or SAND FORMATIONS ON MARINE COASTS. 15 



from the land through the erosion and transportation of waves and cur- 

 rents. This material consists usually of coarse or fine sand, but may 

 include some beds of pebbles or stones when the currents are strong. 

 The stratification is comparatively regular and nearly horizontal. 



3. When the accumulations just spoken of increase under wave- 

 action in shallow water, until they rise above low tide level, they form 

 sandy islands. 



3. Sand}' spits are the lengthwise extensions of beaches formed 

 through the waves throwing material on shoals at the turn of the shore. 

 Their composition is similar to that of the above formations. 



'4. Sandy beaches are made by material thrown up on the shore by 

 waves. This material is coarse where the waves break heavily, because, 

 although trituration is going on at all times, the powerful wave action 

 and the undertow carry ofi: the finer material seaward into the offshore 

 shallow waters, where it settles over the bottom or is distributed by 

 currents. It is fine where the waves are gentle in movement, as in 

 sheltered bays, or estuaries, the triturated material accumulating in 

 such places near where it is made. 



As soon as the accumulations of eroded material have increased so 

 far as to rise above the surface of the water, the further growth is 

 similar to that of the beaches, and from these latter other coastal sand 

 formations such as dunes and sand fields are developed by the influence 

 of wind. The development of these two kinds of eolian sand formations 

 will be discussed in detail under separate headings. 



It is a well known fact that the salts of seawater hasten the deposi- 

 tion of sediments, and consequently the shape and formation of sand 

 banks and beaches on marine coasts is somewhat different from those 

 of corresponding freshwater deposits. I have not been able to ascertain 

 whether the seawater acts differently on siliceous material than on clay 

 sediments. We usually find that deposits nearer the shore or the source 

 of the material contain more silica than further out in the deep water, 

 but this may depend on the usually large size and the greater weight 

 of the siliceous fragments, which causes them to sink sooner. 



In order to determine this, experiments were conducted in the labora- 

 tory. I tried ordinary seawater from the Baltic, of a salinit}' of 0.6% 

 measured with areometer, and artificially prepared solutions of resp. 

 2.7% and 3% corresponding to the salt content of ocean water. Finely 

 ground clay and beach sand were stirred in the water samples, and 

 allowed to settle in vessels 25 cm., 50 cm., and 100 em. deep, all being 

 42 cm. in diameter. The following results were obtained: 



