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 

 shallow 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 occm- only to be killed in winter, the dead 

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

 the crest ma}^ become raised to a level several feet above high water, 

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

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

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

 gi'owth of grass and brush, which more or less obscm-es thoir true 

 character. Sometimes the material is thrown up around thoir edges 

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

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

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

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

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



In open waters, not especially sul)ject to freshets, where the cur- 

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

 t«nd to maintain a round or oval outline, with no great difference 

 between the lon^ and short diameters; but where the currents are 

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

 themselves well al)Ove the bottom, they show a strong tendency to 



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

 uch 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, usuallj^ have their shoalest parts trans- 

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

 comnos<>d of a dense mass of shells and fragments. 



Tne 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 wnth well-known laws, cause eddies or 

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

 sheltered from it, in tidal waters the two being piTiodically 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 U])])er and lower sides 

 of the reef, while the somewhat accelerated flow around th(^ ends 

 scours the shells and ke<'ps them clean and fit to receive fresh sets of 

 youn<^ oysters. These factors operate more energetically the more 

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

 perfectly dear water. Not only does heavily silt-laden w^ater deposit 

 more mud wh(;n its velocity is checked, l)ut it scours more energet- 

 ically when its velocity is accelerated, the partichvs 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'(iater 

 volume of water pa'^sing the ends of the reef has still two other 

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

 the sluflls and it (;aiTies morc^ food to tlu^ oysters living there. Clean 

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



