MARINE MARSHES 



493 



as outlined above in Shaler's hypothesis. Such marsh deposits are 

 now forming on the Long Island and New Jersey coasts. As the 

 coast slowly subsides the marsh grasses will encroach upon the 

 dead fresh water swamps, and since the subsidence is a slow one 

 only the plants growing at high water level or above will enter into 

 the constitution of this deposit. Among these on the Massachu- 

 setts coast, the grass Spartina patens Muhl. is the chief type, which 

 will form successive layers if the subsidence is slow, and so a pro- 

 gressive overlap of the Spartina patens layer over the fresh water 

 peat will take place, with the result that this peat thickens seaward 

 because in that direction deposition began earlier. During all this 

 time the lagoon beyond the area of the fresh water peat deposit 

 may be slowly filling up and the ordinary salt peat would form, 

 beginning with the more euryhaline species of salt grasses, such as 

 Spartina glabra, and followed by the more stenohaline species, such 

 as 5". patens. Progressive erosion and landward migration of the 

 outer bar would eventually result in the entire removal of the 

 purely marine peat series, so that, as in the case of the Massachu- 

 setts marshes, the only part remaining behind the bar is the com- 

 pound peat mass, commonly of fresh water peat at the bottom and 

 salt water peat on top. 



In such cases as these the bar has successively migrated shore- 

 ward until it has reached if not transgressed the original shore 

 which existed at the time that the fresh water peat was forming on 

 the coastal plain. 



If the subsidence is more rapid than the upbuilding of the 

 Spartina patens peat, the more frequent submergence of the surface 

 will kill that species and its place is taken by the more euryhaline 

 S. glabra, which, in turn, may be succeeded by a mussel bed or 

 mud flat. 



Conversion of salt peat into coal. The burial of a salt peat 

 marsh under silts of marine or terrestrial origin would tend to its 

 preservation and ultimate conversion into coal. Such coal will, 

 however, be very high in ash, for the high tides, especially during 

 stormy periods, will spread much silt over them, while wind-blown 

 sand from the shore may also add a quantity of mineral matter. 

 Hydrogen sulphide is generated in great abundance in peat beds 

 to which salt water has access. This is apparently due to the action 

 of certain bacteria on the sulphates contained in the water. By 

 reaction with iron compounds of the silt and other mineral mat- 

 ter iron sulphides are formed, which are deposited as iron pyrite. 

 This mineral is abundant in the inorganic constituent of the salt 

 marsh peat. Such formation of hydrogen sulphide does not occur 



