EARTH FISSURES AND SAND CRATERS. 53 



motion m each case is away from a median plane, the momentum of 

 the two zones, acting in opposite directions, tends to rupture the mass 

 along that plane. But the action of this momentum is modified and 

 restricted by three distinct causes : firstly, the molecular cohesion of the 

 clay ; secondly, the inertia of the motionless masses of clay in front of 

 and behind the zones of motion ; and thirdly, by friction on the sandy 

 bed below. As long as the wave is at some considerable distance from 

 the river's bank, the resistance offered by cohesion and inertia is suffi- 

 cient to restrain the force of momentum and prevent the formation of 

 fissures ; but as the wave approaches the bank, the force of momentum 

 remains, up to a certain point, unaltered, and of the various resistances, 

 that of cohesion remains constant, that of friction may be neglected 

 for though varying somewhat, it is always comparatively insignificant, 

 but the inertia of the mass of motionless clays between the front of the 

 anterior zone of motion and the river's bank diminishes as the wave 

 approaches the latter, until at last it disappears just as the front of 

 the wave reaches the river's bank; at this instant the momentum of 

 the zone of forward motion is undiminished, the resistance of inertia 

 has disappeared, and it is only resisted by cohesion and friction. As 

 soon, however, as this critical moment has passed, the momentum begins 

 to diminish as the motion of the wave particles is dissipated at the 

 surface of the river's bank, and the strain rapidly diminishes till, when the 

 central plane of no motion reaches the bank, it vanishes entirely ; after 

 this, as the momentum of the second or hindermost zone is away from 

 the river, there is no tensional strain, and no tendency to fissure. 



By consideration of the above remarks two conclusions are evident : 

 firstly, that fissures will only be formed along the plane of separation 

 between the two zones, and secondly, that the strain tending to produce 

 rupture is greatest at what I have called the { critical moment/ at which 

 moment the plane of separation, in which the particles have attained 

 their maximum forward excursion, or in other words the ' crest ; of the 

 wave, is distant from the river's bank by the thickness of one of the zones 

 of motion or one-half the whole ' length ' of the wave. 



( 53 ) 



