488 PRINCIPLES OF STRATIGRAPHY 



from the sea-bottom. The bar grows until at last it rises above the 

 level of ordinary tides, and thus becomes a barrier beach. Mean- 

 while, at low tide, the sun dries out the upper portion of the sand, 

 which then becomes mobile, and is piled up into shoreward advanc- 

 ing sand-dunes. Thus a barrier-beach of some breadth may be 

 formed. While this is going on, deposition of sediments within the 

 lagoon behind the beach takes place, for here the water is mostly 

 quiet, while sediment is carried in both by streams from the land 

 and by the tide. As soon as an accumulation of mud or fine sand 

 over the bottom has begun, it is taken possession of by eel grasses 

 {Zostera marina, etc.), which soon cover the bottom with a dense 

 growth. These plants, belonging to the pond-lily family, have be- 

 come adapted to a marine habitat, and cannot live outside of the 

 salt water. Wherever these plants grow sufficiently near the sur- 

 face they form at low tide a tangle, passage through which is ac- 

 complished only with difficulty by the swimmer or the oarsman. 

 "A tidal current of two miles an hour, swift enough to carry much 

 sediment, is almost entirely deadened in this tangle of plants." 

 (Shaler-41.) As a result, the sediment will sink down between the 

 leaves of the plant and there will rapidly accumulate a bottom de- 

 posit of mud, which encloses the vertical stems of the plants. With 

 the fine sediment, fronds of sea-weed are carried by the current, 

 and these with pebbles or shells attached to their bases will settle 

 to the bottom and become buried in the mud. The presence of 

 decaying vegetal and animal matter gives a characteristic color 

 and odor to these deposits which may readily be observed wher- 

 ever the mud flats are exposed at low tide. Where the mud is cal- 

 careous, extensive beds of calcilutytes may be formed in this man- 

 ner, which may even retain the vertical impressions of the plants or 

 other organisms which have "combed" out the mud from the sea 

 water. What appears to be a good example of this kind is found 

 in the Ordovicic (Lowville) limestone of the Black River valley, 

 etc.. in New York, where the remains of the vertical stems form a 

 characteristic feature of the rock, the appearance on cross-section 

 having given rise to the term "Bird's Eye," by which the rock is 

 commonly known. 



W^here the mud has accumulated to such an extent that at low 

 water the flat is uncovered the eel grass will die, and its place is 

 gradually taken by the marsh grasses. For a time mussels will 

 occupy patches of the mud flat surface, and a mussel bed of some 

 thickness may be formed on the substratum of black mud. Micro- 

 scopic examination of the mud will show that it consists of ill- 

 assorted and generally angular grains. The marsh grasses com- 



