38 JOSEPH BARRELL 



Their location, volume, and limits in most cases are not known. 

 Their modern and smaller representatives, as they build out into 

 shallow water, do not greatly increase the load upon the crust. 

 Deltas recently drowned are therefore not well adapted to serve 

 as tests of the strength of the crust. 



Deltas which lie in re-entrant angles of the continents are also 

 poorly adapted to be used as a test. Those of the Indus, the 

 Ganges, and the Colorado are illustrations. As they fill up the 

 heads of gulfs and are without the typical convex outhne, it is not 

 only difficult to compute their volume but their situation is such as 

 to suggest that even without the construction of the delta the 

 region might be far out of isostatic adjustment. 



Certain rivers, which face the open ocean, such as the Columbia, 

 do not build deltas because of the power of the waves and currents 

 which sweep laterally the fine detritus. 



Many rivers, however, build considerable submarine deltas 

 even where the in-planing forces of the ocean prevent a terrestrial 

 outward growth. Such submarine deltas, owing probably to the 

 power of the waves rather than to recent submergence, are marked 

 by convexities in the bathymetric contours opposite the river 

 mouths. The Congo, the Orange, and the Zambesi are examples. 

 These hidden deltas which are built out into deep waters cannot 

 reach more than a certain distance from the shore and part of their 

 detritus is carried laterally along shore by the waves, but never- 

 theless they possess a very considerable volume and the convexity 

 which they make upon the ocean floor shows to that degree the 

 rigidity of the crust. 



The maximum test is found where great rivers have carried 

 forward subaerial topset beds of their deltas over what was pre- 

 viously deep ocean. Fluviatile construction in such examples has 

 dominated over marine destruction, giving a convex outline to the 

 shore; but the subaqueous deposits may still make up the greater 

 part of the volume. Even in these cases the question may be raised 

 whether the deltas have attained the maximum possible size per- 

 mitted by the strength of the crust. Their size may, on the con- 

 trary, be limited even here by the balance of the surface agencies 

 and the limited time during which the river has dominated over 



