84 PROF. E. H. L. SCHWARZ ON THE COAST-LEDGES [Feb. 1906, 



In the foregoing table I have cited only the main plateaux ; the 

 figures with a query after them refer to cases where the action 

 of marine denudation is doubtful. The highest undoubted rock- 

 shelf is the Kentani plateau, in which the lower road through the 

 Native Territories is laid, along which are built the villages of Kei 

 Boad, Komgha, Butterworth, Idutywa, Umtata, Flagstaff, and 

 Bizana. 



In Europe we have traces of similar terraces. The edge of the 

 shelf supporting the British Isles is generally known as the 

 100-fathom line. In front of this is the great escarpment of 

 the continent, descending steeply to 7000 or 8000 feet, sometimes 

 precipitously ; the level at the bottom is what I have called the 

 absolute base-level of erosion. Off Scotland, the margin of 

 the plateau lies about 600 feet below the surface ; but in front of the 

 English Channel, the soundings indicate depths of 1000 to 1200 feet. 

 South-west of Spitsbergen lies a plateau depressed 2418 feet 

 below sea-level, with depths immediately beyond of 8100 feet; and, 

 farther off, the sea-floor sinks to 15,900 feet. Between Greenland 

 and Iceland, the submerged plateau is depressed 1500 feet. 



The drowned valleys of Europe open on to the sea-floor at depths 

 of from 7000 to 9000 feet, the greatest of them being the valley of 

 the Adour. 1 South of the Straits of Gibraltar the continental plat- 

 form descends to 7200 feet, with a gradual slope probably, ' consist- 

 ing of a succession of minor terraces breaking off in cliffs.' 2 In the 

 Northern Atlantic and Arctic Ocean, littoral shells have been dredged 

 up from depths of 8500 feet. 3 I would, therefore, make the abso- 

 lute base-level of erosion at an average of 9000 feet on the eastern 

 side of the North Atlantic. Prof. Hull, in the paper just referred to, 

 publishes a chart of the ocean-floor off the mouth of the Congo, and 

 shows a drowned valley opening out on to an absolute base-level 

 of erosion at 12,600 feet, so that there is apparently a depression of 

 the sea-floor. South of this point, I find evidence of elevation of the 

 absolute base-level of erosion, from this great depth to only some 

 1000 feet ; and, in passing, I should like to anticipate the conclusion 

 of this paper by drawing attention to the nature of the river- 

 systems north and south of the Congo : that is to say, the land- 

 relief, in areas where the absolute base-level of erosion is very deep 

 below tide- level, is favourable to broad valleys and permanent 

 rivers, but where it is high the land-surface is furrowed by deep 

 gorges and traversed by dry river-channels. 



On the western side of the Atlantic, there is a corresponding- 

 continental shelf, submerged some 200 or 250 feet below the ocean- 

 surface, with an external fringe submerged another couple of 

 hundred feet. Off this, there is a long, gradual descent to deep 

 water, mostly unbroken by ledges ; but the Blake Plateau interrupts 

 it in places by a shelf from 2500 to 3500 feet deep, with an average 



* Alphonse Milne-Edwards, Bull. Soc. de Geogr. vol. iii (1882) p. 113. 



2 E. Hull, 'Sub-Oceanic River-Valleys of the West- African Continent, &c. 

 Trans. Victoria Institute, vol. xxxii (1900) p. 148. 



3 W. O. Brogger, Norges Geol. Undersogelse. No. 31 (1900-1901) pp. 682-83. 



