PHYSICAL FEATURES OF MEXICO. 15 
descends about 400 feet from the higher elevations, where there are the 
remains of baselevels and also terrace plains up to at least 75 feet above 
the low divide (which is 776 feet above the sea). 
PACIFIC COASTAL PLAIN OF THE TEHUANTEPEC ISTHMUS 
The Pacific side is distinguished by low plains (with great island 
studded lagoons) which extend much farther inland than beyond the 
limits of the isthmus. The plains rise to an elevation of more than 300 
feet at the edge of the mountain zone, which is 25 or 30 miles from the 
sea. Out of the plains occasional island-like bosses, ridges, and cerros 
or isolated dome-like masses rise from a few feet in height to several hun- 
dred. These are the remains of an epoch of erosion when the region was 
reduced to a baselevel of degradation. The floors of the baselevel pla us 
are commonly covered with more recent accumulations of loams and 
gravels. 
HIGH PLATEAUS 
Landward of the Gulf coastal plain the escarpments rise rapidly to 
the elevated plateaus. Thus, back of Vera Cruz, in ascending a valley 
in a direct line of 40 miles from the foot of the mountain, the floor of the 
tableland is reached at an elevation of 8,000 feet. Along the Interoceanic 
railway, a little to the north, the same altitude is reached more quickly. 
Southward of Monterey the plains send fingers among the mountain 
spurs. Ata distance of 60 or 70 miles farther south the summit of the 
plateau is reached at 6.000 feet. After ascending the valley or the steeper 
slopes of the escarpments the margins of the plateaus are usually very 
abrupt, but their surfaces are remarkably level plains, out of which rise 
ranges of hills and interrupted cerros, older mountain chains, and vol- 
eanic cones, such as Popocatapetl and Iztaccihuatl, illustrated in plate 2, 
which reach an elevation of 8,000 or 10,000 feet above the plateau. The 
extensive plateau thus diversified produces distinctive landscapes. The 
abrupt margins of the plateaus, whether at less elevations or at 6,000, 
8,000, or 10,000 feet above the sea, irrespective of their origin, are incised 
with canyons often of considerable depth, although not of great length. 
MOUNTAIN REGION OF THE TEHUANTEPEC ISTHMUS 
This zone is reduced to the remarkably narrow width of only 25 miles, 
bounded by the two coastal plains. The high plateau region of Mexico, 
with the mountains rising to an elevation of from 6,000 to 10,000 feet or 
more, is here broken down for a distance of 60 or 80 miles, so that the 
higher points do not exceed 4,000 feet, and for a distance of perhaps over 
25 miles the ridges are not more than 2,000 feet in height, with base- 
levels among them. The divide is from 900 to 1,000 feet above sealevel 
