316 MARKINGS, FOOTPRINTS AND FUCOIDS 



numerous small fish of various species occupy the ground and 

 may leave marks of their fins and tails as they gambol or seek 

 their food. Shell fishes, worms, and Crustaceans scramble 

 over the same surface, or make burrows in it. As the tide 

 recedes flocks of sandpipers and crows follow it down, and 

 leave an infinity of footprints, and even quadrupeds like the 

 domestic hog go far out at low water in search of food. It is 

 said that in some parts of the Bay the hogs are so assiduous 

 in this pursuit that they even awake and go out on the flats in 

 the night tide, and that they have so learned to dread the 

 dangers of the flood, that when in the darkness they hear the 

 dull sound of the approaching bore, they squeal with fear and 

 rush madly for the shore. 



If we examine it minutely, we shall find that the tidal de- 

 posit is laminated. The tidal water is red and muddy, and 

 holds in suspension sediment of various degrees of coarseness. 

 This, undergoes a certain process of levigation. In the first 

 run of the flood the coarser material falls to the bottom. As 

 its force diminishes the finer material is deposited, and at full 

 tide, when the current has ceased, the finest of all settles, 

 forming a delicate coat of the purest and most tenacious clay. 

 Thus, if a block of the material is taken up and allowed to 

 dry, it tends to separate into thin laminae, each of which re- 

 presents a tide, and is somewhat sandy below, and passes into 

 the finest moulding clay above. The tracks and impressions 

 preserved are naturally made on the last or finest deposit, and 

 filled in with the coarser or more sandy of the next tide. But 

 this may take place in - different ways. Impressions made 

 under water at flood tide, or on the surface left bare by the 

 ebb, may in favourable localities be sufficiently tenacious or 

 firm to resist the abrading action of the flood, and may thus 

 be covered and preserved by the next layer, and in this way 

 they may be seen on splitting up a block of the dried mud. 

 But in shallow places and near the shore, where the deposit 



