THE STRENGTH OF THE EARTH'S CRUST 



39 



the sea. It is a fair presumption, however, that the largest deltas 

 have reached a size where subsidence keeps pace with added 

 volume. 



The deltas of the Nile and Niger. — Only the most powerful rivers, 

 laden with abundant waste and protected by their situation from 

 the heavier wave and current action, can build deltas of this last 

 class directly into ocean basins. Perhaps the two best of the few 

 good examples are those of the Nile and the Niger. Both have 

 built out great deltas from regularly curving shores of the Atlantic 

 type — the type where recent folded mountains do not mark the 

 line between continent and ocean, the type where tangential forces 



Fig. I. — -Delta of the Nile. 

 Handatlas, vierte Auflage. 



Scale 1 : 10,000,000. From Andree's Allgemeiner 



cannot be supposed to have disturbed recently the isostatic balance 

 of continent and ocean. 



To determine the areas, depths, and volumes of the deltas from 

 the standpoint of isostasy, a smooth curve, as shown in Figs, i and 

 3, was continued through them from the shore beyond. The sub- 

 marine contours were also projected in dotted lines, giving the form 

 of the bottom as it presumably would now be if no rivers at these 

 places had entered the sea. The volume of the deltas may then be 

 determined by computing the volume included between these two 

 sets of contour lines. 



In both cases, in so far as the positions of the hypothetical 

 bottom contours are open to doubt, they have been located some- 

 what above a most probable position, so as to tend to throw the 

 error of computation in the direction of too small rather than too 



