44 JOSEPH BARRELL 



seen that in contrast to the Nile delta the slopes are much steeper 

 between the i,ooo- to 2,000-meter than between the 200- to 1,000- 

 meter contours. This can be explained by assuming that the steep 

 slope lying below and beyond a flatter slope was once a foreset slope 

 just below wave base, whereas it now lies at least 800 meters below. 

 If such a subsidence has occurred, it appears, however, to have 

 been confined to within the limits of the delta; since a peripheral 

 overdeepening of the ocean floor is not evident. On the other hand, 

 it is noted by Penck, but probably too sweepingly, that all bathy- 

 metric curves have their steepest slopes between 1,000 and 2,000 

 meters in depth.^ Such a phenomenon might be due to lateral flow 

 of sediment under a certain depth of load and without relation to 

 subsidence of the base. The question whether the load of the 

 Niger delta is as great as the crust can bear is therefore an open one. 



The Gulf of Guinea, where now the delta is built, is regarded 

 by many geologists as having originated since the Middle Mesozoic 

 by a breaking-down from the continent of Gondwana, but the 

 presence of Middle Cretaceous marine beds skirting much of the 

 coast of West Africa suggests perhaps that the delta in its con- 

 struction does not go back of the Tertiary. In fact it would seem 

 possible from the youthful rehef of the continental plateau that 

 the delta built from its waste is of Upper Tertiary and Pleistocene 

 growth. 



A single delta might happen to be a mere veneer of sediment 

 upon an originally slightly submerged projecting part of the coast. 

 Such a fortuitous coincidence of unrelated circumstances may, 

 however, be dismissed as highly improbable in the case of two 

 great rivers draining in opposite directions from the same continent. 

 The conclusion that these deltas are really externally constructive 

 features and measure a real strain upon the crust is strengthened 

 by noting the submarine deltas opposite the other great rivers of 

 Africa, built into the ocean, even though the waves and currents 

 have limited them by preventing their subaerial seaward growth. 



In the mechanics of the relation of the delta to the stresses in 

 the crust an important factor is the nature of the marginal land. 

 Shores of the Pacific type have great mountain systems marginal 



^ Morphologic dcr Erdoherjidche, I (1894), 146. 



