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 aceumulations of volcanie intrusions 
and ejecta, 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 ejecta; but instead of dominating a 
continental region, practically constitutes a mountainous perimeter sur- 
rounding the depressed basin of the Caribbean. 
Upon this arrangement 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 oceupied 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 cirele enelosing 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 voleanie 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. 
