August ii, 1892] 



NA TURE 



'» tr 1 



round the Gulf of Mexico, but in the Sierra Nevada or Northern 

 Colombia and the Cordilleras of Venezuela Tertiary strata 

 are ridged up into true mountains of elevation. Thus the Mexi- 

 can-Caribbean depression, like that of the Mediterranean, is 

 characterized not only by its irregular depths and its volcanic 

 phenomena, but by the propinquity of recent mountains of up- 

 heaval, which bear the same relation to the Caribbean Sea that 

 the mountains of North Africa do to the Mediterranean. 



We may now compare the Atlantic coasts of South America 

 with those of Africa. The former coincide in general direction 

 with the edge of the continental plateau, to which they closely 

 approach between Cape St. Roque and Cape Frio. In the 

 north-east, between Cape Paria, opposite Trinidad, and Cape 

 St. Roque, the continental shelf attains a considerably greater 

 breadth, while south of Cape Frio it gradually widens, until, in 

 the extreme south, it runs out towards the east in the form of a 

 narrow ridge, upon the top of which rise the Falkland Islands 

 and South Georgia. Excluding from consideration for the pre- 

 ■;ent all recent alluvial and Tertiary deposits, we may say that 

 the coast lands from Venezuela down to the South of Brazil are 

 composed principally of Arch^an rocks ; the eastern borders of 

 the continent further south being formed of Quaternary and 

 Tertiary accumulations. So far as we know, igneous rocks are 

 of rare occurrence on the Atlantic seaboard. Palaeozoic strata 

 approach the coast-lands at various points between the mouths 

 of the Amazons and La Plata, and these, with the underlying 

 and surrounding Archaean rocks, are more or les> folded and 

 disturbed, while the younger strata of Mesozoic and Cainozoic 

 age (occupying wide regions in the basin of of the Amazons, and 

 here and there fringing the sea-coast), occur in approximately 

 horizontal positions. It would appear, therefore, that no great 

 axial uplifts have taken place in those regions since Palaeozoic 

 times. The crustal movements of later ages were regional 

 rather than axial ; the younger rocks are not ilexed and mashed 

 together, and their elevation (negative or positive) does not 

 -eem to have been accompanied by conspicuous volcanic action. 



The varying width of the continental shelf is due to several 

 uises. The Orinoco, the Amazons, and other rivers descend- 

 iig to the north-west coast, carry enormous quantities of sedi- 

 ment, much of which comes to rest on the submerged slopes 

 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 mouth of the Kio 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 chiefly of Archrean and Palaeozoic rocks, 

 tbe low shore-lands showing here and there strata of Mesozoic 

 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 relatively 

 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, however, notwith- 

 standing 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 be- 

 tween 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 Palaeozoic times. The 

 iater 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 compara- 

 tively horizontal positions. Between Cape Nun and Cape 



Spartel the rocks of the maritime tracts range in age from 

 Palaeozoic to Cainozoic, and have been traced across Morocco 

 into Algeria and Tunis. They all belong to the Mediterranean 

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

 of that inland sea extended from a point opposite the Canary 

 Islands along the southern margin of Morocco, Algeria, and 

 Tunis. Towards the close of the Tertiary period the final up- 

 heaval 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 co- 

 extensive with the continental plateau, but it also results in 

 large measure 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 protecting 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 the bordering continents the last series of 

 great mountain-uplifts took place in Palaeozoic times. This 

 is true alike for North and South America, for Europe and 

 Africa. 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 Caino- 

 zoic rocks, which now and again form the shore- 

 lands, occur in more or less undisturbed condition. 

 The only great linear uplifts or true mountains of eleva- 

 tion which have come into existence in Western Europe 

 and Northern Africa since the Palieozoic period trend approxi- 

 mately at right angles to the direction of the Atlantic trough, 

 and are obviously related to the primitive depression of the 

 Mediterranean. The Pyrenees and the Atlas, therefore, although 

 their latest elevation took place in Tertiary times, form no ex- 

 ceptions to the rule that the extreme flexing and folding of 

 strata which is so conspicuous a feature in the geological struc- 

 ture of the Atlantic sea-board dates back to the Palaeozoic era. 

 And the same holds true of North and South America. There 

 all the coastal ranges of highly flexed and folded strata are of 

 Palaeozoic age. The Cordilleras of Venezuela are no doubt a 

 Tertiary uplift, but they are as obviously related to the Carib- 

 bean depression as the Atlas ranges are to that of the Mediter- 

 ranean. .A.gain, we note that volcanic activity along the borders 

 of the Atlantic was much less pronounced during the Mesozoic 

 period than it appears to have been in earlier ages. Indeed, if 

 we except the great Tertiary basalt-flows of the Icelandic ridge 

 and the Arctic regions, we may say that volcanic action almost 

 ceased after the Palaeozoic era to manifest itself upon the Atlantic 

 coast-lands of North America and Europe. But while volcanic 

 action has died out upon the Atlantic margins of both continents, 

 it has continued during a prolonged geological period within 

 the area of the Mediterranean depression. And in like manner 

 the corresponding depression between North and South America 

 has been the scene of volcanic disturbances from Mesozoic down 

 to recent times. Along the African coasts the only displays of 

 recent volcanic action that appertain to the continental margin 

 are tho-;e of the Gulf of Guinea and the Cape de Verde Islands. 

 The Canary Islands and Madeira may come under the same 

 category, but, as we have seen, they appear to stand in relation- 

 ship to the Mediterranean depression and the Tertiary uplift of 

 North Africa. Of Iceland and the Azores I have already 

 spoken, and of Ascension and the other volcanic islets of the 

 South Atlantic it is needless to say that they are related to 

 wrinkles in the trough of the ocean, and therefore have no im- 

 mediate connection with the continental plateau. 



Thus in the geographical development of the Atlantic coast- 

 lines we may note the following stages : — First, during 

 Palaeozoic times a series of great mountain-uplifts, which were 

 frequently accompanied by volcanic action. Second, a pro- 

 longed stage of comparative coastal tranquillity, during which 

 the maritime ranges referred to were subject to such excessive 

 erosion that they were planed down to low levels, and in certain 

 areas even submerged. Third, renewed elevation (negative or 

 positive) whereby considerable portions of the much denuded 

 Archjean and Palaeozoic rocks, now largely covered by younger 



NO. I 189, VOL. 46] 



