OSTRACEJE ; OYSTERS. 399 



the mouth, they are evidently very susceptible of the influence 

 of light, having been observed to close their shells when the 

 shadow of a boat passes over them. 



938. The enormous number of Oysters which exist on our 

 shores, may be best judged of by considering the extent of the 

 banks which they form. These beds occupy portions of the sea, in 

 shallow parts, extending for miles in each direction; and in some 

 places, the depth of the stratum is very considerable. A remark- 

 able growth of them exists along the alluvial shores of Georgia, in 

 North America ; and their influence in preventing the encroach- 

 ments of the sea is very important. The marsh land extends 

 inwards for a space of from twelve to eighteen miles ; and it is so 

 soft, that an iron rod might be pushed into it without difficulty 

 to the depth of 18 or 20 feet. A great number of large creeks 

 and rivers are found meandering through these marshes ; and the 

 bends of these rivers would in a short time cut through the 

 adjoining land to such an extent, that the whole seaboard would 

 become a quagmire. But wherever the tide directs its destroying 

 force, its effects are counteracted by walls of living oysters, which 

 grow upon each other from the beds of the rivers to the very 

 verge of the banks. These hillocks are often found in bunches 

 among the long grass growing upon the surface of the soil. They 

 are in such abundance, that a vessel of a hundred tons might 

 load herself in three times her own length. These banks are 

 the favourite resort of fish and birds, as well as of the racoon 

 and some other quadrupeds. The neighbouring inhabitants will 

 sometimes light a fire upon the marsh-grass, roll a bunch of 

 oysters upon it, and then eat their contents. This barrier of 

 oysters, like rocks of coral, must offer the strongest resistance to 

 the force of the tide. Such immense collections of shells are 

 very interesting when viewed in relation to Geological pheno- 

 mena; since whole strata of rock are often found entirely 

 composed of shells thus aggregated ; which probably occupied, 

 on the shores of the land then upraised above the surface of the 

 ocean, a position similar to that in which we find the oyster- 

 beds at the present time. 



939. Notwithstanding the enormous number of Oysters 



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