SHALLOW-WATER DEPOSITS 193 



plane surface, though there are some notable departures from flat- 

 ne>s. The steep slopes <>l" the delta fronts and of wave-built terraces 

 have already been noted (pp. 187, 191). Barriers may shut in de- 

 pressions, and the disposition of sediment may be uneven, owing 

 to shore and tidal currents. The result is that the surfaces of shal- 

 low-water deposits are affected by low swells and shallow sags. 

 The swells and sags may be elongate, circular, or irregular in outline. 

 This topography is in some cases preserved on newly emerged lands. 

 Chemical and organic deposits in shallow water. There is no 

 sharp line of distinction between the deposits classed as chemical 

 and those classed as organic. The latter are chemical in the broader 



1 iic. 199. Cross-bedded sandstone, Dells of the Wisconsin. The strata are 

 horizontal The laminae within each stratum dip notably. (Atwood.) 



sense of the term, but as they are directly associated with life 

 and arise from it, it is a matter of convenience to separate them. 

 Chemical deposits made in shallow sea- water embrace (i) those 

 due to evaporation, and (2) those due to chemical reactions between 

 constituents so brought together that new and insoluble compounds 

 are formed and precipitated. 



The chemical deposits made in the shallow water of the sea, 

 or in bodies of shallow water isolated from the sea, are chiefly 

 precipitates resulting from evaporation. All substances in solution 

 are necessarily precipitated on complete evaporation; but since the 

 sea-water is in general far from saturation, so far as all its leading 

 salts are concerned, only a few are thrown down in quantity suffi- 

 cient to be of geologic importance where evaporation is incomplete. 



