"MACKEREL SKY" 359 



some separation of sand from water, the upper 

 parts becoming more watery. 



Near by I found a mass of wet mud which had 

 flowed down the lower slopes of the cliff as far 

 as the beach, where it had come to rest with a 

 sloping surface. I inserted the plunger and worked 

 it up and down as before, but no air-bubbles came 

 off and there was no separation of the water from 

 the earthy matter. 



When sand dries, air enters its pores which, if 

 the sand be pure, readily escapes when it is again 

 wetted, but an admixture of fine earthy particles 

 prevents or retards the escape of air. The tenacious 

 character of the sands of Morecambe Bay (where 

 the neighbouring rocks are of limestone) doubt- 

 less causes the retention of the gases evolved by 

 decomposition of the cockles. It may also cause 

 an imprisonment of air during the rapid rise of 

 the tide, thus co-operating with the upwelling of 

 the water to deprive the sands of the power to bear 

 a load. I have already described a case in which 

 the flood-tide entrapped the air in a sandbank, and 

 in all such cases the sands would become unstable. 



On the Rippled Clouds called " Mackerel Sky.'' 



Ruskin wrote of " mackerel " clouds that " the 

 vapour . . . falls into ripples like sand." An 



