EARTH FISSURES AND SAND CRATERS. 57 



more recedes. The action by which the water is forced up is two-fold. 

 In the first place there is set up in the soft yielding bed of ooze a mo- 

 lecular motion precisely similar to that of the clays above and below, in 

 virtue of which there are established two zones whose momentum drives 

 them in opposite directions; and as the ooze is comparatively free to 

 yield to any strain set up in it, the water contained in the bed, both in 

 front of and immediately behind the wave will be subjected to an aug- 

 mentation of pressure, and so the flow of a natural spring is increased, 

 or in the case of an artificial well the level of the water rises and may 

 even, if the water stands near the surface, rise above the level of the 

 ground and overflow. But though this undoubtedly does take place to 

 some extent, it may be, and in this case was, masked and eclipsed by a 

 far more powerful action yet to be described. As the wave travels 

 along, the lower clays are rapidly pressed upwards, and neglecting 

 the horizontal component as not producing any effect in this case, the 

 thrust is conveyed through the semi-liquid ooze to the mass of clays 

 above, whose inertia prevents their rising at once, and so a sudden 

 and tremendous pressure is brought to bear on the water-logged ooze; at 

 the same or almost the same instant cracks are opened in the superincum- 

 bent mass of clays, and through these cracks the water is immediately 

 forced out if the pressure is sufficient. . But such a large quantity of semi- 

 liquid matter, when once in motion, possesses a very considerable momen- 

 tum and therefore cannot be suddenly brought to rest ; so that, though 

 the pressure due to the passage of the shock is but transient, the effects 

 endure after it has passed away, the water still continues to rise 

 in the fissure, and if it rises high enough will pour out on the surface. 

 Further if, while the fissure is still full of water, the effect, whether 

 primary or secondary, of the earthquake causes its sides to approach each 

 other they will exert a pressure on the water contained in the fissure 

 quite independent of that on the water-bearing bed, and thus the water 

 will be forced to rise and overflow to a still greater extent. It 

 can easily be understood that if the water-bearing stratum was impreg- 

 nated with decomposing organic matter, this opening of connection 



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