The Earthquake in relation to Geological Structure. 173
present earthquake geologists naturally called attention80 to
the older, harder, and more elastic rocks which underlie the
newer and softer formations of this country. In suggesting
that some kind of connection might exist between the dis-
tribution of the shock and the presence of the older rocks
near the surface at any place, it is not intended to convey the
impression that these older formations are in any way con-
cerned with the production of the earthquake ; of such casual
connection there is at present no evidence, chiefly owing to
the great difficulty attending the determination of the depth
of origin of earthquakes. The connection suggested is in
fact a purely mechanical one, depending upon the superior
conductivity of the older rocks ; or, in other words, in passing
through such a material as clay more heat is developed, and
the mechanical vibrations die out more rapidly than when a
series of seismic waves traverse a stratum of hard crystalline
rock.
After carefully considering all the facts available we think
that it may be safely concluded that the westward extension
of the shock was as it were exaggerated by the spreading of
the older rocks in this direction. The connection in question
is not, however, very clearly defined,—a fact which is not
surprising when it is considered how slight were the effects
towards the extreme limits of the disturbance—and can only
be shown by a special method of treatment, which to many
may appear arbitrary, but which, we are convinced, reveals a
real physical connection between the extension of the harder
rocks and the distribution of the disturbance.
In presenting the facts necessary for the present dis-
cussion it will perhaps be most instructive in the first place
to give broadly the extreme western limits of the shock,
because it happens that in this direction the older rocks corne
nearer to or actually crop out at the surface. Thus there
are records from :—
80 W. Topley, F.G.S., in 'Nature,' May 1st, 1884, p. 17; and Map,
ibid., May 15th, p. 61. See also a letter by " Seismos " in the ' Standard '
of April 26th, 1884; and J. E. Taylor, F.G.S., in ' Hardwicke's Science
Gossip,' June 1884, p. 122.