TRANSACTIONS OF SECTION E. 805 



The varying widtli of the continental shelf is due to several causes. Tiie 

 Orinoco, the Amazons, and other rivers descending to the north-east coast, carry 

 enormous quantities of sediment, much of which comes to rest on the submerged 

 elopes of the continental plateau, so that the continental shelf tends to extend 

 seawards. The same process takes place on the south-east coast, where the River 

 Plate discharges its muddy waters. South of latitude 40° S., however, another 

 cause has come into play. From the month of the Rio Negro to the terminal point 

 of the continent the whole character of the coast betokens a geologically recent 

 emergence, accompanied and followed by considerable marine erosion. So that in 

 this region the continental shelf increases in width by the retreat of the coast-line, 

 while in the north-east it gains by advancing seawards. It is to be noted, however, 

 that even there, in places where the shores are formed of alluvia, the sea tends to 

 encroach upon the land. 



The Atlantic coast of Africa resembles that of South America in certain 

 respects, but it also offers some important contrasts. As the northern coasts of 

 Venezuela and Colombia must be considered in relation rather to the Caribbean 

 depression than to the Atlantic, so the African sea-board between Cape Spartel and 

 Cape Nun pertains structurally to the Mediterranean region. From the southern 

 limits of Morocco to Cape Colony the coastal heights are composed chie9y of 

 Archaean and Paleozoic rocks, the low shore-lands showing here and there strata of 

 Meaozoic and Tertiary age together with still more recent deposits. The existing 

 coast -lines everywhere advance close to the edge of the continental plateau, so that 

 the submarine shelf is relati%'ely narrower than that of Eastern South America. 

 The African coast is still further distinguished from that of South America by the 

 presence of several groups of volcanic islands — Fernando Po and others in the Gulf 

 of Guinea, and Cape Verde and Canary Islands. The last-named group, how- 

 ever, notwithstanding its geographical position, is probably related rather to the 

 Mediterranean depression than to the Atlantic trough. 



The geological structure of the African coast-lands shows that the earliest to 

 come into existence were those that extend between Cape Nun and the Cape of Good 

 Hope. The coastal ranges of that section are much denuded, for they are of very 

 great antiquity, having been ridged up in Palnsozoic times. The later uplifts 

 (negative or positive) of the same region were not attended by tilting and folding 

 of strata, for the Mesozoic and Tertiary deposits, like those of South America, lie 

 in comparatively horizontal positions. Between Cape Nun and Cape Spartel the 

 rocks of the maritime tracts range in age from Palteozoic to Cainozoic, and have 

 been traced across Morocco into Algeria and Tunis. They all belong to the Medi- 

 terranean region, and were deposited at a time when the southern shores of that 

 inland sea extended from a point opposite the Canary Islands along what is now 

 the southern margin of Morocco, Algeria, and Tunis. Towards the close of the 

 Tertiary period the hnal upheaval of the Atlas took place, and the Mediterranean, 

 retreating northwards, became an almost land-locked sea. 



I need hardly stop to point out how the African coast-lines have been modified 

 by marine erosion and the accumulation of sediment upon the continental shelf. 

 The extreme regularity of the coasts is due partly to the fact that the land is 

 nearly coextensive with the continental plateau, but it also results in large mea- 

 sure from the extreme antiquity of the land itself. This has allowed of the cutting- 

 back of headlands and the filling up of bays and inlets, a process which has been 

 going on between Morocco and Cape Colony with probably little interruption for 

 a very prolonged period of time. We may note also the effect of the heavy rains 

 of the equatorial region in washing down detritus to the shores, and in this way 

 in'otecting the land to some extent from the erosive action of the sea. 



What now, let us ask, are the outstanding features of the coast-lines of the 

 Atlantic Ocean ? We have seen that along the margins of each of tlie bordering 

 continents the last series of great mountain-uplifts took place in Paheozoic times. 

 This is true alike for North and South America, for Europe and .'Vfrica. Later 

 movements which have added to the extent of land were not marked by the extreme 

 folding of strata which attended the early upheavals. The Mesozoic and Cainozoic 

 rocks, which now and again form the shore-lauds, occur in more or less undisturbed 



