THE STRENGTH OF THE EARTH'S CRUST 41 



Buchanan states that this gentle bottom slope extends for 

 1,100 miles along the coast, and computes the volume contained 

 between the steep gradient presumably once existing and the flatter 

 gradient of the present bottom. This represents a deposit of 66,- 

 000 cubic nautical miles of detritus due principally to the Niger 

 and the Congo.' This great volume cannot be used safely, however, 

 as the measure of a load upon the crust, since a believer in the 

 theory of close isostatic compensation could claim with some degree 



Fig. 3. — -Delta of the Niger. Scale 1:10,000,000. From Andree's Allgemeiner 

 Handatlas, vierte Auflage. 



of reason that the initial slope of the concave shores of the Gulf of 

 Guinea need not have been as steep as the bold convexity of 

 Africa to the west, or that the load may have depressed the 

 bottom so as to have equalized the pressures. Furthermore, 

 Buchanan does not include any of the land area of the Niger delta. 

 The following estimates will give the volume only of the clearly 

 constructional part of the Niger delta, including both the land and 



' Op. cit., p. 8 and Fig. 3. The volume stated by Buchanan appears to be correct 

 if the two profiles have a common point taken upon the shoreline. In his figure, how- 

 ever, the common point A is shown as upon the loo-fathom contour. From this error 

 in the diagram given by Buchanan the volume estimated from the diagram would be 

 much less than 66,000 cubic nautical miles. 



