1872.] OLDHAM AND MALLET CACHAR EARTHQUAKE. 265 



At the side A. the wave-energy is traiasraitted from particle to 

 particle through the clays and all beneath, until it arrives at the 

 termination of the mass by the steep river-cliff at h ; and there it is 

 expended (or part of it) in breaking through the bed of clay at a, 

 and throwing off more or less the mass a h towards the river. This, 

 resting on its bed of unstable ooze, not only slides forward towards 

 E, but, as it compresses the ooze-bed unequally and the mass of 

 the ooze-bed itself is forced forward in like manner but unequally, 

 descends also, and, dependent on many accidental conditions of the 

 adhesion of the surfaces of fissure, the shape of the mass detached, 

 the direction more or less curved of the fi.ssure itself, and many more 

 existing in nature, may tend to turn over likewise, and so descend 

 unequally, and thus when come to next may have no longer a level 

 top, as seen in several of the photographs. 



For though the throwing-off force must always act in the line 

 of the wave-path and through the centre of gravity of the detached 

 mass, the resultant of aU the retaining forces, whether of support or 

 of cohesion, that kept it in its place may not pass through that same 

 point nor be in the direction opposite to that of the wave-path ; so 

 that rotation either in a vertical, inclined, or horizontal plane, or in 

 all, may take place in the movement of the mass before coming to 

 rest. Several fissures, more or less parallel, will nearly always be 

 formed, for reasons that we need not stop to detail ; but the place 

 of the great separating fissure a, or where the whole of the nearly 

 parallel fissures will most closely congregate, will be at a distance 

 back from the edge of the cliff h, about equal to c d — that is to say, 

 at about half the amplitude of the material in elastic wave- motion 

 at the same instant, as ef in the diagram, wherein the ordinate ef 

 represents the amplitude, and the vertical abscissae the velocities of 

 the wave-pai'ticle, mcreasing in the first semiphase ey up to the 

 maximum y c, and then diminishing to zero in the second semiphase 

 yf, the fissure being formed and the mass being thrown off in most 

 eases just at the instant when, and at the point where, the wave- 

 particle had its maximum velocity. In this diagram the wave-path 

 is taken as normal to the face of the river-cliff; but, as is obvious, 

 it may meet that at any angle at different places of the winding- 

 river ; and new conditions of fracture and of dislocation of the 

 severed masses will then arise, often producing phenomena of great 

 perplexity to the eye, but into which we cannot here afford space 

 to enter. 



A large portion of the elastic wave of shock which affected 

 the masses above the level of the river-bed, is thus at the side A 

 superficially extinguished. But the earthquake-wave exists far 

 below this ; and so a large portion is transmitted beneath the river- 

 bed in the wave-path tn to the masses at the side B. 



These are shaken, whether directly or not is dependent on ques- 

 tions of relative levels ifec, but chiefly through the wave-movement 

 of the subjacent strata ; and thus fissures and dislodged masses may, 

 and generally must, occur at g also. 



Thus, from whichever side of the river the shock comes, or in 



VOL. XXVIII. PART I. V 



