cretaceous rocks being "lumped"' for the
purposes of the present comparison, because these are, as a
whole, harder than the overlying beds. The few stations on
the N.W. area which happen to be on softer formations do
not therefore materially affect the comparison, and in nearly
all cases the Palaeozoic rocks—the hardest and most elastic
of all rock materials—lie at no very great depth beneath the
surface. The superior conductivity of these last rocks appears
also to have exerted a distinct influence in spreading the
shock widely round the centre in a manner which may now
be considered :—
In considering the action of the older rocks as specially
good conductors of the vibrations, it is assumed that these
rocks underlie the whole of the seismic area, or, in other
words, that the Palaeozoic rocks which crop out at the north-
west, west, and south-west of England, and in the north of
Prance and Belgium, are in seismic and geological continuity.
That such geological continuity exists was first surmised by
the late Godwin-Austen in 1856, in a paper on the possible
extension of the Coal-measures beneath the south-eastern
parts of this country,83 a view which receives ample support
from the well-borings that have been made in the area under
consideration. Thus the Palaeozoic rocks have been proved
to exist beneath Northampton; at Meux's Brewery, Totten-
ham Court Road, the Devonian rocks were reached at a
depth of about 1066 feet; at Turnford (Cheshunt) a boring
reaches the same formation at a depth of 980 feet; at Ware
a boring 831.5 feet in depth reached Silurian rocks ; and at
Harwich carboniferous rocks were reached at a depth of
1025.5 feet. Of other borings in the London Basin, that at
Kentish Town reached the Gault at a depth of 130.5 feet, and
then passed through 188.5 feet of doubtful beds between the
New Bed series and the Carboniferous. The recent boring at
Richmond (Surrey), at a depth of 1447 feet, terminated in red
sandstone of Carboniferous or Old Bed Sandstone age.
83 Quart. Journ. Geol. Soc, vol. xii., p. 38.