286 Our Food Mollusks 



and mud, and most of these were permanently destroyed. 

 Storms of such magnitude occur only once in a life- 

 time, but more common ones, if less terrible, are often 

 destructive. 



Soil is rendered tenacious in several ways. First, 

 when sand is found to be mixed with fine sediments, its 

 grains are often held together by this cementing sub- 

 stance. Clay, the finest of sediments, is very resistant 

 to the erosive action of water, and is often found on 

 flats and beaches. Another important agency in render- 

 ing the surface tenacious, and thus preventing the shift- 

 ing of particles, is the growth of algae, which forms a 

 close, thin mat over some surfaces. The presence of 

 this dark, green crust gives a flakey or cake-like ap- 

 pearance to the bottom. The plants do not extend deep 

 into the sand, but bind the surface grains closely enough 

 to prevent their movement even in strong currents. The 

 growth of alga? seems to be best where tides move with 

 some force, and such a combination of firm bottom and 

 fairly strong currents, bearing abundant food, affords 

 the best conditions for clam growth. When such a bed 

 is dug, the coating of algae again forms over the surface 

 within a very short time. 



Again, under natural conditions, one observes that 

 sometimes a growth of thatch plants will convert a waste 

 of sand into a clam bed. Thatch is found on many flats 

 between tide lines. The plants grow close together, their 

 blades rising to a height of two or three feet, and their 

 roots forming a feltwork beneath the surface. In this 

 mass of vegetation, clams are often abundant, even when 

 the soil is almost pure sand and the currents rapid. They 

 are able to establish themselves because the plants pre- 

 vent a shifting of sand. On account of the wire-like 



