266 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 10, 



whatever direction (unless pai'allel) it meets the river-banks, fissures 

 may be formed, though the direction of these, their extent, the size of 

 masses dislodged, &g. must vary much as these conditions alter. 



All that has been thus explained might have been given much 

 more elegantly, succinctly, and accurately by the use of dynamic 

 formulo3, but, probably, not so satisfactorily to many geologists as in 

 the way the writer has endeavoured to put it. 



An inspection of Dr. Oldham's photographs wiU sufficiently indi- 

 cate the prodigious scale on which these forces may act ; and the 

 physical geologist will readily deduce for himself the consequential 

 effects of these huge dislocated masses of mud when thus prepared 

 for the erosive and transporting action of a great tropical river. 



It remains to make a few remarks upon the so-called mud erup- 

 tions, and upon the inverted sand cones, formed in holes through which 

 such mud, sand, and water have issued. 



If the explanations that have already been given as to the dis- 

 placement of solids by the passage of the shock be applied to the 

 case of a bed of semiliquid ooze, or to any water-bearing bed of 

 gravel or sand supposed hoi'izontal, and the wave-path also very 

 nearlj' horizontal, it will be obvious that the elastic wave must tend 

 to pack up the water contained in the bed during the first semi- 

 " phase, and, where an aperture from the water-bed to the surface 

 exists, may cause the level of the water in this to rise, or even mo- 

 mentarily to overflow, or to spout up above the surface, so that 

 neighbouring wells of a shallow class thus alter their levels, or 

 overflow ; and the level of the fluid once raised may take some time 

 to subside. 



Where the water-bearing bed or that of semiliquid ooze is still 

 nearly level, or the wave- path (as is generally the case) is an 

 emergent one, the last conditions still come into play ; but others 

 are now added. 



Just as it was explained, that the masses r, on the sloping flank N, 

 (fig. 2) are, in the first semiphase of the wave, pressed more firmly 

 against their rocky beds, so the heavy masses of clay (of the 

 Caehar shock) reposing horizontally upon the nearly level bed of 

 ooze beneath, are during the first semiphase pressed downwards 

 upon the latter. 



The amount of this pressure for the moment is enormous ; for 

 it is the same as would have sufficed to raise the whole mass of 

 the superincumbent clay affected by shock through a height equal 

 to the wave's semiamplitude, and with a velocity equal at least to 

 the mean velocity of its wave-particle resolved in the vertical. 



Thus, for the moment, the ooze or water-bearing bed is squeezed 

 like a layer of saturated sponge between the beds below and the 

 clay masses above, by a force far in excess of the ordinary statical 

 pressure due to the weight of the latter. Wherever there may be 

 a sufficiently weak point in the clay-m.asses, or a well dug through 

 them, or the elay may pass into incoherent sand, the semiliquid ooze, 

 or the water in it, will start up, and ma)" reach the surface under 

 tliis momentary pressure. 



