804 KEPOKT— 1892. 



tion across Asia Minor with the Indian Ocean, and with the Arctic Ocean athwart 

 the low-lying tracts of North-western Asia. Similarly, it would seem, the Mexican- 

 Caribbean Sea is the remaining portion of an ancient iuknd sea wliich formerly 

 stretched north through the heart of North America to the Arctic Ocean. Like 

 its European parallel, it has been diminished by sedimentation and crustal move- 

 ments. It resembles the latter also in the greatness and irregularity of its depths, 

 and in the evidence which its islands supply of volcanic action as well as of very 

 considerable crustal movements within recent geological times. Along the whole 

 northern borders of the Gulf of Mexico the coast-lands, like those on the Atlantic 

 sea-board of the Southern States, are composed of Tertiary and recent accumulations, 

 and the same is the case with Yucatan ; while similar young formations are met 

 with on the borders of the Caribbean Sea and in the Antilles. The Bahamas and 

 the Windward Islands mark out for us the margin of the continental plateau, which 

 here falls away abruptly to profound depths. One feels assured that this portion 

 of the plateau has been ridged up to its present level at no distant geological date. 

 But notwithstanding all the evidence of recent extensive crustal movements in this 

 region, it is obvious that the Mexican-Caribbean depression, however much it may 

 have been subsequently modified, is of primitive origin.' 



Before we leave the coast-lands of North America I would again point out 

 their leading geological features. In a word, then, they are composed for the most 

 part of Archoean and Palaeozoic rocks : no great linear or axial uplifts marked by 

 much flexure of strata have taken place in those regions since Palaeozoic times ; 

 while igneous action virtually ceased about the close of the Palaeozoic or the com- 

 mencement of the Mesozoic period. It is not before we reach the shores of the 

 Southern States and the coast-lands of the Mexican-Caribbean Sea that we 

 encounter notable accumulations of Mesozoic, Tertiary, and younger age. These 

 occur in approximately horizontal positions round the Gulf oV Mexico, but in the 

 Sierra Nevada of Northern Colombia and the Cordilleras of Venezuela Tertiary strata 

 enter into the formation of true mountains of elevation. Thus the Mexican-Caribbean 

 depression, like that of the Mediterranean, is characterised, not only by its irregular 

 depths and its volcanic phenomena, but by the propinquity of recent mountains of 

 upheaval, 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. 

 la the north-east, between Cape Paria, opposite Trinidad, and Cape St. Roque, the 

 continental slielf 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 present 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 Archsean 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 sea-board. 

 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 less folded and disturbed, while the younger strata of 

 Mesozoic and Cainozoic age (occupying wide regions in the basin 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 flexed and mashed together, 

 and their elevation (negative or positive) does not seem to have been accompanied 

 by conspicuous volcanic action. 



' Professor Suess thinks it is probable that the Caribbean Sea and the Mediter- 

 ranean are portions of one and the same primitive depression which traversed the 

 Atlantic area in earlv Cretaceous times. He further suggests that it may have been 

 through the gradual widening of this central Mediterranean that the Atlantic in later 

 times came into existence. 



