EARTH FISSURES AND SAND CRATERS. 55 



Geological Society, Vol. XXVIII, p. 264 ; in it s s represent the lower 

 strata of clay, o o represent the bed of sandy ooze lying on them, and 

 on it lie the upper clays g g } in which the fissures were formed and 

 through which the river bed is cut, the arrows serving to denote the 

 direction of wave path. The line ef'\s meant to represent the length 

 of the wave, while the ordinates of the curve e c f show graphically 

 the relative amounts of motion of the particles from their normal posi- 

 tions, in the various phases of the wave, starting from zero at e gra- 

 dually reaching the maximum at c and then diminishing to zero at f; 

 in the semiphase represented by f c, which is the first semiphase, the 

 particles are all in motion forwards, while in the semiphase c e the 

 motion of the particles is backwards, in both cases away from c, at 

 which point the particles are momentarily at rest, and which represents 

 the 'crest' of the wave, along which the fissures are always formed. 

 In the figure the wave is supposed to have reached that point at which 

 the internal strain is greatest and the resistance least, here the front of 

 the wave has just reached the river bank at b and the ( crest ' of the 

 wave is at a distant from the river's edge just half the length of the 

 wave. 



As yet we have only considered the hypothetical case in which the 

 clays are perfectly homogenous and . the direction of the wave-path is 

 normal to the river bank; but as this can never be the case in nature, 

 the fissures can never form mathematically straight lines normal to the 

 wave-path, but will vary indefinitely from them. The first and foremost 

 cause of this variation is of course that want of homogeneity which must 

 be found in the clays forming the banks, in consequence of which the 

 fissures will be formed along planes of weakness which need not be coin- 

 cident with what would have been the planes of Assuring in a perfectly 

 homogenous mass, thus giving rise to the irregularity and often branch- 

 ing or inosculation of the cracks as found in nature. Again if the wave 

 course is not normal to the course of the river, the fissures will not be 

 formed normal to the wave-path, but will tend to arrange themselves 

 more or less parallel to the river course, and in this case any want of 



( 55 ) 



