HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 161 



The Andean region of the South American continent is one of north 

 and south folded sedimentaries plus accumulations of volcanic intrusions 

 and ejeeta, and dominates a continental area. 



The Caribbean region, including Central America, the Antilles and the 

 Windward Islands, and most of the Venezuelan and Colombian coast of 

 South America, is one of east and west folded sedimentaries plus accumu- 

 lations of volcanic intrusions and ejeeta ; but instead of dominating a 

 continental region, practically constitutes a mountainous perimeter sur- 

 rounding the depressed basin of the Caribbean. 



Upon this ai'rangement of the three systems of mountain folds are 

 chiefly dependent the great physical differences between the lands 

 bordering the Gulf of Mexico and the Caribbean Sea. The former in 

 their geognostic aspects and relations are North American, while the 

 latter are distinctly Central American. 



The Gulf of Mexico, with the single exception of its extreme south- 

 western indentation upon the coast of Mexico, is surrounded by gently 

 tilted plains composed of great sheets of subhorizontal sediment largely 

 deposited by its own waters when they occupied a larger area than at 

 present. The entire Gulf margin 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. 



The Central American region as above outlined, — i. e. that portion of 

 the American hemisphere extending from the southern termination of 

 the Rocky Mountain region to the northern termination of the South 

 American Andes, including the southern border of Mexico, the Republics 

 of Central America, and the Isthmus of Panama proper, — constitutes the 

 western perimeter of the mountainous circle enclosing the Caribbean. 

 As a whole it is called by some writers the American Isthmian region. 

 This greater Isthmian (Central American) region is marked by its nar- 

 row elongated outlines relative to the broadening areas of the adjacent 

 continent, and the completely mountainous character of its entire 

 topography. Its conspicuous characters are : — 



1. The volcanic plateau lying nearer the Pacific coast, from its com- 

 mencement in Guatemala to its eastern termination in Costa Rica, 

 which is composed of accumulated material extruded across the western 

 termini of the Antillean trends. 



2. The lower but mountainous portions of the Caribbean side, com- 

 posed of folded mountain axes extending in east and west directions 

 in conformable direction with the Antillean uplifts, accompanied by 

 eruptive extrusions of past geologic time. 



