684 DYNAMICAL GEOLOGY. 



have their bars and barriers, the great Mississippi sends its many arms 

 far out into the Gulf, prolonging its channels in the face of winds, 

 waves, and tides (Fig. 1093, p. 659). Incipient sand-bars at times 

 form; but these serve only to divide one of the great channels, and 

 make a new branch. 



3. Structure of Deposits. 



The kinds of deposits are as follows : — 



1. Beach Deposits : Consist generally of sand or gravel, sometimes 

 of large stones, the sands most mobile when wholly siliceous and when 

 not very fine ; often contain worn shells and other sea-relics ; stratifica- 

 tion irregular above high-tide level, but, below this, parallel to the 

 sloping surface of the beach; often have rill-marks where half buried 

 shells or stones have interrupted the flow of the waters down the 

 beach. 



2. Sand-flats, Sand-bars, Sand-reefs: Consist of sand, but sometimes 

 of coarser material, and often blend with the beach ; also graduate into 

 mud-flats where sheltered from the open sea ; upper surface commonly 

 between high and low-tide level, but often below the latter, also some- 

 times raised by the waves and winds to a higher level ; stratification 

 horizontal or parallel to the upper surface ; often covered by ripple- 

 marks, when exposed at low tide, from the sweep of the tide over the 

 surface, the waters pushing the sand into a ridgelet, as high as the 

 force can make, and then plunging over the little elevation to begin 

 another, thus making the first slope gradual and the closing steep ; 

 often have vertical borings made by sea-worms, mollusks, or certain 

 kinds of Crustaceans (as species of the Callianassa family). Dead and 

 worn as well as living shells, are common over its surface, and if 

 emerged at low tide, the tracks of worms, mollusks, and, if a mud- 

 bank, of birds. 



3. Bottom Deposits on Soundings outside of strong tidal currents : 

 Consist commonly of mud or silt, but often also of sand, and sometimes 

 of gravel ; stratification horizontal or nearly so ; contain beds of living 

 shells, and other marine species, differing in kind with the depth. 

 The plateau off the Atlantic coast, 50 to 80 miles wide, Long Island 

 Sound, and the bays of the coast, have deposits of this kind. Occa- 

 sionally they contain objects of fresh-water origin contributed by the 

 rivers, as floating plants, logs, transported shells, etc. 



4. Bottom Deposits beneath strong tidal currents: Consist of mud, 

 sand, or coarser material ; stratification horizontal, but owing to the 

 strong flow during the incoming tide and its alternation with high- 

 water and low-water quiet and the ebbing tide, layers that are obliquely 

 laminated alternate with ordinary layers. This oblique lamination 



