THE OYSTER AND THE OYSTER INDUSTRY. 23 



ing generations, raising the living parts of the bed higher above the 

 bottom, while the interstices beneath become filled with old shells, 

 fragments, sand, and mud to form a compact mass. Eventually, in 

 shfulow water, the living oysters approach low-water mark or in some 

 parts of the coast rise above it, where their progress is arrested by 

 cold or long periods of exposure to the air. 



Each year a set may occur only to be killed in winter, the dead 

 shells, fragments, sand, and mud piling up under wave action, until 

 the crest may become raised to a level several feet above high water, 

 producing a shell island usually surrounded by a more or less dense 

 growth of live oysters. Such islands are not uncommon in the South 

 Atlantic and Gulf States, and they frequently accumulate in time a 

 growth of grass and brush, which more or less obscures their true 

 character. Sometimes the material is thrown up around their edges 

 atoll-like, leaving a depression in the middle in which muddy deposits 

 collect and support a growth of brush. In places where the bottom 

 is composed of very soft mud the sides of these lumps are compara- 

 tively steep and soundings will change 1 or 2 feet within a few yards, 

 the difference being due to the depth of shells and oysters. 



In open waters, not especially subject to freshets, where the cur- 

 rents are moderate and the silt carried not excessive, such lumps 

 tend to maintain a round or oval outline, with no great difference 

 between the long and short diameters; but where the cm-rents are 

 rapid or the bodies of water constricted, there, as soon as they rear 

 themselves well above the bottom, they show a strong tendency to 

 grow transversely to the tides, especially if the water be silt-laden. 

 Such long, narrow reefs are common in the rivers of North Carolina 

 and in the bays and rivers of Florida, Alabama, Louisiana, and Texas. 

 In James River, Va., and probably in other rivers of the Chesapeake 

 region, the beds, while often showing their greatest extent in the 

 direction of the current, usually have their shoalest parts trans- 

 versely to it or are made up of a series of transverse shoals and ridges 

 composed of a dense mass of shells and fragments. 



The reasons for this transverse development are as follows: The 

 upgrowing reefs form partial dams or obstructions to the flow^ of the 

 currents, and, in accordance with well-laiown laws, cause eddies or 

 backwaters on both the side presented to the current and on that 

 sheltered from it, in tidal waters the two being periodically reversed 

 with the reversal of the tide. When the velocity of a silt-laden stream 

 is checked, it deposits part of its load in the slack water, and, under 

 the conditions stated above, mud falls on the upper and lower sides 

 of the reef, while the somewhat accelerated flow around the ends 

 scours the shells and keeps them clean and fit to receive fresh sets of 

 young oysters. These factors operate more energetically the more 

 heavily silt-laden the water, and they would become nonoperative in 

 perfectly clear water. Not only does heavily silt-laden water deposit 

 more mud w^hen its velocity is checked, but it scours more energet- 

 ically when its velocity is accelerated, the particles of sand and other 

 materials carried in suspension, acting as so many small brushes to 

 rub off such materials as may have previously lodged. The gi'eater 

 volume of water passing the ends of the reef has still two other 

 effects — it brings a large number of swimming larva? in contact with 

 the shells and it carries more food to the oysters living there. Clean 

 cultch, abundant larvie, and ample food, three principal factors in 



