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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; 

 and let the wave-path of shock be in the direction Tc to I and of m to s. 



