CONDITIONS OF SEDIMENTARY DEPOSITION. 493 



probable that the influence of tides is often subordinate to that 

 of winds, of the difference of density between fresh and salt 

 water, of mechanical and chemical reactions of salt water on 

 sediments, and of currents prevailing along shore. - 



The influence of tides upon undertow, tending alternately to 

 retard and accelerate the seaward current, may be important and 

 may lead to alternate episodes of deposition and scouring as it 

 does in estuaries ; this is probably the case on all submerged 

 continental platforms, and particularly where tides sweep in from 

 a great expanse of ocean, as on the Atlantic coast of the United 

 States. The effect, where conditions favor it, would be more 

 regular than among the shoals and channels of an advancing 

 delta, and the alternation of strata would be more distinct and 

 even ; it is possible that thinly interbedded strata of unlike 

 character may be thus interpreted. 



The well recognized characteristics of tidal formations are 

 the evidences of shallow water, ripple marks, sun cracks, organic 

 trails, etc., peculiar to sections of the shore where sediment is 

 abundant. The strata are shales, and shaley sandstones 

 irregularly bedded and often red. Such deposits are direct evi- 

 dence that : 



( 1 ) The land from which they came presented gentle slopes 

 and was mantled in residual formations to a distance from the sea. 



(2) Since the zone of tide-flats along any shore is limited in 

 width, if the distribution of such strata be wide, either great 

 rivers gradually filled a shallow basin, as the Mississippi, the 

 Amazon and Parana have done, or the sea transgressed upon a 

 low-level land. In the former case the land was built outward 

 by volumes of muddy fresh water, and the deposits would be of 

 fresh or brackish water types. In the latter case the sea pre- 

 vailed and the deposits would be of marine character. 



(3) Since the level of tidal deposits is near the surface of 

 the water, and they are therefore limited in thickness, if a con- 

 siderable thickness shows the characteristic marks throughout, 

 the area of deposition subsided at a rate approximating to that 

 of accumulation. 



