60 ALLUVIUM. 



of the ocean. Alluvium is foniied upon marshes, 

 near the coast, by the decay of vegetation, by ma- 

 terial retained after the flow of the tide, and by 

 that deposited by streams. In northern latitudes, 

 peat is thus formed. In the muddy deposit at the 

 bottom of lakes, ponds, or inlets, aquatic plants 

 begin to flourish, and by their decay increase the 

 depth of the deposits ; at last it reaches the surface 

 of the water, and becomes a marsh. The top of 

 this deposit is composed of vegetable fibre hardly 

 decomposed at all, and from this is a regular grada- 

 tion to perfectly-formed peat in the lower strata. 



Dr. James E. De Kay, Zoologist for the State of 

 New York, says : " In a calm, still day I have fre- 

 quently noticed the surface of the water covered 

 with patches of sand, varying in extent from one to 

 six or eight inches square. These patches are com- 

 posed, of course, only of the finer portions of sand, 

 adhering to each other by a thin film of gelatinous 

 matter, which gives buoyancy to the mass. I have 

 been surrounded frequently by patches of this kind 

 in tolerably close contact, and covering a surface of 

 several hundred acres. The lightest touch of an 

 oar, or a slight breeze, causes them to sink immedi- 

 ately. The rationale of their formation I conceive 

 to be this : the shore we know to be peopled with 

 myriads of minute mollusca, furnishing, either by 

 their excrements or their own proper bodies, a 

 gelatinous substance, which hardens upon exposure 



