CHAPTER V — 
Submerged River -Valleys off the Coast of West Africa 
EXPLANATION TO ACCOMPANY PLATE VI 
FTER passing the Straits of Gibraltar, and following the coast of the African Continent, a 
change takes place in the general character of the submerged region between the coast and 
the abyssal ocean beyond. The Continental Platform loses much of its uniformity and 
importance, and sometimes altogether disappears, or passes into a more or less gradual slope 
from the 1oo-fathom contour to that of 1200 fathoms. This slope, in all probability, consists of a 
succession of minor terraces breaking off in steps, which it would be difficult to determine with certainty, 
unless’with the aid of maps on a large scale containing very numerous soundings. The change seems 
to be indicated by the contraction of the shelf on approaching the vicinity of Cape St. Vincent, where 
its breadth varies from 10 to 20 miles only. Directly opposite the Straits, and again along the coast of 
Morocco, between 31° 10’ N. and the Canary Islands, there occurs a narrow terrace, with a steep slope 
from the 100- to the 614-fathom contour, and from this latter a gently shelving terrace of prolonged 
extent is continued down to the abyssal region of the ocean. 
Along other parts of the coast, however, the descent into deep ocean is very rapid ; sometimes 
precipitous. Such is the case off the coast of Cape Coast Castle (Ashanti), and again off Dahomey. From 
Cape Lopez southwards as far as Cape Lombo, the soundings on the charts are few, except in the vicinity 
of the Congo; but they indicate a wide expansion of the Continental Platform from the coast to the 
100-fathom contour—in striking contrast with the precipitous coast described to the north of this tract. 
Of the several great rivers entering the ocean along the western coast of Africa, including the 
Senegal, the Niger (or Quarra), the Congo, the Coanza, the Cunene, and the Orange, only the Congo 
can be traced from its source to its original outlet'at the margin of the abyssal ocean. ‘There can be no 
question but that the channels of all of these rivers are continued under the. ocean ; but they cannot be 
traced on the map owing to the insufficiency of the soundings on the charts. Such is the case with the 
Grand Bassam (in lat. 5° N.), described on the charts as the “ Bottomless Pit,” which was sounded by the 
officers on board the ss. Buccaneer in 1886. ‘The submerged channel is only traceable to the depth of 
the 400-fathom contour, and crosses the Continental Platform, which is here from 40 to 50 miles broad. 
It was originally described by Mr. Edward Stallibrass before the Society of Telegraphic Engineers ; and 
more recently by Mr. Henry Benest. 
The Niger or Quarra.—In the case of this great river we are met by a similar disappointment owing 
to the absence of soundings failing to reach the bottom of the old channel; and we are only able to 
follow its course to the depth of about 270 fathoms, or 1620 feet. For the purpose of the soundings, 
this was doubtless considered sufficient by the officers employed, and it is not surprising that the experi- 
ments were not followed to a greater depth.' 
The Orange River.—This important river, in a position not very distant from the southern end of the 
African Continent, would, if the data for determining its channel under the ocean had been available, have 
1 Fourn. Soc. Teleg. Eng. vol. xvi. (1887). From a statement made by Mr. Henry Benest (Transactions of the Victoria Institute, vol. xxxii. 
p. 161), it would appear that the soundings taken over the “ Bottomless Pit” by the Silvertown Company’s steamer Bree, in 1886, were under the 
supervision of Mr. J. Y. Buchanan. The latest observations show the width of the channel at less than a mile and a half from the shore to be 
170 fathoms. At about 7 miles from the coast the depth is 327 fathoms, and at 9 miles, 452 fathoms (or 2712 feet). Soundings are, of course, 
undertaken with the one object, for the information of navigators ; and when they fail to reach the bottom, at depths much below the possibility 
of interfering with the course of ships, or offering fz ble ground for anch they are discontinued. 
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