54 OLDHAM THP, CACHAR EARTHQUAKE OF 10TH JANUARY 1869. 



If the force of momentum be only just sufficient to overcome the 

 resistances of cohesion and friction, only one fissure will be formed, and 

 that at the critical moment and at a distance of one-half the length of 

 the wave from the river's edge ; but if the momentum of the zone of 

 motion towards the bank be more than sufficient to overcome these two 

 resistances, it will be able to overcome the inertia of a certain mass of 

 clay in addition, or else it may suffer a greater or less reduction before 

 it ceases to have the power of overcoming them, and in that case fissures 

 will be formed both nearer to and farther from the river's bank than the 

 main fissure or fissures, which will always lie at one-half the whole 

 length of the wave from the river's edge. 



As the wave passes on its way, the motion of the particles is extin- 

 guished by the breach of continuity caused by the riverbed; but though 

 the motion is thus extinguished in the superficial stratum of clay, the 

 impulse, being of deep-seated origin, passes under the bed of the river 

 and emerges on the opposite bank, where motion is once more commu- 

 nicated to the superficial stratum, and the train of phenomena is just 

 the reverse of what happened as the wave approached the river. At 

 first the motion of the particles is away from the river, and consequently 

 there is no tendency to fracture in the mass ; but as the wave progresses, 

 the particles of clay begin to move backwards, and a strain is set up, 

 as when the wave approached the river, which gradually augments till 

 it reaches its maximum as the rear of the wave just reaches the river's 

 edge, the critical moment in this case, and then diminishes. The same 

 reasoning and considerations which applied to the case of the wave ap- 

 proaching the river apply to that of its passing away from the river, the 

 position of the main and secondary cracks being in every way similar. 1 



The explanation given above may perhaps be rendered more intelligible 

 by a reference to PL VI, fig. 1, taken from the Quarterly Journal of the 



1 Though this is qualitatively it is not quantitatively true, the upper stratum not being 

 able to take up immediately the whole motion impressed on the lower clays, owiug to the 

 intervention of the soft yielding bed of sand, and it is the vertical motion— that which has 

 least effect in producing these fissures — which would be most rapidly acquired. 



Cf. remarks on angle of emergence and velocity of wave particle at Silchar, pp. 68, 71. 



( 5^ ) 



