38 THE BIOLOGY OF THE SEA-SHORE 



The mud, again, shows all degrees of texture and com- 

 position according to homogeneity, or admixture of vegetal 

 or other matters. These two, sand and mud, may be in situ, 

 or transported from some distance by current or river 

 action, and may (together with shingle derived from glacial 

 drift) obscure the local substratum. 



Mud deposition denotes shelter, and is chiefly found in 

 estuaries, creeks, and other inlets, while sand may occupy 

 great stretches of open coast, though requiring shelter and 

 the accompanying stability in order to show its maximal 

 faunal development (within the tidal range). 



Plants may play an important part in causing changes, 

 chiefly within estuaries, as in the case of Spartina townsendii 

 at Poole, and other places, where it rapidly advances, con- 

 verting the mud flats into firmer ground and raising its level 

 above marine influence. The Salt-wort {Salicornia) and 

 others may have a distinct eff^ect, while, on open shores, stony 

 algae, such as Lithothamnion, form considerable, hard masses. 

 Rocks give shelter in their crevices to various animal forms, 

 and, on the other hand, compact colonies of sedentary 

 species, such as Balanus, Mytilus, and encrusting algae, 

 protect the surface of rocks from destructive marine action 

 (Issel, 1918). 



Davenport {op. cit.) describes a constant struggle taking 

 place at the top of the Cold Spring Sand Spit, between the 

 upbuilding tendencies of the Spartina and the scouring- 

 away tendencies of the tide. There is a tendency to form 

 channels at weak spots in the Spartina. These channels 

 widen and anastomose, and thus little hummocks are formed ; 

 but these are prevented from being washed away by an 

 interesting association. The current that rushes through 

 the channels carries with it an abundant supply of micro- 

 scopic food such as Lamellibranchs can make use of. The 

 food is taken advantage of by the mussels which come to 

 line the muddy banks of the channels and form so close a 

 wall that erosion is almost completely stopped. 



The importance of the activity of rock-boring organisms 

 in the gradual disintegration of rocks must not be overlooked, 



