Vol. 62.'} IX THE SOUTH-WEST OF CAPE COLONY. 85 



of 2700 feet. The submarine valleys which cut through the 

 plateaux reach to depths of 12,000 feet below sea-level. Even at 

 these great depths there still appear to be plains, which probably 

 were once near or above the surface of the water; for instance, south- 

 east of ~New Jersey the contour-lines between 9000 and 10,500 feet 

 are some 60 miles apart. 1 The greater depths of 20,000 feet, such as 

 are reached in the Bartlett Deep south of Cuba, probably indicate 

 structural depressions below the absolute base-level of erosion. 



If we admit the expression absolute base-level of erosion, we 

 have then evidence which goes to show that this surface is depressed 

 4000 feet more on the western shores of the North Atlantic than 

 it is on the eastern, and that the same depression is found off the 

 mouth of the Congo on the eastern shores of the South Atlantic, but 

 that south of this there is a rapid elevation to within 1000 feet of 

 ocean-level. We can, then, compare the following plateaux : — 



North Atlantic, 

 American side. 



i 



Feet 

 Sea-level — 



North Atlantic, 

 European side. 



Feet 

 Sea -level — 



South Afric; 



Kentani plateau . 

 De Vlugt plateau. 

 Uplands plateau . 



Sea-level 



Agulbas plateau . 

 A.B.L.E 



f erosion.'] 



Feet 

 2500 

 1500 



700 



Coast-shelf 300 



Blake plateau . 2700 



A.B.L.E 12,000 \ 



[A.B.L.E 



Coast-shelf 600 



Iceland shelf ... 1200 

 A.B.L.E 9000 



. = ' Absolute base-level o: 



600 

 1200 



Whatever may be the objections to the term absolute base-level 

 of erosion (and I myself can see many), it expresses some surface 

 that must exist ; and once we grasp this fact, we have a most fertile 

 medium through which to look at the unexplained differences in the 

 surface-conditions of continental areas. By its means we determine 

 that Europe and America are, on the Atlantic coast, on the down- 

 grade of the oscillatory motion which all land-masses are under- 

 going, and have very nearly reached bottom. South Africa, on the 

 other hand, is on the upgrade, and probably near the top. Compare 

 the topography of Europe and South Africa. In Europe, one finds 

 flowing streams which have cut their beds so that adjoining stream- 

 systems meet at the crest of the dividing-ridge : iu South Africa, 

 the stream-beds are separated by great distances of level country, 

 the actual river-valley occupying only a narrow V-shaped gorge 

 or channel. In Europe, the valleys are wide, great tracts of 

 alluvium spread along the borders of the streams, and deltas form 

 at the mouths : in South Africa, the valleys are cut almost always 

 in rock, and at the mouths the rivers run out to sea through rock- 

 bound gates. In Europe, when a dam is to be built, a moderate 



1 J. W. Spencer, Bull. G-eol. Soc. America, vol. xiv (1903) p. 213 ; see also ibid, 

 vol. vi (1895) p. 103. 



