826 THE EEGOLITH 



and moUnsca which live on its stalks or on the bottom contribute 

 to the deposit, so that it thickens with considerable rapidity. 



When the bed thus formed has risen to the point where it is 

 dry at low water of the ordinary run of tides, the eel-grass 

 can no longer maintain itself, but gives place to other groups 

 of sea-weeds and grasses. These plants find their place first 

 near the shore line, where the eel-grass platform is naturally the 

 highest. At first the vegetation is quite sparse, owing to the 

 difificulty with which they endure the depth of water at high tide. 

 There is often, indeed, a considerable difficulty in establishing the 

 growth of the second group of plants, and for a while the de- 

 posit takes the shape of bare mud-flats, dependent in the main 

 for their accumulation of detrital matter on the growth of 

 certain mollusea, especially of the genera Mytilus and Modiola. 



Pig. 35.— Cross-seetion of marine marsh, a, original surface of shore line; 

 }), grassy marsh; Cj mud-flats; d, eel-grass; e, mud accumulated in eel- 

 grass. 



When, as is usually the ease, the more highly organized plants 

 have difSculty in establishing themselves over the broad surface 

 of the mud-flat, they win their way to it in the following manner. 

 From the vantage ground of the shore line, where are found 

 the conditions of submergence which suit their needs, the plants 

 slowly extend the front of their bench out over the mud-flats. 

 (See Fig. 35.) This process of growth can be more easily studied 

 than that of the earlier or eel-grass stage of the marshes, for it is 

 visible along miles of our sea-shore. The higher grasses have even 

 more thick-set stems than those of the eel-grass flats; they en- 

 tangle sediment even more effectively. At first their stems are 

 covered for a few hours at each ordinary tide; they gather 

 waste rapidly, and soon lift the plain which they are constructing 

 up to the point where only at the highest tides are the tops 

 covered by water. At this stage the growth of the deposit is 

 practically arrested, there being no means of increase save from 

 the decay of the grasses themselves. 



