GEOLOGICAL IMPORTANCE OF SEDIMENTATION 541 



chances of temporary preserval until buried, but since the surfaces 

 upon which the fine-grained river waste is deposited are ordinarily 

 near the level of the sea and are also in the case of the greater deposits 

 frequently regions of subsidence, the chances for ultimate preserval 

 of the bulk of the formation is, in such cases, as favorable as that of 

 the true marine shallow- water deposits. 



Again, flood-plain surfaces are not of a transitional or temporary 

 geological nature, like the margins of interior lakes, or the borders 

 of the sea, but they are the ultimate physiographic forms toward 

 which both lakes and shallow seas tend by the filling-in of river 

 waste. They are of broad occurrence at all times of continental 

 extension and erosion, and should be looked for in the geological 

 column as only second in importance to the off-shore deposits of the 

 continental shelves and seas. But although flood-plains are most 

 commonly built near the margins of the land and encroaching as 

 deltas upon shallow seas, they are also found to occur over the 

 regions of graben or troughs of subsidence, such as those of the 

 Rhine, and of other tectonic valleys, and also over interior basins. 

 Murray has estimated the desert areas, that is those which do not 

 drain to the sea, as one-fifth of the continental areas. Doubtless, 

 at least another fifth is possessed of a climate marked by sufficient 

 seasons of drought to allow the broad formation of mud-cracks upon 

 flood-plains, following the subsidence of the flood waters. 



This natural condition is, however, largely modified at the present 

 time by the agency of man, since, by regulating the floods and by 

 systems of irrigation, such regions become the seat of populous 

 societies. It" has been shown in the previous article that the deposits 

 of flood-plains should enter more largely into the geological record 

 than is usually appreciated. Combining this conclusion with these 

 considerations in regard to climate it is seen that in those past times, 

 which corresponded in general to present conditions, an appreciable 

 fraction of argillaceous deposits should be characterized by mud- 

 cracks formed upon flood-plains, and, on the other hand, in regions 

 where the mud-cracks of the period are missing, another appreciable 

 fraction should by their carbonaceous and organic contents bear 

 witness of the verdure which prevails upon the river plains of pluvial 

 climates. 



