42 JOSEPH BARRELL 



the sea portion. But it will be seen, from Buchanan's statements, 

 that this is a minimum estimate of the areal load imposed by the 

 rivers, for a more or less continuous burden on the crust would 

 appear to stretch for a thousand miles along this African coast, 

 reaching a maximum unit value, however, in the great delta of the 

 Niger. 



The outer limits of the deltas were taken where the convex 

 slopes fade out into the general ocean bottom. 



The results of computing the volumes shown between the two 

 sets of contour lines are as follows: 



TABLE I 



Delta of the Nile 



Area within i,ooo-m. contour 71,000 sq. km. (27,400 sq. mi.) 



Area within 2,000-m. contour 106,000 sq. km. (38,800 sq. mi.) 



Radius of equivalent circle 175 km. (no mi.) 



Equivalence in equatorial square degrees 8.6 sq. degr. 



Average thickness within assumed limits 0.84 km. (2,800 ft.) 



Equivalence in rock upon land o. 46 km. (1,540 ft.) 



Ratio to 76 miles of crust i to 260 = 0. 0038 



Maximum thickness 2 . 0-2 . 3 km. (6,600-7,600 ft.) 



Equivalence in rock upon land i . i-i . 3 km. (3,600-4,200 ft.) 



Volume within assumed limits (extending oh 



the east to somewhat below 2,000 m.) 89,000 cu. km. (21,300 cu. mi) 



Equivalence in rock upon land 50,000 cu. km. (11,700 cu. mi.) 



TABLE II 



Delta oe the Niger 



Area within the assumed limits 195,000 sq. km. (75,300 sq. mi.) 



Radius of equivalent circle 250 km. (155 mi.) 



Equivalence in equatorial square degrees 15.8 sq. degr. 



Average thickness within assumed limits i . i km. (3,600 ft.) 



Equivalence in rock upon land 0.6 km. (1,980 ft.) 



Ratio to 76 miles of crust i to 200= .005 



Maximum thickness 3.0 km. (9,900 ft.) 



Equivalence in rock upon land 1.65 km. (5,450 ft.) 



Volume within assumed limits 217,000 cu. km. (52,000 cu. mi.) 



Equivalence in rock upon land 120,000 cu. km. (27,000 cu. mi.) 



The deltas in their growth had displaced their volume of water. 

 The added loads which they throw upon the crust are measured by 



