46 JOSEPH BARRELL 



forms spread over areas of 40,000-75,000 square miles and reaching 

 thicknesses in air over considerable areas of 4,000-5,000 feet. 



The tabulation of the data regarding the deltas shows the area 

 of the Niger delta to be equivalent to a circle 310 miles in diameter 

 and that over this area the load of the delta is one two-hundredths 

 the weight of the crust to a depth of 76 miles, this being the depth 

 of the zone of isostatic compensation given by the latest determi- 

 nation of Hayford. 



According to Hoskins, in a calculation made for Chamberlin 

 and SaHsbury,' 



a dome corresponding perfectly to the sphericity of the earth and formed of 

 firm crystalline rock of the high crushing strength of 25,000 pounds to the 

 square inch, and having a weight of 180 pounds to the cubic foot, would, if 

 unsupported below, sustain only 5+5 of its own weight. This result is essen- 

 tially independent of the extent of the dome, and also its thickness, provided 

 the former is continental and the latter does not exceed a small fraction of the 

 earth's radius. 



The delta, though large, is so limited in size in comparison with 

 continental areas that it would be somewhat more effectively sup- 

 ported, but its externally convex form can hardly be supposed to 

 give it added domal strength, since it consists of more or less uncon- 

 solidated material piled upon a concave floor. 



The theory of isostasy holds that at a certain depth in the crust 

 there is an approach to equal pressures, the larger relief of the sur- 

 face being balanced in large part by subsurface variations in 

 density. The larger segments of the crust tend to rise or sink until 

 the elevations are in adjustment to the density beneath. A corol- 

 lary of this theory is that unbalanced surface loads are largely 

 sustained by the strength of the crust above this level of equal 

 pressures; in other words, but little of the load is transmitted to 

 the deeper earth below. For purposes of discussion it may then be 

 assumed that the load of the Niger delta is supported by the outer 

 76 miles of crust. This depth is one-fourth of the diameter of the 

 circle equivalent in area to the delta. The load over this area, as 

 stated, is one two-hundredths of the weight of the supporting crust. 

 Allowing something for the lunited area of the delta, it is seen never- 



' Geology, I, 555, 1904. 



