246 SEA AND LAND 



shore is largely to be exphiined by the great and swiftly accu- 

 mulated deposits of such shells which are formed in the south- 

 ern district. 



In proportion as we go southward the share of vegetable 

 matter which is built into the marine marshes rapidly dimin- 

 ishes until in the Georgia district, although the species of 

 the salt-loving grasses and rushes, as well as other plants, 

 are more numerous and, as regards growth, more vigorous 

 than in the northern districts, the contribution which they 

 make in the way of sediments is inconsiderable. The reason 

 for this is easily seen. Owing to the greater warmth of 

 the southern realm the process of decay of the dead vege- 

 tation is much more rapid and complete than in the north. 

 On the other hand, the warmth of the water in the southern 

 field favors the rapid development of shell-bearing animals, 

 particularly of the oysters, which contribute vast amounts of 

 debris to the growth of the marsh deposits. The result is 

 that the southern growths of this nature are formed with far 

 greater rapidity than those of the northern realm. Along 

 the o-reater part of the coast south of Charleston the devel- 

 opment of these organic benches has attained the point 

 where their further extension is arrested on account of the 

 energy of the tidal movement. In fact the conditions have 

 come to the balanced state, which, as we have seen, is the 

 natural result of the struggle between the organic actions 

 which tend to construct marine marshes and the physical 

 forces which contend against their growth. 



There are many other marine animals which here and 

 there in varying quantities form organic sediments which 

 lead to the shallowing of harbors. Of these we can only 

 incidentally note a few of the more important. The common 



