The Earthquake in Relation to Geological Structure. 165
given in the future, when further advances have been made
in experimental seismology.
Effects of Free Margins, Boundaries, Topographical Situation,
and other Details of Geological Structure.
Junctional Vibrations.—When an earthquake-wave passes
from one formation into another of different elasticity, a
portion of the wave is reflected at the surface bounding the
two formations, and the remainder becomes refracted in
accordance with the general laws of wave-motion. The
shock thus tends to become exaggerated along such boun-
daries, owing to the interference of the direct and reflected
waves; and as such reflexion is most likely to occur along the
lines of junction of different geological formations, we propose
for brevity to speak of these effects as the result of "junctional
vibrations."
A careful examination of the geological map of the area of
structural damage rather favours the view that the destructive
effects may have been in some instances increased by the
situation of buildings near the junction of clay with drift-sand
or gravel, although this part of the subject is surrounded by
such difficulties that we by no means wish to put forward
this statement as a positive assertion. It may, however, be
pointed out that those portions of Colchester most severely
visited are represented in the map as being situated on or
near such junctions, and the same remark applies to Rowhedge
and West Mersea. It is likewise possible to name numerous
isolated buildings which may have suffered from the effects of
junctional vibrations, but the evidence in such detached cases
is of course less satisfactory than in the case of larger aggre-
gations of houses. An attempt has been made also to connect
the occurrence of the shock at the more distant stations with
the situation of these places in reference to lines of junction,
but the results are of a negative character. Some few towns
and villages might be mentioned which, by their geological
position, would favour the view that the shock experienced
was the result of junctional vibrations, but their number is
too small as compared with those stations far removed from
lines of junction to warrant the assumption of any such