File:The Quarterly journal of the Geological Society of London (12512912173).jpg

264
PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .April 10,
wave returning into the mass in the reverse direction ; but with
that we are not here very much concerned.
This is rendered evident to the senses by the analogous movements
of a line of billiard balls (fig. 3). "When a line of such equal balls
Pig. 3. — Transfer of Elastic Wave in Ivory Balls
in close contact, 1, 2, 3, 4, 5, 6, 7, B, is struck by a ball A moving
in directum with the line, each elastic ball in succession from A and 1
to 7 and B is deformed by the blow, and in regaining form transmits
the elastic wave from ball to ball. All the balls, 1 to 7, however, •
remain at rest in space, the transmitted energy being equilibrated
between each pair in succession ; but the last ball B is thrown off
from its contact with 7.
Now, if the balls were all adherent to each other at their points
of contact, instead of being free and merely touching, the same result
would follow, provided the energy transmitted from 7 to B were
sufficient not only to move it off but to overcome the adhesion. And
this would be equally true if the whole line from 1 to B had been
a solid cylindrical or prismatic bar of elastic material, provided that
the energy of any impulse delivered by a blow into the bar and com-
pressing it at the end 1, were sufficient to overcome the cohesion of
the elastic material in some cross section through the point of contact
of 7 and B, or nearer to the extremity at B, the bar being thus
torn asunder and its extremity thrown forward by the otherwise
unbalanced energy of the wave accumulated there.
Now this illustrates in a common-sense way what actually happens
when an earthquake-wave having a nearly horizontal wave-path or
an emergent one through an extended horizontal mass, such as the
Cachar clay -beds, suddenly arrives at an abrupt termination of the
mass by their ending in a vertical or nearly vertical cliff, such as
that forming the river-banks described by Dr. Oldham.
Thus, let fig. 4 be an ideal section of the river-bed, with the low-
water and wet-season water-levels shown ; g the masses of clay
Fig. 4. — Ideal Section of Biver-hed.
resting upon the ooze-bed o, and this on subjacent incoherent strata s;