altitude is attained, tlic L^rowtii of tlui plants becomes retarded ; 

 the species of animals which aid in the formation of the marsh 

 deposits cease to live in the localit\- and the rate of decay 

 of the vegetable matter is sufficientl\' rapid to balance the 

 deposition which growth brings about. The fict that the 

 marshes cannot attain to the level of high tide is of mo- 

 mentous consequence to the condition of our harbors, for 

 the reason that it insures a permanent and considerable flow 

 of water through the remaining tidal channels. But for this 

 limitation many of our harbors which have attained to the 

 state where the marshes can no longer develop would already 

 have been closed. 



The marginal growth of the marshes when the develop- 

 ment of the held is near maturity comes to a point where 

 the efforts of the vegetation to win new ground are successfully 

 opposed by the flow of the tide that is made more energetic by 

 the extent to which it is confined within narrow limits through 

 the growth that has fenced it in. When the development is 

 complete there is no mud bank left for the roots to extend over ; 

 the slope toward the stream descends steeply into tolerabK- 

 deep water which often affords an excellent haven for ships. 

 So far as possible the grcnvth of the marsh plants anil the 

 formation of their dense mat of root fibers deepen the tidal 

 stream by narrowing the horizontal distance between the banks. 

 In fact, the ultimate conditions of the area are not completely 

 attained until the principal creeks have been forced to excavate 

 their beds to a depth below that which they had before the 

 shelf of the marsh began to press upon the streams. Hence it 

 comes about that in most cases the depth of water in their chan- 

 nels is considerably greater than in the part of their path which 

 lies beyond the limits of the grassed fields. 



