234 SEA AND LAND 



Wherever the shore is of an earthy nature at high-tide level 

 a number of species of flowering plants, principally belonging 

 to the grasses, take root a little below high-tide mark, grow 

 luxuriantly, and develop a dense mat of lowly vegetation. The 

 tops of these plants die down each winter, but their roots 

 are perennial and closely interlaced, forming a very dense 

 and elastic tangle, which is well fitted to resist the blows 

 of the waves. In course of time the interwoven living and 

 dead roots form a mass, having a thickness of from one to two 

 feet. Its upper surface is always so placed as to be covered 

 by each high tide, except perhaps on a few days in the year. 

 In most cases the water rises high enough to cover all but 

 the tops of the full grown stems. When the tide is low the 

 whole of these marine marshes is laid bare ; generally, in- 

 deed, they do not extend below the level of half tide. 



In the process of its growth the mass of marine marsh 

 pushes horizontally outward toward the mud flats. The margin 

 of the accumulation is commonly somewhat undermined by 

 the action of the waves, so that it falls as a sloping curtain, 

 and in a measure protects the bank from such surges as may 

 form in a tolerably well sheltered basin. From time to time 

 the waves and ice break off masses from the low cliff which 

 borders the mud flat. This debris is commonly distributed 

 over the bottom next the growing marsh and helps to shal- 

 low the water to the point where the grasses can take root. 

 In this manner the growth is extended over the deposits 

 of ooze, it may be for a distance of miles. The limitation 

 in the spread of the marsh is effected in some cases by the 

 fact that the margin comes into a position where the waves 

 break it up, but more commonly the restriction is brought 

 about by the action of the tide in keeping open channels by 



