442 STUDIES FOR STUDENTS 



slightly above the level of the sea, and subsidence in such regions is 

 observed ordinarily to go forward with accumulation. This will 

 indefinitely protect the formation until some new and adventitious 

 geological activity reverses the processes which resulted in the accumu- 

 lation. 



To sum up, then, it is seen that the geological processes which 

 result in the accumulation of desert deposits, if carried to their limit, 

 will ultimately destroy those same deposits, as Passarge has recently 

 shown. ^ The same is apt to be largely true of Piedmont plains of 

 river deposits ; but in the case of interior basins, and more especially 

 in deltas, the conditions are most favorable both for temporary and 

 ultimate preserval of the land-surface record. 



The same is, of course, true of the record made on the ocean 

 bottom, since this is normally the region of deposit and not of erosion. 



CONDITIONS FOR PRESERVAL OF THE LITTORAL RECORD 



In regard to littoral deposits it will be seen, however, that the 

 chances are frequently unfavorable for the preserval of its deposits 

 until burial. 



1. On an emerging land, such deposits would form a surface 

 veneer and be the first layer to suffer erosion, before even a chance 

 for lithification had occurred. 



2. On a stationary or slightly subsiding coast, where delta deposits 

 are encroaching upon the sea, the fresh-water material will fill up 

 the estuaries and lagoons, covering and preserving their records, and 

 crowding the beaches farther out to sea. The littoral deposits in 

 that case would be preserved as old beach, lagoon, and estuarine 

 deposits transitional, in a vertical section, between the off-shore 

 marine deposits below and the fresh- water land-surface record above. 



3. Where an old land surface is slowly subsiding, the weak ero- 

 sive power of the upper portions of the rivers is no longer able to 

 supply sediment for building out deltas against the sea, and there is, 

 on the contrary, a transgression of the sea across the land. Assuming 

 that the land surface slopes gently seaward, the depth of the littoral 

 deposits behind the barrier beach where such exists cannot in general 



I " RumpfBachen und Inselberg," Zeitschrijt der deutschen geologischen Gesell- 

 schaft, Vol. LVI (1904), Protokol., 193-209; review by W. M. Davis, Science, Vol. 

 XXI (1905), p. 825. 



