THE GEOGRAPHIC RELATIONS OF THE WEST INDIES 5 



The great physical differences between the lands bor- 

 dering the G-ulf of Mexico and the Caribbean Sea are 

 chiefly dependent upon the arrangement and relation of 

 the Rocky Mountain, Andean, and Antillean systems of 

 mountain folds. The first of these in its geognostic as- 

 pects and relations is distinctly North American, the 

 second South American, and the third is peculiarly Cen- 

 tral American. The Gulf of Mexico is an indentation 

 into the North American continent the restricted sur- 

 vival of a great interior sea which once extended over the 

 Great Plains region of the United States, which at one 

 time almost, if not entirely, separated North America 

 into two great prehistoric continents, the Appalachian 

 and Cordilleran. The basin of the Gulf is still filling up 

 from the sediments brought down by rivers which drain 

 nearly one fourth the area of the United States. With 

 the single exception of its extreme southwestern indenta- 

 tion upon the coast of Mexico, the Gulf is surrounded by 

 low plains composed of great sheets of subhorizontal and 

 unconsolidated sediments deposited when its own waters 

 occupied a larger area than at present. The entire sea 

 margin of the Gulf region of the United States and most 

 of Mexico is of this nature, while the north coasts of 

 Yucatan and portions of Cuba, although modified, are 

 related phenomena. Thus the Gulf of Mexico, instead 

 of having a mountainous periphery like the Caribbean, 

 is bordered by plains. 



There is still another class of tropical mountains, distinct 

 from those made of folds of the earth's sedimentary crust. 

 These are the volcanoes which have grown by extrusion 

 and accumulation. Sometimes they are parasitic upon 

 the folded mother systems, sometimes independent of 

 them. They belong to the great area of igneous erup- 

 tivity which, at least since the beginning of Tertiary time, 

 has marked the western half of the North American con- 

 tinent, the northern and western sides of South America, 

 and the eastern side of the Caribbean region. Although 



