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STUDIES FOR STUDENTS 



Gommon to the littoral zone may occur between truly marine forma- 

 tions. Thus, first, the perception of the principle that erosion domi- 

 nates the land and sedimentation is largely restricted to the sea; 

 second, the fact that in Europe and eastern America river aggradation 

 is much less important that in many other regions; third, the confu- 

 sion of littoral and flood-plain deposits ; and, fourth, the grouping of 

 litt^.'ral with marine formations, render it probable that in the past 

 certain unfossiliferous river deposits have been misinterpreted as 

 marine or estuarine. 



It is intended to show that the area of the zone of true littoral 

 deposits is always a small fraction of the area of shallow- water marine 

 deposits, and usually but a small fraction of the area of various forms 

 of land deposition; that, furthermore, the chances for the preserval 

 of littoral deposits is slight in comparison with those of either marine 

 or continental origin, and consequently that, unless an ancient forma- 

 tion is clearly of littoral origin, it is more likely to be either marine or 

 continental. For that reason it will be necessary to discuss the rela- 

 tive areas of deposition of the three classes of deposits, the charac- 

 teristics which they hold in common, some of their distinctive features^ 

 and especially their relative chances for preserval. 



THE REGIONS OF CONTINENTAL SEDIMENTATION 



Formations made upon the land may be classified under several 

 divisions, as follows: 



Desert deposits. — Typically where the evaporation exceeds the 

 precipitation and no outflowing drainage results. 



Piedmont river deposits. — Built up by rivers or shallow lakes upon 

 the foreland plains or piedmont belt fronting high mountain ranges. 



Basin deposits 0} pluvial climates.— The deposits laid down by" 

 rivers or in lakes in down- warped basins, such as these of the Great 

 Lakes, situated in continental interiors, but not necessarily asscciated 

 with mountains. If a large river, laden with sediment, flows acrcss 

 such a region, a lake condition can hardly arise, but, on the contrary,, 

 a broad river plain is more likely to be found, constantly built up as 

 subsidence takes place. 



Suhaerial delta deposits. — Where powerful and sediment-laden rivers 

 meet the sea, especially if the latter is shallow and protected from 



