Uplift and Subsidence. 13 



the Niger and the Nile in his tests. He showed that the 

 present configuration of the bottom of the ocean at the 

 mouth of the Niger indicates that there is much material 

 present in a space formerly occupied by water. He 

 concluded that this material is an extra load and is a 

 measure of the earth's strength. It appears to the writer 

 that we may have an explanation of the presence of this 

 sedimentary material which will be consistent with the 

 theory of isostasy and the view that the earth's crust is 

 not excessively strong. 



The apparent maximum depth of the sediments in the 

 delta of the Niger is approximately 10,000 feet. The 

 density of this material is probably about 2.4. The 

 density of sea-water is slightly over 1.0. Therefore the 

 weight of water displaced by sediments is approximately 

 the weight of 40 per cent of the material deposited. There 

 remain 6000 feet of sedimental material which Barrell 

 claimed is the overload. 



If we should have a sinking of the sedimentary material 

 in the isostatic shell, as we find has taken place in many 

 parts of the earth, then we should expect to have some 

 sediment above the former position of the bottom of the 

 ocean. The sedimentary material is approximately 20 

 per cent lighter than the material whose space it occupies 

 and, therefore, as a thousand feet of sediment is deposited, 

 we can expect 800 feet of this material to sink into the 

 earth and 200 feet to stand out. We can explain the ten 

 thousand feet of material above the former ocean bed 

 if the total thickness of sediments under the surface of the 

 Niger delta is 34,000 feet. There would be 24,000 feet 

 below the original position of the base of the sediments. 

 This does not seem to be an excessive depth of sediment, 

 judging from what has been found in other parts of the 

 earth. 



Barrell has found a depth of 7000 feet of sedimentary 

 material under the surface of the delta of the Nile. If 

 we use the same reasoning in regard to the Nile delta as 

 for the Niger, we shall require only 24,000 feet of sedi- 

 ments to enable 7000 feet to project above the surface 

 comcidmg with the former bottom of the Mediterranean. 



A test of this theory that the delta formation is not an 

 overload could be made by gravity observations. Unfor- 

 tunately, we have only one gravity station on a well- 

 defined delta. This is at New Orleans, La. Here the 



