33^ STUDIES FOR STUDENTS 



of river characteristics, it is seen that at least in North America pied- 

 mont slopes of alluvial waste are now frequently undergoing dissec- 

 tion, but that the great delta regions, such as those of the Mississippi 

 and the Colorado, are receiving annual accessions of fresh-water 

 deposits. In many cases, especially on the eastern coast, subsidence 

 . has been so recent that the rivers have only begun the work of filling 

 their embayed valleys, and notable deltas have not yet been con- 

 structed. On the whole, however, the work at the river mouths 

 stands in contrast to that of their middle and upper courses. Extend- 

 ing the view to the greater rivers of the world, it would seem to be a 

 safe conclusion that, as a rule, they are building up their flood-plains, 

 even if not encroaching upon the sea. 



The problem next following is whether this is due to a local station- 

 ary attitude, or even subsidence of the delta regions, or to an average 

 general stationary attitude or subsidence of the lands as a whole. 

 In answer to this general problem there appears to be no unanimity 

 of opinion. The continental margins frequently show drowned coastal 

 shelves cut across by river gorges, but this is no indication as to the 

 character of present movements. In Andree's Hand- Atlas, p. 4 

 (Leipzig, 1904), movements of elevation are indicated as taking place 

 at the present time at many places along the shores of all continents, 

 while movements of depression are indicated only along the eastern 

 coast of North America, the coasts of France and the Netherlands, 

 the eastern shore of the Adriatic, the delta regions of the Nile and 

 Amazon, and more than half of the oceanic islands. While there are 

 undoubted errors of detail in this map, it may be assumed as a first 

 hypothesis that the general result is true, and that the continents at 

 the present time are in a general stage of upward movement; or, per- 

 haps speaking more correctly, the oceans, by the subsidence of their 

 basins, are receding from the lands. 



If this conclusion be true, the subsidence of regions of sedimenta- 

 tion at the present time is probably less conspicuous than in the 

 average of past time, since it is only where the local downward move- 

 ment is more pronounced than the general regional upward move- 

 ment of the lands that actual subsidence would result. 



Turning from induction to deduction, subsidence of deltas as dis- 

 tinct from the surrounding regions is to be anticipated only where the 



