180 GEOGRAPHIC RELATION OF THE THREE AMERICAS 
the lands bordering the gulf of Mexico and Caribbean sea ; the 
former in its geognostic aspects and relations is North American, 
while the latter is distinctly Central American. 
Tlie gulf of Mexico, with the single exception of its extreme 
southwestern indentation of the coast of Mexico, is surrounded 
l)v gently tilted plains, composed of great sheets of sul)horizontal 
sediment, largely dej)osited by its own waters when the}' occu- 
})ied a larger area tlian at present. 
The Central American region as above outlined — i. e., that 
l)ortion of tlie American hemisphere extending from the south- 
ern termination of the Rocky Mountain region to the northern 
termination of the South American Andes, including the south- 
ern border of Mexico, the Republics of Central America, and the 
isthmus of Panama proper — constitutes the western perimeter 
of the circle of mountains inclosing the Caribbean. As a whole 
it is called by some writers the American Isthmian region,* and 
can be genetically separated into two conspicuous regions : 
1. The recent volcanic plateau lying nearer the Pacific coast 
from its commencement in Guatemala to its eastern termination 
in Costa Rica, which is composed of accumulated material ex- 
truded across the western termini of the Antillean trends. 2. The 
lower but nevertheless mountainous iiortions of the Caribbean 
side, composed of folded mountain-axes extending east-west in 
conformable direction with the Antillean uplifts, accompanied 
by old eruptive extrusions of past geologic time. The most 
cons])icuous eminences are the grand volcanic peaks of Guate- 
mala, San Salvador, and Costa Rica. These rise to an average 
height of 10,000 feet, in irregular masses standing nearer the 
Pacific coast than the Atlantic until reaching the borders of 
C’osta Rica, Avhen they sweep diagonally toward the Caribbean 
side, again assuming in the southern j)ortion of that republic a 
central continental position. These great eminences are built 
up of accumulations of volcanic debris, which have buried and 
largely concealed a most interesting antecedent geologic structure 
that must be interpreted before the complete history of the re- 
gion can be written. These mountains, being largely extrusions 
of volcanic material instead of regular folds or })lications of 
stratified rock, ])roduce irregularities of surface which defy the 
ordinary modes of classification. 
* The conspicuous features of this greater Isthmian (Central American) region are 
its narrow, elongated outlines relative to the bro.idening areas of the adjacent conti- 
nent and the completely mountainous character of its entire area, which is void of 
coastal plains. 
