30 
PEOF. MELDOLA-THE EAETHQUAKE 
In trying to find out the origin of the shock, that is, the point 
where it happened, the evidence leads me to believe that it probably 
originated somewhere beneath the villages of Abberton and Peldon, 
and that it apparently suffered a considerable amount of reflection 
at Wivenhoe. 
One very common form of damage which we constantly observed, 
was the peculiar twist given to chimneys, which were not thrown 
down, but broken right across and the upper part twisted round; 
but whatever its explanation may be, I think it perfectly conclusive 
that we could not arrive at the direction in which the earthquake 
travelled by the direction in which the chimneys were twisted. 
[On considering the damage done by the earthquake in relation to 
the geological structure of the area affected, it will be evident to 
geologists that there is no marked difference in hardness and 
elasticity in the surface-formations of this area. The whole area 
of damage is on the London Clay, covered here and there with 
occasional patches of drift sands or gravels and alluvium. It seemed 
likely that the vibrations would produce a greater effect upon 
buildings situated on a damp coherent formation like clay than 
upon those on loose gravel, and we were not surprised, therefore, 
to find that at Colchester many of the residents had formed the 
opinion that the chief damage was confined to the clay areas. A 
certain amount of damage, however, occurred on the drift and 
alluvium, and we believe that the greater destruction occurring 
on the clay was not due to the dynamical effect produced by the 
difference between clay and drift, but was a pure geological acci¬ 
dent, due to the fact that the initial blow was given beneath the 
London Clay in the Peldon-Abberton district. 
The lines of junction of different geological formations probably 
had some effect upon the earthquake-wave, and free margins, such 
as coast-lines, river-valleys, and lines of outcrop, certainly had. 
When an earthquake-wave passes from one formation to another of 
different elasticity, a portion of the wave is reflected at the surface 
bounding the two formations, and the remainder is refracted in 
accordance with the laws of wave-motion. The shock thus tends 
to become exaggerated along such boundaries, owing to the inter¬ 
ference of direct and reflected waves, giving rise to what we have 
termed “ junctional vibrations.” In this instance those portions of 
Colchester, and also of Rowhedge and West Mersea, most severely 
visited, are situated near the junction of clay with drift-sand 
or gravel; but at the more distant stations junctional vibrations 
cannot be clearly traced to have had any effect, for there is no 
preponderance of records from the various towns and villages 
situated along the junction of the Tertiaries with the Chalk either 
north or south of the London Basin. 
Along free margins there is no resistance offered, in one direction, 
to the vibrating particles in their outward movement. This may be 
illustrated by placing a row of marbles in contact with each other 
and delivering a blow at one end of the series, which will cause the 
last marble at the other end to move outwards. “ Marginal 
