RIVER VALLKYS OK THE WEST AFRICAN CONTINENT, ETC. 



149 



the 500 and the 1000-fathom contours, which may be taken 

 to represent the Continental Platform at a lower level {see 

 Fig. 1), and indicates a prolonged pause in the process of 

 elevation or depression of the crust at this stage. 



Along some parts of the coast, however, the descent into 

 deep water is very rapid, sometimes precipitous. Such is 

 the case along the Ivory Coast and the Bight of Benin, 

 where the descent from the 100 to the l,2U0-fathom contour 

 is precipitous. Off Cape Coast Castle the base of the 

 escarpment descends to over 2,000 fathoms (Figs. 2 and '6). 



ricj C Oections fo illustrate form of Ocean bcfl . W. Africa. 

 S. /I 7 J. •' N 



j± 5 n a rzt L . 



A t I a. rt-i L C O C & a n SSt ^,rf777/7?y>rTrfT7h 



l^^^vH ^.■rr-rr/m^///////m////////A 



^'9=^ Dahorrzey 



Atlantic Ocean. \ ^^/-^ 



From Cape Lopez southwards along the French Coast and 

 as far as Cape Lombo in Loanda, the soundings on the 

 cliarts are few, except in the vicinity of the Congo, but they 

 indicate a wide expansion of the Contiiiental Platform from 

 the const to the 100-fathom contour — in striking contrast 

 with the precipitous coast described to the north of this 

 tract. 



2, Base-level of Erosion, or Old Coast Line. — Assuming the 

 base of escarpment, or declivity, wliich descends from the 

 margin of the Continental Platform, to be the original land 

 margin at the time that the sub-oceanic rivers were eroding 

 their valleys and pouring their waters into the ocean of that 

 period, we are always able to define its depth with great 

 accuracy when we know the level at which the rivers open 

 out on tile sloping plane of the abyssal region, and we 

 have seen this level to be at (approximately) 1,200 fathoms 

 (7,200 feet) below the surface oft' the coast of AVestern 

 Europe. But Avhere rivers are absent, as along a large 



